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METHODOLOGIES FOR ECOLOGICAL
MONITORING IN BIOENERGY CROPS
A review and recommendations
ADAS Contract Report for the Department for
Environment, Food and Rural Affairs
Defra Project NF0408
Chris Britt
August 2003
(Revised December 2003)
CONTENTS
CONTENTS .................................................................................................................................... 1
EXECUTIVE SUMMARY ................................................................................................................ 2
Objectives and scope ...................................................................................................................... 2
Biomass crop production ................................................................................................................. 2
Environmental impacts of biomass crops ....................................................................................... 2
Objectives of ecological surveys and monitoring ............................................................................ 2
1.
1.1
1.2
1.3
1.4
INTRODUCTION ................................................................................................................. 8
Objectives and scope .......................................................................................................... 8
Biomass crop production ..................................................................................................... 8
Environmental impacts of biomass crops ............................................................................ 9
Objectives of ecological surveys and monitoring .............................................................. 11
2.
2.1
2.2
ECOLOGICAL MONITORING .......................................................................................... 13
Practical considerations ..................................................................................................... 13
Ecological monitoring programmes and networks ............................................................. 13
3.
3.1
3.2
3.3
3.4
PLANTS............................................................................................................................. 15
Botanical species diversity in biomass crops .................................................................... 15
Standard botanical survey methods .................................................................................. 16
Botanical survey methods applied in studies of biomass crops ........................................ 19
Advantages and disadvantages of different methods in relation to surveys in biomass
crops .................................................................................................................................. 21
4.
4.1
4.2
4.3
4.4
INVERTEBRATES ............................................................................................................ 23
Invertebrate species diversity in biomass crops ................................................................ 23
Standard invertebrate survey methods .............................................................................. 24
Invertebrate survey methods applied in studies of biomass crops .................................... 33
Advantages and disadvantages of different methods in relation to surveys in biomass
crops .................................................................................................................................. 34
5.
5.1
5.2
5.3
5.4
BIRDS ................................................................................................................................ 36
Bird species diversity in biomass crops ............................................................................. 36
Standard bird survey methods ........................................................................................... 38
Bird survey methods applied in studies of biomass crops ................................................. 41
Advantages and disadvantages of different methods in relation to surveys in biomass
crops .................................................................................................................................. 41
6.
6.1
6.2
6.3
6.4
MAMMALS ........................................................................................................................ 43
Mammal species diversity in biomass crops ..................................................................... 43
Standard mammal survey methods ................................................................................... 44
Mammal survey methods applied in studies of biomass crops ......................................... 47
Advantages and disadvantages of different methods in relation to surveys in biomass
crops .................................................................................................................................. 48
7.
7.1
7.2
7.3
7.4
REPTILES AND AMPHIBIANS ........................................................................................ 50
Reptile and amphibian species diversity in biomass crops ............................................... 50
Standard reptile and amphibian survey methods .............................................................. 50
Reptile and amphibian survey methods applied in studies of biomass crops ................... 51
Advantages and disadvantages of different methods in relation to surveys in biomass
crops .................................................................................................................................. 52
8.
RECOMMENDATIONS ..................................................................................................... 53
9.
ACKNOWLEDGEMENTS ................................................................................................. 55
10.
REFERENCES .................................................................................................................. 56
1
EXECUTIVE SUMMARY
Objectives and scope
1.
The main objectives of the work described in this report were to review
methodologies used for ecological monitoring in short rotation poplar and willow,
and other biomass (bioenergy) crops, and to recommend standardised methods for
future projects.
2.
This report considers currently available information on the biodiversity of bioenergy
crops and reviews the methods employed in previous research, survey and
monitoring projects.
It also summarises standard ecological monitoring
methodologies for plants, invertebrates, birds, mammals, reptiles and amphibians –
and evaluates the practical advantages and disadvantages of different techniques
for studies of species diversity in bioenergy crops.
3.
The report recommends that new projects for the ecological monitoring of bioenergy
crops in the UK should be encouraged to adopt a standard suite of protocols,
although this should not preclude originality or the inclusion of additional species or
groups.
4.
The report does not prescribe suitable protocols, but sets out 11 basic principles to
be considered (see paragraph 48 below) and provides clear guidance on the likely
suitability, or otherwise, of many commonly used ecological monitoring methods.
Biomass crop production
5.
Bioenergy crops, which include short rotation coppice (SRC) and energy grasses,
are likely to make a major contribution to Government targets for increased use of
renewable energy sources and reduced carbon dioxide emissions.
6.
The main bioenergy crops likely to be grown in the UK are willow and poplars SRC,
Miscanthus grass and whole crop cereals. Other potential candidates, for the
longer-term, include other grass species such as reed canary grass (Phalaris
arundinacea), switchgrass (Panicum virgatum) and giant reed (Arundo donax).
Environmental impacts of biomass crops
7.
Bioenergy crops provide several potential environmental benefits, including
greenhouse gas reduction, reduced nutrient leaching, reduced soil erosion and
phytoremediation of contaminated soils.
8.
One very important consideration, before widespread planting of any new crop is
actively encouraged, is the probable impacts on biodiversity. This aspect is only
partially (in the case of SRC) or poorly (in the case of energy grasses) understood.
However, in most cases, biodiversity is likely to be low in comparison with most
semi-natural habitats – but will vary considerably, depending on factors such as
management practices, plantation size and location.
9.
Impacts on biodiversity should be considered on a landscape scale and viewed in
the context of the crop or other land-use that it is replacing.
Objectives of ecological surveys and monitoring
10.
The objectives of any ecological survey or monitoring must be considered at the
outset, as these will have an important influence on methodologies used.
Objectives might include: describing the interest of sites, estimating population size,
2
monitoring population changes, determining the habitat requirements of a species,
determining why species have declined, monitoring habitat management and
studying population dynamics.
Ecological monitoring
11. The methodologies used for ecological monitoring will be determined primarily by the
scientific objectives, available resources and practical considerations.
12.
Bioenergy crops are usually very densely planted and may be difficult to walk
through, particularly when carrying bulky equipment. Their height and density will
also restrict visibility through the crop for much of the year. Crop plants may impede
the placement of quadrats. Relocation of ‘permanent’ quadrats or recording points
may be difficult.
13.
Previous ecological monitoring of bioenergy crops in the UK, almost exclusively in
SRC, has focused primarily on plants, arboreal invertebrates and birds. More work
is needed to broaden our understanding of the ecological function of SRC crops
within the broader landscape. Studies on the ecology of Miscanthus and other
energy grasses are urgently needed.
Plants
14.
The ground flora of SRC plantations on ex-agricultural sites tends to be dominated
by a small number of competitive or ruderal weed species. In dense, well managed
plantations, ground cover is sparse and shade-tolerant species relatively important.
In less dense, poorly managed plantations (with inadequate weed control),
percentage ground cover can be much higher and open-ground species
predominant. Other factors affecting botanical composition include plantation age,
previous land use and the proximity of woodland.
15.
Species lists can be compiled for any site, or part of a site, but more useful data will
be obtained from assessments of cover, biomass or frequency. Another alternative
is vegetation mapping. Careful thought must be given to the choice of survey period
and the possible need for surveys in more than one period each year.
16.
Previous botanical surveys in SRC have used linear quadrats (e.g. 4 x 1 m or 10 x 1
m) and simple estimates of cover (e.g. DAFOR scale) and frequency. Plant
frequency measurements using ‘nested’ quadrats were used in one study of UK
poplar plantations. Classification of species present using primary ecological
strategies, or plant communities using the National Vegetation Classification (NVC)
system, have been used to aid analysis and interpretation of ground vegetation
data.
17.
Botanical monitoring in bioenergy crops should consider the practical difficulties of
placing quadrats across crop rows, but also take account of the fact that surveys
confined to inter-row areas may provide data that are not representative of plant
species diversity across the whole site. For example, in SRC, different physical
conditions within rows may create a microhabitat that supports communities that are
distinct from those of the wider alleyways.
18.
Large quadrats, or larger numbers of small quadrats, will be required in crops where
ground vegetation is sparse or heterogeneous. The preferred option is probably
more small quadrats, allowing more meaningful calculation of species frequencies.
19.
Botanical methodologies must also take account of edge effects and other potential
causes of transitional change in species composition across a cropped area.
3
20.
Samples must be large enough, and suitably stratified, to calculate (or at least allow
for) effects of geographical/climatic variation, crop species/varieties, crop density
and layout, different management systems, plantation age and stage in the rotation,
previous land-use and adjacent/nearby land-uses or habitat types.
Invertebrates
21.
Native willow species - which include common osier (Salix viminalis), a parent of
most commercially available SRC willow varieties - support a great diversity of
invertebrate species.
22.
Most surveys and research in the UK have, however, focused on leaf beetle species
that are significant pests of SRC crops. The blue willow beetle, in particular, is very
widely distributed in willow SRC crops and is often present at very high densities.
The brassy willow beetle and a sawfly species (Nematus melanaspis) are
sometimes abundant In poplar SRC.
23.
One survey, in the UK and Ireland, recorded invertebrates from 48 taxonomic
groups on the foliage of SRC crops. Willows had a greater diversity of species and
more individual invertebrates than poplars.
24.
Another study, at five sites in southern England, recorded 21 species of
conservation concern – including species of beetle, fly, spider, sawfly, alderfly, bee
and wasp. Ground beetle (Carabidae) species diversity and total catches (in pitfall
traps) declined as SRC plantations aged. Species assemblages for ground beetles,
rove beetles (Staphylinidae) and spiders (Arachnidae) also showed a change from
those of agricultural or ruderal habitats towards those more typical of less disturbed
habitats.
25.
Willow flowers may provide an important source of pollen for bumblebees and other
insects in early spring.
26.
Standard invertebrate survey methods suitable for use in bioenergy crops might
include: soil sampling (soil-dwelling species e.g. earthworms), pitfall trapping (soil
surface species e.g. adult beetles, spiders and ants), beating (species on crop
foliage and stems e.g. spiders, true bugs, leaf beetles, butterfly and moth larvae),
water traps (flying insects e.g. adult flies and sawflies) and direct searches (species
under rocks, in leaf litter or at base of tree stems/stools).
27.
Methods employed in previous surveys of SRC in the UK have included stem
beating, pitfall trapping, direct searches of plant stems and foliage, and counts of
earthworms by soil sampling or application of an irritant chemical to the soil surface.
28.
The use of cumbersome equipment, such as a D-vac suction sampler, is likely to be
very difficult in SRC and Miscanthus crops. Ground vegetation would be more
easily sampled by sweep netting.
29.
Although positioning of collection sheets will be restricted in SRC, stem beating is
considered a good method for the sampling of many foliar invertebrate groups. It is,
however, not considered feasible in Miscanthus crops. Leaf sampling from the
upper canopy, for direct examination, will be more difficult when crops are taller than
2 m.
30.
Digging large soil pits (e.g. for earthworms) may be restricted by dense root
networks and dry soils. Installation of pitfall traps should, however, be readily
achievable although re-location of traps and removal of trays of samples from
plantations could be difficult, particularly in Miscanthus.
31.
Water traps for flying insects (e.g. Diptera and Hymenoptera) may be more suitable
than sticky traps.
4
Birds
32.
The bird species present within bioenergy crops will vary according to the stage of
growth.
33.
Among the most commonly recorded species in willow SRC during the
spring/summer are migrant warbler species (e.g. willow warbler , sedge warbler and
garden warbler), reed bunting and pheasant; although in the early stages of crop
development species such as corn bunting, skylark and linnet may be present. In
winter, SRC is used by several species including pheasants and various thrushes,
finches and tits. Several species of conservation concern have been recorded in
SRC, in addition to those already mentioned.
34.
Little is known about the use of Miscanthus by birds. It is likely to provide a much
less important source of food for insectivorous species, but could provide a suitable
nesting habitat for reed-bed species.
35.
Standard bird survey methods include territory mapping (e.g. BTO’s Common Birds
Census), point counts, line transects, mist netting and ringing (mark-releaserecapture).
36.
Territory mapping, point counts and mist netting have all been used in surveys of UK
SRC plantations. Adapted bird census methodologies have also been used to map
the distribution of winter bird populations in SRC. US studies in poplar biomass
crops have used a line transect method. Sand quadrats have been used to monitor
the movement of gamebirds into and out of SRC plots.
37.
Most of the standard bird survey techniques should be suitable for bioenergy crops,
although severely restricted visibility (particularly in Miscanthus crops) in established
plantations will increase the dependence on expert recognition of bird calls/song.
Mist netting and ringing can only be employed by trained and licensed staff.
Mammals
38.
Although several mammal species have been recorded in willow and poplar
biomass crops, only small mammals (mice, voles and shrews) have been properly
surveyed in the UK. From the limited data available, SRC does not appear to
provide a particularly good habitat for small mammals, although weedy plantations
hold larger populations than relatively weed-free sites.
39.
There have been no surveys of mammals in Miscanthus.
40.
Standard methods for mammal surveys include total counts (e.g. deer or hares),
counts of breeding sites (e.g. badgers, rabbits, foxes or squirrels), counting and
mapping calls (e.g. bats), trapping (e.g. mice, voles and shrews), dung counts (e.g.
rabbits, deer, foxes, otters) and feeding signs (e.g. dormice).
41.
A few surveys of small mammals in SRC have been conducted in the UK. These
have all used Longworth box traps, or similar, to capture live mice, voles and
shrews. US studies in hybrid poplar plantations have used baited ‘snap traps’ that
kill trapped animals. Bats have been surveyed using a tuneable, heterodyne bat
detector.
42.
Small mammal trapping, using grids or transects of live traps, will be appropriate for
surveys of bioenergy crops – but must be repeated over several years, to allow for
annual fluctuations in population size. Trap number and the duration of trapping
periods should be sufficient to ensure meaningful data. Trapping in spring/early
summer and autumn is preferred.
5
43.
The abundance of canopy invertebrates may make SRC an important food source
for bats, and surveys using acoustic methods (tuneable bat detector) should be
encouraged.
Reptiles and amphibians
44.
There have been no formal surveys of reptiles or amphibians in bioenergy crops, but
species observed in or near SRC plantations have included common toad (Bufo
bufo), common frog (Rana temporaria) and common newt (Triturus cristatus). Most
bioenergy crops are, however, unlikely to provide good habitat for amphibians.
Similarly, these crops will not be suitable for most reptiles, but woodland edge/scrub
species such as slow worm (Anguis fragilis) and smooth snake (Coronella austriaca)
may possibly occur in and around the edges of some SRC plantations.
45.
Standard survey methods for amphibian species include night-time, torchlight
searches (preferably in wet weather); measurement of area/volume of frog spawn;
pitfall trapping, with ring-fences or drift fences, if required; and netting or bottletrapping newts. Standard methods for reptiles include direct searches and use of
refugia traps (‘tinning’).
46.
The potential use of bioenergy crops by amphibians, perhaps migrating to a nearby
pond, could be confirmed by night-time torchlight searches or (if it can be justified)
by installing pitfall traps with drift fences. Probably more usefully, ‘tinning’ (refugia
trapping) could be operated to ascertain the presence and distribution of reptile
species. ‘Tinning’ should be carried out between late April and late June or between
late August and late September.
Recommendations
47.
This review of methodologies for ecological monitoring in bioenergy crops has led to
recommendations for a basic framework of techniques for future surveys and
monitoring in the UK.
48.
It is recommended that new UK monitoring projects in energy crops should be
encouraged to adopt a standard suite of protocols (although this should not preclude
originality or inclusion of additional species), and to follow 11 basic principles:
i.
Surveys/monitoring should, wherever possible, include a few ‘core groups’ considered indicative of overall biodiversity. These should include ground flora
species, ground beetles, arboreal insects, breeding birds and small mammals.
The list of core groups should be short, but a secondary list should be
published, for measurement where resources allow. This list should include soil
fauna and might also include butterflies, moths, bats and deer.
ii.
Surveys must not focus solely on groups for which bioenergy crops are known
to provide valuable habitats. An ‘independent’ and wide-ranging approach is
necessary, although it is important to monitor usage of energy crops by species
of conservation concern e.g. Biodiversity Action Plan species.
iii.
Methodologies should be based upon widely accepted techniques - preferably
those ‘tried and tested’ in surveys of bioenergy crops. For the core groups
suggested above, plant frequency counts, pitfall trapping, stem beating, bird
censuses and live-trapping in Longworth box traps are considered suitable.
iv. Methods used must fully consider the practicalities of sampling/recording within
the bioenergy crop concerned. Protocols must allow for row spacings when
specifying the location of recording/sampling positions, and should consider
6
restrictions on access and movement, limited visibility in spring and summer
and potential difficulties in digging soil pits.
v.
Surveys should be designed to facilitate valid and dependable statistical
analyses. This will require care in selection of sample sites and levels of
stratification, and consideration of the use of ‘controls’ - as well as robust field
methodologies. Samples should be representative and take account of
important influences such as climate, altitude, soil type, adjacent land-uses and
plantation size.
vi. New surveys should make full use of any opportunities for comparisons with
‘baseline data’ – either newly collected or already available.
vii. Methodologies should, where applicable, be compatible with those used by
national surveys or national environmental monitoring networks.
viii. Surveys should, wherever possible, allow direct comparisons between the
biodiversity of energy crop plantations and other habitats within the local
landscape – particularly the land-use that it has replaced.
ix. Survey plots must incorporate field margins and field boundaries. Grassy
headlands might provide much of the site’s biodiversity. One important impact
of bioenergy crops may result from removal of the negative effects of
agrochemicals and livestock on hedgerows and watercourses.
x.
Surveys should also consider the wider, landscape-scale impacts of biomass
crops on farmland ecology – taking account of adjoining and nearby habitats,
and linkages with semi-natural habitats, and potential species movements
across the landscape.
xi. One-off surveys have a value, but it is always desirable to repeat surveys over
at least three years, to allow for temporal changes. Populations of many
species change dramatically from year-to-year, as a result of climatic factors,
food availability, predator-prey interactions, etc.
7
1.
INTRODUCTION
1.1
Objectives and scope
The main objectives of the project documented in this report were:

To review methodologies used for ecological monitoring in short rotation
poplar and willow, and other biomass crops.

To recommend standardised methods for future projects.
Consideration of suitable methodologies for ecological surveys in biomass, or
bioenergy, crops requires not just a knowledge of methods used in previous studies
but also a comprehensive overview of standard assessment methodologies and a
detailed understanding of the habitats concerned and the plant and animal
communities within them. These aspects are, therefore, also fully reviewed.
Although other bioenergy crops are mentioned within the report, the focus of the
review is on short rotation coppice (SRC) and Miscanthus. Other crops are either
of relatively minor importance as potential sources of renewable energy in the UK
or are, as in the case of whole-crop cereals, conventional agricultural crops grown
under a slightly modified production system.
1.2
Biomass crop production
Concerns about the effects of global warming have led to international agreements
to reduce consumption of fossil fuels and the resultant emissions of carbon dioxide
– one of the most important of the ‘greenhouse gases’. Renewable energy
technologies, including the use of biomass crops to fuel heat and electricity
generators, are a vital component of strategies to achieve these reductions. Crop
residues, such as cereal straw and forest thinnings, may provide important sources
of fuel, but dedicated biomass crops will also be necessary.
Although the renewable energy industry in Europe is still in a relatively early stage
of development, the most important biomass (or bioenergy) crops at present are
short rotation coppice willow (Salix spp.) or poplar (Populus spp.); Miscanthus
grass; and whole-crop cereals. Other potential crops include energy grasses such as
reed canary grass (Phalaris arundinacea), switchgrass (Panicum virgatum) and
giant reed (Arundo donax).
The agronomy of SRC, or arable energy coppice, has been described in detail by
Britt et al. (1995) and several other authors. However, as the physical structure of
bioenergy crops, the composition and density of associated vegetation, and the
frequency and timing of harvesting and other management operations all have a
major influence on the diversity and ecology of species within these crops, a short
description of crop agronomy is considered appropriate.
8
1.2.1 Agronomy of short rotation coppice
SRC crops are usually mixtures of 3-6 varieties (clones) of high yielding willows
(Salix viminalis or S. viminalis x schwerinii hybrids) or poplars (Populus
trichocarpa, or P. trichocarpa x deltoides or P. deltoides x nigra hybrids). They
are mechanically planted in March or April, as unrooted cuttings. Willows are
usually planted at densities of 12-20,000 cuttings per ha and poplars 10-12,000 per
ha – in double rows, that can be straddled by a large mechanical harvester. Spacing
between rows is usually 0.8 to 1.5 m. Weeds are controlled by pre-planting
cultivations, pre-and post-planting herbicides and by the competition and shading
from the dense crop. Crops are mechanically harvested, using heavy adapted
forage harvesters, in the winter months, every three years (occasionally two or four
year rotations are used).
Well managed SRC crops may be over one metre tall by June in the planting year,
and two to three metres tall by October. In subsequent years, after cut-back or
harvest, growth can be even more rapid. At harvest, crops may be 5-7 m tall.
The effects of residual herbicides and the dry, shady conditions beneath SRC crops
result in a sparse and species-poor ground flora in most crops.
1.2.2 Agronomy of Miscanthus
Miscanthus crops are planted as rhizome pieces, or occasionally as
micropropagated plants. As for SRC, crops are planted into weed-free cultivated
soil in early spring. The densely planted crops grow very vigorously and rapidly
cover the ground, suppressing ground vegetation.
Like SRC, Miscanthus crops are perennial, but one important difference is that
Miscanthus crops are harvested annually. Crops are mechanically cut in the winter
months, after the onset of dormancy and desiccation of the foliage. In spring,
plants quickly re-grow from the crowns – reaching heights of 3-4 m by autumn.
1.3
Environmental impacts of biomass crops
1.3.1 Greenhouse gases
The substitution of renewable biomass crops for fossil fuels in the generation of
heat or electricity, if undertaken on a major scale, will bring important, and fairly
obvious, environmental benefits. For example, it has been calculated that every
hectare of short rotation woody energy crops, grown as a direct substitute for coal,
could displace 5.2 tonnes of fossil carbon in CO2 per annum (Graham et al., 1992),
as well as reducing air pollution from SOx and NOx emissions (Graham et al., 1995;
Muskett, 1996).
There are also other, more direct, effects on greenhouse gas emissions when
bioenergy crops are grown on agricultural land. For example, Börjesson (1999)
stated that “greenhouse gas emissions from arable land can be reduced in three
different ways when annual crops are replaced by perennial energy crops, through
9
(i) accumulation of soil carbon in mineral soils, (ii) reduced carbon dioxide
emissions from organic soils, and (iii) reduced nitrous oxide emission caused by the
use of fertilisers.” If livestock numbers were to be reduced, to accommodate an
increased area of bioenergy crops on grassland, then methane and ammonia
emissions would also be reduced.
However, the picture is not always clear. Zemanek & Reinhardt (1999) used ‘life
cycle assessments’ to compare biofuels and fossil fuels, and to make comparisons
between the relative values of certain biofuels. For example, their comparison of
short rotation forestry (SRF) woodchip with fuel oil in a heating plant led them to
conclude that SRF use saved ‘finite primary energy’ and reduced the ‘greenhouse
effect’ (emission of fewer CO2 equivalents); whereas fuel oil produced fewer
emissions of NOx, SO2 equivalents (responsible for acidification) and total N
(responsible for eutrophication). Use of a system of weighting of factors for
ecological significance, produced a final assessment in favour of the biofuels.
Comparisons between biofuels led Zemanek & Reinhardt (1999) to the conclusion
that cultivated solid biofuels were best if the main objective is to reduce emissions
of greenhouse gases, or to reduce primary energy consumption. Furthermore, they
rated SRF woodchip as “clearly the best” among cultivated biofuels. Liquid
biofuels were considered to compare badly to solid biofuels.
1.3.2 Ecological impacts
Their physical structure, rapid growth rates and the way that they are grown (i.e. at
close spacings, with chemical weed control, fertiliser applications and regular
harvests) mean that the habitat within bioenergy crop plantations is not generally
favourable for plant and animal diversity (Beaumont, 1993; Britt et al., 1995).
However, as these crops usually replace species-poor agricultural monocrops, of
cereals or perennial ryegrass (Lolium perenne), provide a contrasting habitat, and
are managed with relatively low pesticide inputs (Sage, 1998), they can increase
biodiversity at the landscape scale (Beaumont, 1993; Paine et al., 1996; Perttu,
1999). The structure of SRC crops is similar to that of young traditional coppice
plantations (Sage & Robertson, 1994), which frequently support large and diverse
bird populations. There may also be opportunities to enhance the biodiversity of
bioenergy crops through the introduction of woodland ground flora species (Tucker
et al., 1997), although the additional costs and probable incompatibility with
routine management are likely to severely restrict such practices.
In many cases, even though biodiversity within SRC and energy grass crops is
limited, the wide grass headlands (usually 6 m and 12 m) may have greater wildlife
value. With appropriate management for botanical and invertebrate species
diversity, these headlands could potentially provide a useful habitat for a number of
vertebrate species, including several declining farmland birds.
There are numerous willow species native to the UK, including the common osier,
Salix viminalis, which is a parent species of most commercial SRC willow
varieties. Two poplar species are also native. Black poplar (Populus nigra ssp.
betulifolia) occurs naturally across most of England and Wales, and aspen (Populus
tremula) throughout the British Isles (Stace, 1991). Several poplar hybrids and
basket willow clones have also been cultivated here for many years.
10
Most willows support a large number of invertebrate species (Kennedy &
Southwood, 1984), including many actual or potential pest species (Sage, 1994).
Populations of invertebrates in SRC canopies can also be relatively large and
diverse, resulting in a habitat that is attractive to insectivorous birds and, possibly,
bats (although there is currently no direct evidence to support this assumption).
In contrast to SRC, Miscanthus species are recent introductions from Asia and very
few invertebrate species feed on this crop. Similarly, some other non-native
‘energy grasses’ being considered for cultivation in the UK (e.g. switchgrass,
Panicum virgatum, and giant reed, Arundo donax) are likely to support relatively
few phytophagous invertebrate species.
The location of biomass crops relative to other habitat types and land uses can also
strongly influence biodiversity within those crops - and their contribution to
biodiversity on a landscape scale (Christian et al., 1994; Graham et al., 1996).
Woody energy crops such as willow or poplar SRC, if planted adjacent to existing
woodland or forest, can increase the extent of the ‘forest habitat’ and may provide
corridors between fragmented woodland patches in agricultural landscapes
(Graham et al., 1996). On the other hand, any ecological benefits of planting
biomass crops must be weighed against the negative impacts e.g. destruction of
existing habitat, removal of food resources, etc.
Agronomic methods and the intensity of management will heavily influence
biodiversity within biomass crops (Perttu, 1999), with less intensively managed,
weedy crops generally having more plant and animal species and larger
populations. Any major deviation from the ‘standard’ systems of crop agronomy,
as briefly outlined in Section 1.1 of this report, is likely to have significant impacts
on biodiversity value. For example, McLaughlin & Walsh (1998) have pointed out
that cutting switchgrass crops twice per year, instead of annually, could have
negative impacts on nesting birds.
1.3.3 Other environmental impacts
Börjesson (1999) has summarised a number of other potential environmental gains
when willow SRC crops replace annual arable crops. These include reduced
nutrient leaching, reduced soil erosion (by water and wind), uptake of heavy metals
and increased soil organic matter. Börjesson (1999) also reviewed the potential use
of SRC in the treatment of municipal wastes – waste-water, sewage sludge and
landfill leachate.
1.4
Objectives of ecological surveys and monitoring
The objectives of any ecological survey or monitoring may vary in different
situations, but it is vitally important that these are clearly determined at the outset
and that appropriate sampling procedures and methodologies are selected. KovacsLang & Simpson (2000), discussing reasons for ecological monitoring, stated that
“Monitoring systems provide users with information about the state of… biological
resources; trends of natural changes; and effects of human interventions on
11
populations, communities and landscape mosaics. Data from monitoring can be
used to assess priorities for conservation, for land use, for environmental impact
assessments, and for informing policy-makers and the general public on the state of
biodiversity and the environment.”
Sutherland (1996a) sets out a number of general objectives that might apply to
ecological studies:
1. Describing the interest of sites e.g. for the identification of those sites within
an area that have the greatest biodiversity or conservation value.
2. Estimating population size e.g. to estimate the numbers of a given species
within an area, or even to estimate the total population size.
3. Monitoring population changes e.g. for the determination of annual changes
in the population of a particular species.
4. Determining the habitat requirements of a species e.g. comparisons between
various attributes of habitats with and without the species of interest, to
determine particular characteristics associated with success of that species.
5. Determining why species have declined e.g. comparisons between sites where
a species still occurs and sites where it is known to have disappeared;
estimation of fecundity, seedling/juvenile or adult survival; measurement of
limiting factors.
6. Monitoring habitat management e.g. use of experimental designs (including
replication, randomisation and use of control treatments) to measure the impacts
of management practices.
7. Population dynamics e.g. annual counts to allow estimation of the population
size. If estimates of population size are combined with measurements of key
‘life history parameters’ then the causes of fluctuations in population size might
be deduced.
All of these objectives may be applicable to studies within, or incorporating
bioenergy crop sites. Describing the interest of sites is, however, probably most
applicable to surveys that aim to describe the relative importance of SRC and
Miscanthus crops within the wider agricultural landscape, and to compare their
biodiversity with that of other farm crops and with semi-natural habitats.
12
2.
ECOLOGICAL MONITORING
2.1
Practical considerations
The methodology and sampling strategy will be largely determined by the scientific
objectives of the research, survey or monitoring programme and will also be
constrained by resource availability. There will also be important practical
considerations, with certain ecological monitoring methodologies being generally
unsuitable for use in biomass crops, because of difficult access, movement or
vision.
Biomass crops such as willow and poplar SRC and Miscanthus are usually very
densely planted crops. They may quickly become difficult to walk through and
some may be almost impenetrable by late summer. Certainly, in SRC, movement
across rows can be severely impeded. Their rapid rates of growth mean that
biomass crops commonly attain heights above the sight line of most surveyors (1.51.8 m) quite early in the growing season. These restrictions on visibility and
movement make some ecological assessment methods impractical.
Transects through crops may be difficult or impossible to walk, particularly if they
cross crop rows. Visual detection and accurate identification of animals on either
side of the transect may not be possible, particularly when surveying small, highly
mobile species in very dense biomass crops during the summer months.
The placing of quadrats may be very difficult for vegetation recording. If they are
to be used, careful consideration needs to be given to quadrat size, positioning and
whether they need to have a frame that can easily assembled and dismantled in the
field. Random positioning is unlikely to be feasible.
Fixed markers (e.g. for the ends of transects, or corners of quadrats or grids) may
prove difficult to relocate. Horizontal measurements, by whatever method (e.g.
pacing, tape measure, pedometer, hodometer) are likely to be very difficult and,
consequently, inaccurate. However, as most biomass crops will have been
mechanically planted at fairly accurate spacings, distances along lines running
parallel to crop rows, or at 90o across rows, may be reliably estimated by counting
the number of stools/crowns passed or rows crossed, respectively.
It will be difficult to move through biomass crops carrying heavy and bulky
equipment. This may restrict or prevent use of equipment such as D-vac suction
samplers. Vehicular access beyond crop headlands will not be possible, except in
the first few weeks after planting and in the period between crop harvest (during
winter) and early May.
2.2
Ecological monitoring programmes and networks
Ecological monitoring in energy crops has focused almost exclusively on SRC and
primarily on three biological groups: plants, arboreal invertebrates and birds. UK
studies undertaken by the Game Conservancy Trust, ERM (Environmental
Resources Management), CSL (Central Science Laboratory) and other
13
organisations, have employed a range of methods – largely based upon standard
techniques, described by Southwood (1978) and Sutherland (1996a).
These techniques have included the use of quadrats for the estimation of plant
species frequency and % cover in the ground vegetation; beating of stems for
collection of invertebrates in the crop canopy; and census and point count methods
for assessment of breeding and over-wintering bird populations. Other animal
groups have been studied to a lesser degree. These include small mammals,
surveyed using standard live-trapping, mark-recapture methods; and bats,
monitored using acoustic methods (tuneable bat detector).
Occasionally,
researchers have developed novel techniques for monitoring of animal activity in
and around energy crops. One example of this is the use of sand quadrats for the
recording of partridge and pheasant footprints, on the edges of coppice plantations.
Previous studies have provided a good insight into the ecology of SRC, but more
work is required to broaden our understanding of SRC habitat use – and the
ecological function of SRC crops within the broader landscape. Studies on the
ecology of Miscanthus crops are also urgently needed. In the longer term, if
research suggests that other grasses might be potentially viable bioenergy crops,
then the impacts of planting crops such as Phalaris, Phragmites and Arundo on
farmland biodiversity will also require full evaluation.
Studies of the ecological effects of modified cereal management for biomass
production (‘whole crop’ cereals), for example potential reductions in fertiliser and
agrochemical inputs, would be useful – although much can probably be deduced
from the many agricultural research projects on a similar theme.
14
3.
PLANTS
3.1
Botanical species diversity in biomass crops
The ground flora of SRC plantations established on ex-agricultural sites tends to be
dominated by a small number of competitive or ruderal weed species, such as those
typical of disturbed ground. In dense, well-managed plantations, ground cover will
be sparse and shade-tolerant species relatively important. In poorly managed
plantations, where early weed control has been deficient, cover can be very high
and open-ground species predominant. Few species of significant conservation
value are likely to be found, although rare or declining arable plants may persist
(Coates & Say, 1999).
Surveys of 29 SRC plantations across the UK and Ireland in 1993 (Sage et al.,
1994), and a re-survey of 21 of the same sites in 1996 (Sage & Tucker, 1998a),
indicated that the composition of the ground flora was largely dependent upon
factors such as age of plantation, previous land use, crop management and the
proximity of existing woodland.
The ground flora of most SRC plantations were classified as either tall herb or
ruderal communities, although there was a marked increase in the proportion
classified as woodland communities in 1996 (Sage & Tucker, 1998a). The most
frequently recorded species was common nettle (Urtica dioica); followed by
creeping buttercup (Ranunculus repens), creeping thistle (Cirsium arvense),
cleavers (Galium aparine), broad-leaved willowherb (Epilobium montanum),
broad-leaved dock (Rumex obtusifolius), bramble (Rubus fruticosus agg.), rough
meadow-grass (Poa trivialis), spear thistle (Cirsium vulgare), rosebay willowherb
(Chamaenerion angustifolium), annual meadow-grass (Poa annua), dandelion
(Taraxacum officinale agg.), groundsel (Senecio vulgaris), Yorkshire fog (Holcus
lanatus), curled dock (Rumex crispus), fat hen (Chenopodium album), soft rush
(Juncus effusus) and prickly sow-thistle (Sonchus asper). In the three year interval
between surveys, there was a sharp decline in the frequency of rosebay willowherb
and perennial rye-grass and increases in some ruderal species, such as wavy
bittercress (Cardamine flexuosa) and scentless mayweed (Tripleurospermum
inodorum). There were also apparent increases in the frequency of ivy (Hedera
helix) and elder (Sambucus nigra) and grasses such as annual meadow grass,
creeping bent (Agrostis stolonifera), common bent (A. tenuis), creeping soft-grass
(Holcus mollis), couch (Elytrigia repens) and tufted hair-grass (Deschampsia
flexuosa) – although most of these species were still recorded at only a small
number of sites. Sage & Tucker (1998a) also reported that plant species more
characteristic of woodland or woodland edge habitats were more frequently
recorded in western SRC sites, where landscapes were generally more wooded.
Coates & Say (1999), in a survey of five young SRC plantations in England, found
that the ground flora was dominated by semi-ruderal species. The extent of ground
cover varied greatly, from 0-15% where the crop canopy was closed to 100% in
very sparse, open crops. In the more open crops, docks (Rumex spp), thistles
(Cirsium spp.) and common couch tended to dominate. Where crop cover was
more comprehensive, common nettle and rough meadow-grass were the most
frequently recorded species. Shade-tolerant willowherbs (Epilobium spp.) were a
15
constant and locally dominant feature of the ground vegetation. Over the first six
years of the cropping cycle, there was a sharp decrease in annual ruderals and a
tendency for woodland and marsh species to increase.
Surveys in Swedish willow SRC plantations have recorded 125 vascular plant
species and 18 mosses (Gustavsson, 1987; cited by Börjesson, 1999). This
compares with a typical 50 weed species in Swedish cereal crops, of which about
30 occur only sporadically (Börjesson, 1999).
The results of surveys in England (Coates & Say, 1999) and Sweden (Börjesson,
1999), clearly illustrate the influences of site management and previous and
adjacent land uses on ground flora composition – and demonstrate the importance
of carefully documenting such factors. Coates & Say’s results were undoubtedly
affected by inconsistent management regimes and the fact that several plots
remained unharvested throughout the survey period. Significantly, one site (in
Oxfordshire) was adjacent to the River Thames and had two nearby ponds and
another (in Cornwall) had woodland on three sides of the plantation, with several
nearby ancient semi-natural woodland sites (including an SSSI). These two SRC
sites accounted for much of the recorded increase in marsh and woodland species
respectively, within the sample of sites as a whole. Neither was managed according
to normal guidelines. For example, neither site received fertilisers and the
Oxfordshire site was not sprayed with herbicides. The Cornish site, which was
predominantly poplar, was planted at a relatively low density. Both of these sites
were cut back after the first year, but neither was subsequently harvested (although
the Cornish site was partially thinned with a chainsaw).
The surveys mentioned previously have all focused on botanical diversity within
bioenergy crops. For various reasons, already described, the ground flora beneath
SRC or energy grasses is usually of limited ecological value. In many instances,
the relatively wide grassy headlands maintained around bioenergy crops will be
significantly more diverse. Consequently, it is important that botanical monitoring
also includes crop edges and headlands.
3.2
Standard botanical survey methods
Plant species diversity may be simply measured by compiling a list of all species
found across a site, or within one or more sample plots. Although this information
can be interesting and provide a good indication of a habitat’s ‘conservation
importance’ relative to other habitats, it is usually necessary to collect data that
provide a measure of the relative prominence of individual species within the
sampled vegetation. This is usually achieved through assessments of cover,
biomass or frequency (Bullock, 1996).
Although the sedentary nature of most plants makes them easier to observe than
most animal species, it is important to remember that most are very different in
appearance, size and visibility at different times of year. Careful thought must be
applied to the choice of survey period and repeat surveys at different times of year
are frequently advisable (e.g. surveys in May and August). At certain times of year,
16
some species will be difficult to identify reliably. Other species have no aboveground organs for part of the year.
Estimates of cover
Cover is usually defined as the percentage of an area covered by the above-ground
parts of a plant if viewed directly from above. As the foliage of plants overlaps, the
total percentage cover can, and very often does, total more than 100.
The proportion of ground covered by individual plant species within frame quadrats
or across the whole site can be visually estimated. An estimate of percentage cover
can be assigned to each species present, or some sort of scale (either existing or
devised specifically for the task in hand) might be used.
Percentage cover can also be assessed using point quadrats, which are particularly
useful for assessments in grasslands and other short vegetation types. These
provide a more objective measure of percentage cover, as only the species that are
in direct contact with the vertical metal pin (or other ‘point’) are recorded, with no
requirement for visual estimates of relative cover percentages. However, the use of
point quadrats is very time consuming because a very large number of points is
needed to obtain an accurate record of infrequent species.
The simplest scale in frequent use is the DAFOR scale. Using the DAFOR system,
plant species are subjectively classified as ‘dominant’, ‘abundant’, ‘frequent’,
‘occasional’ or ‘rare’ – although each class has no strict definition, or associated
range of percentage cover values, and depends on each surveyor’s interpretation.
Alternatively, researchers can devise their own percentage classes or use widely
accepted systems such as the Domin or Braun-Blanquet scales (Table 1).
The main advantage of visual estimates of cover is speed. However, there are
several disadvantages, including subjectivity and difficulties in accurately
estimating cover percentages – particularly for tall vegetation that you cannot look
down on from above. The most reliable estimates can be obtained when low
vegetation within a quadrat is viewed directly from above. In most instances, the
use of very narrow ranges of percentage cover (e.g. 5% or less) probably cannot be
justified by the achievable accuracy of the recorder’s estimates.
Although the Domin scale has often been used in surveys, it is of very limited use
in scientific studies. It is a quick technique, for use in the field when available time
is very limited, but data collected are non-linear and difficult to analyse. It is
possible to convert Domin scales to percentage cover values, but there are
advantages in simply estimating cover in the first instance.
Data from plant cover estimates of this type are scores and should only be analysed
using non-parametric statistical techniques (Bullock, 1996).
The accuracy of cover estimates for ground flora can be increased by training staff
and by providing calibrated ‘templates’ for standard amounts of cover (i.e. small
sub-quadrats representing a known proportion of the total quadrat area). As visual
cover estimates are, however, subjective it is also important to maintain
consistency, both between assessors and between different occasions e.g. repeat
17
surveys. Wherever possible, repeat surveys should be undertaken by the same
individuals involved in the previous survey. Teams of assessors should mutually
calibrate at regular intervals (e.g. once every two or three days for lengthy surveys),
by assessing one or two quadrats jointly, to ensure continued consistency between
them.
Table 1. Domin and Braun-Blanquet scales for visual estimates of plant
cover (from Bullock, 1996)
Value
+
1
2
3
4
5
6
7
8
9
10
% cover or other description
Braun-Blanquet
Domin
< 1%
1-5%
6-25%
26-50%
51-75%
76-100%
1 individual, with no measurable cover
< 4%, with few individuals
< 4%, with several individuals
< 4%, with many individuals
4-10%
11-25%
26-33%
34-50%
51-75%
76-90%
91-100%
Biomass
The above-ground biomass of plants can be readily calculated by harvesting (cut
flush with, or close to, the soil surface) and weighing. Normally, this involves
cutting the base of all plants within a quadrat, putting the cut plants into polythene
bags (to conserve moisture) and returning them to the laboratory for weighing. If
necessary, plants can be oven dried, for the determination of dry weight as well as
fresh weight. Dry weights are always preferable (Bullock, 1996). Samples can be
weighed whole – to calculate the total plant fresh/dry weight per unit area – or subdivided first (before drying) into species or species groups (e.g. grasses and forbs).
Sorting harvested material can, however, be very difficult, particularly if plants fall
apart after cutting.
Large samples may be weighed whole and sub-sampled in the field, and the subsamples taken to the laboratory for botanical separation and/or dry weight
percentage determination.
Attempts to measure the biomass of roots and other underground organs are made
much less often in ecological studies and are more usually restricted to studies of
plant physiology.
Measurements of biomass involve destructive sampling and should not normally be
used in areas of high conservation value – unless the scale of sampling is really of
no consequence. This is unlikely to be an issue, however, in bioenergy crops.
18
Frequency
The frequency of occurrence of any plant species can most simply be measured as
the percentage of quadrats in which that species was recorded. Quadrats might be
distributed across the survey area in either a random or non-random fashion (e.g. on
a grid or along transects).
One alternative is to use quadrats that have been subdivided into smaller squares
and, for each species present within the quadrat, to record the number of smaller
squares in which that species was present. For example, a 0.5 x 0.5 m quadrat
might be sub-divided into 25 squares of 10 x 10 cm. A development of this
approach involves the use of ‘nested’ quadrats, each nest containing a series of cells
of increasing size (Hodgson et al., 1993). Each species present is recorded against
the cell number in which it first occurs, with the higher numbers representing the
largest cells. Consequently, dominant species will tend to have a high proportion of
1 and 2 scores.
This approach has been further developed into the ADAS Plot Method (Critchley &
Poulton, 1998; Burke & Critchley, 1999; Critchley et al., 2002) which has been
widely used in the monitoring of the impacts of UK agri-environment schemes.
This method uses a number of nested quadrats placed together at a site in a single
block or ‘plot’. A location for the plot is chosen as representative of the site as a
whole. Data are analysed in a way that combines objectivity with a high level of
sensitivity for detecting vegetation change. Use of nested quadrats per se
effectively minimises variation between recorders, a very important consideration
in national surveys or monitoring programmes involving several ecologists.
However, one disadvantage of the ADAS Plot Method is that its complexity and
intensity of measurement mean that it can be relatively costly to implement.
Vegetation mapping
This involves the classification and mapping of vegetation communities, either into
broad categories based on simple characteristics observed in the field or by
statistical ordination of species data. One widely used system of vegetation
mapping using broad habitat categories is English Nature’s Phase 1 Habitat Survey.
An example of the more complex type of vegetation classification is the National
Vegetation Classification (NVC) e.g. Rodwell (1991 & 1992).
3.3
Botanical survey methods applied in studies of biomass crops
The Game Conservancy Trust’s surveys of SRC sites in the UK and Ireland (Sage
et al., 1994; Sage, 1995; Sage & Tucker, 1998a) used linear quadrats for
assessments of the abundance of all higher plants within each quadrat. In each
surveyed SRC plot, five 10 x 1 m quadrats were randomly positioned in the
alleyways between crop rows. These relatively large quadrats were used to ensure
that all of the more frequently occurring species were recorded – even in plots
where the ground vegetation was very sparse. Linear quadrats were used to avoid
the need to position quadrats and record vegetation across tree rows, which would
be more difficult and time consuming. Another concession to time/cost restrictions
was the omission of bryophytes from the species recorded. All plants recorded
were assigned an abundance ‘score’, based on a slightly adapted DAFOR system
19
(see Section 3.2), with the addition of a ‘V’ category for very rare species that were
estimated to cover less than 1% of the total quadrat area. ‘Rare’ species covered 15% of the quadrat, ‘occasional’ species 5-10%, ‘frequent’ species 10-25%,
‘abundant’ species 25-50% and ‘dominant’ species 50-100%. Data were analysed
using TABLEFIT (Hill, 1996), to assign plant communities to NVC types, and
DECORANA (detrended correspondence analysis) (Hill, 1994) programmes.
DECORANA allowed the vegetation communities of different sites to be plotted on
a single scatter graph (ordination diagram), and the similarities between different
sites to be viewed pictorially – those with strong similarities being plotted close
together. Sage & Tucker (1998a) also classified ground vegetation at different sites
according to the primary ecological strategies of the dominant species present. The
ecological strategies of individual plant species were determined as ‘competitor’,
‘ruderal’ or ‘stress-tolerator’ (or an intermediate category) according to the system
devised by Grime et al. (1988).
In their studies of five SRC sites in southern England, ERM (Coates & Say, 1999)
also used linear quadrats. The main differences from the methods used by Sage et
al. (1994) were that quadrats were smaller (4 x 1 m); located permanently on a grid,
rather than randomly, so that the same quadrats could be assessed annually for six
years; and located in adjacent field edge, hedgerow and farmland habitats, as well
as SRC.
Noticeable differences in botanical species composition can be expected along a
gradient from the crop edge towards the centre. This will be due to decreasing light
levels and wind exposure - and probably also soil moisture – along this gradient,
combined with increasing distance from adjacent habitats that may provide sources
of incoming propagules. This effect has been extensively documented in woodland
and forests (as well as many other habitats, including arable fields and set-aside)
and many botanical surveys have utilised methodologies that were designed to
measure the effects of distance from the field or woodland edge. For example,
Buckley et al. (1997) placed 1 x 1 m quadrats at intervals along transects that ran
from a woodland ride into the woodland, spanning all of the expected vegetation
zones.
A survey of hybrid poplar plantations in four countries (Britt, 1999; Weih et al.,
2003) also used quadrats spaced at pre-determined distances along transects
running from the woodland edge into the plantation. Similar transects ran from the
edges of adjacent or nearby agricultural fields, to collect comparative data on the
floral composition of fields that had not been planted with poplars. Plant
frequencies were assessed using nested quadrats. A 0.25 m2 (0.5 x 0.5 m) frame
quadrat was sub-divided to allow each species within it to be assigned a score of 14, where 1 was allocated to the species (or other cover variable e.g. leaf litter)
directly beneath a point in one corner of the quadrat. Scores of 2, 3 or 4 were then
given to all additional species that first occurred within the 10 x 10 cm, 25 x 25 cm
or 50 x 50 cm sub-divisions of the quadrat.
20
3.4
Advantages and disadvantages of different methods in relation
to surveys in biomass crops
Any methodologies used for surveys of the flora within biomass crops must take
full account of the physical difficulties of placing quadrats across crop rows and the
distribution of ground vegetation – as well as the scientific objectives of the
research and the proposed methods of statistical analysis. The use of large
quadrats, or a large number of smaller quadrats, may be required in biomass crops
where ground vegetation is sparse or particularly heterogeneous.
The relatively large, rectangular quadrats (10 x 1 m) used by the Game
Conservancy Trust (Sage et al., 1994; Sage, 1995; Sage & Tucker, 1998a) allow
ready placement between pairs of rows in SRC crops and ensure inclusion of a
representative range of species, even in very sparse vegetation. Such large quadrats
are, however, quite difficult to work with and accuracy of recording can be lower
than when working with smaller quadrats.
The smaller, rectangular quadrats (4 x 1 m) employed by Coates & Say (1999)
probably offer a better compromise between the advantages of many small quadrats
and the need for larger quadrats when surveying low density ground vegetation.
Both of the above, however, lend themselves primarily to the compilation of
species lists and subjective estimates of percentage ground cover and relative
species abundance scores (e.g. DAFOR, Domin or Braun-Blanquet). The use of
nested quadrats and their placement at pre-determined intervals along transects
running from the headland or crop edge into the biomass crop, as used by Britt
(1999) and Weih et al. (2003), provides more objective data for analysis of relative
species frequencies and allows measurement of edge effects, which can be very
important.
To ascertain the true floral diversity of a biomass crop, measurements should not be
confined to the most easily accessible areas e.g. between twin-rows of SRC.
Surveys restricted to the larger alleyways (often 1.5 m wide) between pairs of rows
are relatively simple, and therefore relatively quick and cheap. However, there is a
danger in this approach in that some species prevalent in areas adjacent to tree
stems/stools might be under-represented or omitted from the survey sample. It
seems likely that plant communities in areas closer to crop plants will differ from
those in the wider alleyways – due to reduced disturbance and, possibly, to lower
light, soil moisture and nutrient levels – although there is currently no evidence to
support this assumption. It is also very possible that certain species (e.g. some
bryophytes) may be present only within the microhabitat provided by tree stem
bases.
Large quadrats can incorporate intra-row, as well as inter-row, ground vegetation,
but they would have to be of a type that can be easily constructed in the field,
around crop plants. For all situations where taller, or woody, vegetation is likely to
seriously impede quadrat positioning, fixed frame quadrats will be unsuitable.
Alternatives to be considered will include a quadrat with only three fixed sides,
which is slid into position around or beneath the obstructing vegetation, or pegs and
string.
21
Surveys of vegetation within SRC must take account of variation between sites, by
including several contrasting sites in the sample. The effects of vegetation
succession, as a plantation ages, and the effects on the ground flora of rapidly
changing conditions after crop harvests must also be accounted for – by repeating
assessments over at least three years. Repeat assessments should be performed
within quadrats placed in permanently marked positions.
22
4.
INVERTEBRATES
4.1
Invertebrate species diversity in biomass crops
Native willows are known to support a great diversity of invertebrate species, but
most studies of the invertebrate fauna of SRC crops have focussed on pest species,
particularly three species of leaf beetle (Coleoptera: Chrysomelidae): the blue
(Phratora vulgatissima), brassy (P. vitellinae) and brown (Galerucella lineola)
willow beetles (e.g. Kendall & Wiltshire, 1998; Sage & Tucker, 1998a & 1998b;
Sage et al., 1999). Research has included detailed studies of the life cycles and
ecology of leaf beetles (Kendall & Wiltshire, 1998; Sage et al., 1999) and their
preferences for different species and clones (Augustin et al., 1993; Kendall et al.,
1996; Orians et al., 1997).
A review of the status of a wider range of actual and potential insect pests of
willows and poplars was published by Sage (1994).
Sage & Tucker (1997) surveyed the invertebrate fauna in the canopies of 21 SRC
crops across Britain and Ireland. They recorded animals from 48 taxonomic
groups, with a greater diversity of groups in willow plots than in poplars. Leaf
beetles were, by far, the most abundant canopy invertebrates in poplar and willow
plots – although not all invertebrate groups present were efficiently sampled by the
beating method employed. Blue willow beetle was recorded in most willow plots
surveyed, but not in poplar plots. Brown willow beetle was recorded on a few of
the willow plots. Brassy willow beetle was recorded only in poplar plots. The total
number of leaf beetles was sometimes very high – greater than 20 per m2 of crop
(equivalent to 200,000 per ha) on some plots. Larvae of a sawfly species, Nematus
melanaspis (Hymenoptera: Symphyta) were also abundant at one poplar site.
Sage & Tucker (1997) found significantly higher numbers of individual Diptera and
Heteroptera insects in willow plots. Discounting the main pest species, leaf beetles
and sawfly larvae, willow plots also had a higher total number of individual
invertebrates than did poplars.
Male willow plants can provide a useful source of pollen for bumblebees, and other
pollinating insects, in early spring (Börjesson, 1999; Reddersen, 2001); so
assessment of these species will often be worthwhile.
Coates & Say (1999) examined the ground-dwelling invertebrate fauna of five
willow and poplar SRC sites in southern England. They trapped and identified
species of ground beetles (Coleoptera: Carabidae), rove beetles (Coleoptera:
Staphylinidae) and spiders (Arachnidae). There was a general decrease in species
diversity and total numbers of ground beetles as plantations aged. Changes in the
species diversity and total numbers of rove beetles and spiders showed no clear
pattern. Species assemblages for all three groups indicated a change away from
communities typical of agricultural or ruderal habitats, towards those more typical
of less disturbed habitats. For example, the ground beetle Pterostichus melanarius,
which is commonly present in high numbers in arable fields, rapidly declined or
disappeared after SRC establishment. Coates & Say recorded a total of 21 species
23
of beetle, fly, spider, sawfly, alderfly, bee and wasp of conservation importance
(i.e. Red Data Book or Nationally Scarce).
Baxter (1996), in a one-year survey of three SRC sites, recorded 30 species of
ground beetle and 22 rove beetles (7 unidentified). However, the actual number of
species present was probably higher than this, as several were identified only to the
level of genus.
Slater et al. (1997) found that the ground-dwelling invertebrate assemblage in a
willow SRC site in mid-Wales generally reflected that of the preceding land use
(rough pasture).
In some instances, populations of earthworms appear to decline when SRC is
planted on agricultural land (Sage & Tucker, 1998a; Coates & Say, 1999).
Comparisons of the biomass of larger invertebrates (>2 mm in length) in soil
samples from SRC, rushy pasture and improved pasture (at a farm in Devon)
showed that there was a significantly lower total biomass in SRC – with smaller
weights of earthworms, beetle larvae (Coleoptera) and leatherjackets (Tipulidae)
(Sage & Tucker, 1998a). However, in contrast to these findings, research in
Sweden has suggested that the diversity and abundance of soil organisms,
especially decomposers such as earthworms, wood-lice and harvestmen, are
generally higher in SRC than in arable fields (Börjesson, 1999).
It might be presumed that soil-dwelling invertebrates, in general, are adversely
affected by drier soil conditions in SRC crops, which are known to have very high
rates of water use. In their study, Sage & Tucker (1998a) demonstrated that soils
under SRC had a lower penetrability than nearby rush pasture and improved
pasture. These negative factors may be counter-balanced by the positive effects of
reduced soil tillage, reduced agrochemical inputs and increased leaf litter/organic
matter (Börjesson, 1999).
4.2
Standard invertebrate survey methods
Of the many methods listed by Ausden (1996) and Consult-Eco (2000), those most
widely used for invertebrate groups likely to be of major interest to researchers
investigating the ecology of biomass crops are listed in Table 2 and described in
more detail later in this section.
If the objective of a study is to measure the invertebrate diversity at a site, or within
a specific habitat – perhaps relative to another, adjacent site or habitat – then it will
be important to incorporate as wide a range of methods, sampling positions and
sampling times/dates as resources allow. If the aim is to confirm the presence of, or
estimate the population size of, a specific species or species group, then it will be
important to fully understand the specific microhabitat requirements and life cycle
of the target organism(s) before a sampling methodology is confirmed.
24
Table 2. Potentially appropriate sampling/survey methods for some important terrestrial invertebrate groups (from Ausden, 1996).
Species group
Stage
Method
Direct
searching
Ants

Bees and wasps

Beetles
Adults
Larvae


Separation
from soil
Water
traps
Light
traps
Soil
sampling
Adults
Larvae
Adults
Earthworms
Flies
Adults
Larvae

Lacewings, ant-lion, etc
Adults
Moths
Adults
Larvae



Suction
sampling

























Adults



Spiders & harvestmen













Springtails



Slugs and snails
Woodlice & other terrestrial
crustaceans
Sweep
netting


Grasshoppers & crickets
Sawflies & parasitic wasps
Flight interception traps

Centipedes, millipedes, etc
Dragonflies & damselflies
Beating

Bugs
Butterflies
Pitfall
traps





25
26
A survey of invertebrate species diversity within a biomass crop might, for
example, incorporate:

Soil sampling – to identify soil-dwelling species e.g. earthworms (Annelida),
beetle (Coleoptera) and fly (Diptera) larvae.

Pitfall trapping – to trap soil-surface species e.g. adult ground and rove
beetles (Coleoptera: Carabidae and Staphylinidae); spiders (Arachnida:
Araneae), harvestmen (Opiliones); ants (Hymenoptera: Formicidae) and true
bugs (Hemiptera).

Beating – to capture species living on plant stems and foliage e.g. spiders,
harvestmen, true bugs, leaf beetles (Coleoptera: Chrysomelidae), lacewings
(Neuroptera: Planipennia), caterpillars of butterflies and moths (Lepidoptera)
and sawfly larvae (Hymenoptera: Symphyta).

Water traps – to trap flying insects e.g. adult flies and sawflies
(Hymenoptera: Symphyta).

Direct searches – to find invertebrates in favoured microhabitats that might
otherwise remain unsearched e.g. under rocks, in leaf litter, at base of tree
stems/stools, under the bark of any dead and decaying branches. As well as
for those groups shown in Table 2, direct searching can also be used for dayflying moths.
The period of trapping (duration, time of day and time of year) will have a very
important influence on the species and numbers of any invertebrate group captured.
Many species hibernate throughout the winter months – so even if they are present,
they will be inactive and will not enter traps. Many invertebrates may utilise
biomass crops during one part of their lifecycle, but occupy another habitat during
other parts.
Even if present in the biomass crop throughout their lives, they may occupy
different sub-habitats at different times e.g. larvae might be present in the soil or
leaf litter, whilst adults feed in the crop canopy.
Most survey methods are measures of species activity, as well as abundance.
Activity is frequently very heavily dependent on factors such as air temperature,
solar radiation levels, wind strength, humidity and soil moisture. Weather
conditions (recent, as well as present) will also influence the location of many
invertebrate species, and their likelihood of being observed or captured. For
example, in wet weather, earthworms will be nearer the soil surface and molluscs
are more likely to be grazing on taller vegetation; and on warm, calm and sunny
days butterflies are more likely to be found in open positions feeding on flowering
plants. Many standard methodologies (e.g. ‘Pollard Walk’ for butterfly surveys)
take account of this by requiring that surveys are undertaken only in specified
weather conditions. It is important that weather conditions during survey periods
are recorded and, if it is not possible to record different sites simultaneously, that
sites between which comparisons are subsequently to be made are recorded in
similar conditions.
Many invertebrate species are diurnal, whilst others are nocturnal. Levels of
activity might be higher at dawn or dusk. Survey periods should either be extended
27
across a period of at least 24 hours, and preferably over several days, or be timed to
ensure that the target species/groups are trapped or monitored during peak periods
of activity.
Survey methods
A brief summary of each of the important invertebrate survey methodologies listed
in Table 2 is given below.
Direct searching
Targeted searches of suitable habitats. Particularly useful for locating species that
are not widely distributed across a site and are unlikely to be detected by methods
that utilise random sampling, transects or trapping grids. Targeting sampling of
dead wood, leaf litter, mammal dung and carrion may be the optimum method for
the collection of many specialist saprophagous species. Many species, particularly
those that are most susceptible to dehydration (e.g. slugs), may be found
congregated under rocks, logs or dense masses of vegetation - particularly on warm,
dry days. Tussocky grasses can hold large numbers of hibernating invertebrates
during winter months.
Large flying insects such as butterflies, dragonflies and damselflies may be
identified in the field or collected using nets. Smaller insects and spiders uncovered
during targeted searches may be collected using a ‘pooter’ (a simple piece of
equipment for safely sucking invertebrates up through a piece of plastic or rubber
tubing into a collection bottle - see Ausden, 1996) or a fine, moistened paintbrush.
Grass tussocks can be cut off at ground level, taken inside and shaken onto a white
surface for subsequent identification of invertebrates present.
Direct searching can be standardised in some situations to obtain relative
population estimates (Ausden, 1996). Examples include:

Determination of the numbers of specified insects or galls on a pre-determined
number, area or weight of leaves (or a number of whole plants). Data on the
distribution and frequency of leaves or plants can then be used to estimate the
total population of a species or group per plant or per unit area.

Counts of individuals per unit area. May be performed across a defined area
(e.g. a pond) or using quadrats within a selected habitat or microhabitat. Most
useful for large conspicuous species (e.g. dragonflies) or relatively immobile
species such as molluscs. More mobile taxa, such as grasshoppers and
crickets, can be more reliably counted by placing a quadrat and then returning
to it later, or by using ‘box quadrats’ with high sides to reduce lateral
movement (Ausden, 1996).

Counting invertebrates along transects. Useful for the estimation of the
relative sizes of butterfly, dragonfly or damselfly populations through, or along
the perimeters of different habitats. The effects of weather conditions on
numbers of flying insects make it essential for transects to be walked in similar
conditions, if comparisons are to be made between them. The best known
example is the ‘Pollard Walk’ (Pollard, 1977) method, used across Britain in
the Centre for Ecology and Hydrology’s Butterfly Monitoring Scheme (BMS)
(Hall, 1981; Pollard et al., 1986; Sykes & Lane, 1996). The BMS
methodology specifies that surveys, completed between 1 April and 29
28
September, should always be completed between 10.45 and 15.45 British
Summer Time and only when the air temperature is at least 13oC and it is not
raining. If the temperature is 13-17oC (or 13-15oC at northern, upland sites),
then there must be no more than 40% cloud cover. At higher temperatures, the
extent of cloud cover is irrelevant.
Separation from soil, leaf litter or other debris
Invertebrates may be separated from samples of soil, leaf litter or other debris using
a variety of methods, including:

Dry sieving through progressively smaller sieves (Ausden, 1996, suggests
working down from a 3-4 mm to a 0.5 mm mesh sieve) onto a white cloth,
under a strong light.

Wet sieving with running water.

Stirring the sample in a bucket of water and collecting the invertebrates that
float to the surface. Beetles float readily in tap water, but the proportion of
other invertebrate groups collected will be increased if the specific gravity of
the water is increased by adding sugar or salt (Ausden, 1996). Small
invertebrates are collected using a pipette or paintbrush.

Tullgren funnel. This is a ‘desiccation funnel’ used to extract invertebrates
that favour cool, damp or shady conditions from loose, large-particled
substrates e.g. leaf litter (Ausden, 1996). A funnel, ‘plugged’ with a gauze
mesh to prevent the substrate from falling through is filled with leaf litter or
other suitable sample material. A collection bottle, containing moistened filter
paper (for live specimens) or a preservative, is placed beneath the funnel and a
strong light source above it. Invertebrates move downwards, away from the
light source (and the warm, dry conditions that it creates), and fall into the
collection bottle.
Water traps
Coloured bowls filled with water can be used to trap flying insects. The range of
species captured will be heavily influenced by the colour of the bowl and the height
at which it is placed (Ausden, 1996). Yellow plastic bowls are widely used, which
are strongly attractive to flies (Diptera) and wasps (Hymenoptera). White bowls
also attract Diptera, but strongly repel Hymenoptera species. ‘Neutral’ colours
such as brown, grey or blue have little attractant or repellent effect on insects,
reducing the selectivity of samples.
The largest catches are usually obtained from water traps placed just above the
height of surrounding vegetation. If comparisons are to be made between sites,
then traps should be positioned at a standard height at all sites. However, if the
objective is to get as broad a view as possible of the flying insect diversity at a site,
traps should be located at a range of heights and locations across the site.
Insects that land in the water and get trapped there should be removed at least once
per week (by pouring through muslin), and transferred to sample bottles with 70%
alcohol, before subsequent identifications. Alternatively, weekly samples could be
oven dried and the total flying invertebrate biomass (dry weights) of different
habitats compared.
29
Efforts must be made to prevent traps from overflowing with rainwater or drying
out.
Water traps are prone to disturbance by birds (which may eat the trapped
invertebrates), cattle (which use them as drinking bowls) and, because they are so
conspicuous, even humans.
Light traps
These depend upon the attraction of bright lights to moths and some other groups of
night-flying insects (Ausden, 1996). Various types of light trap can be used, from a
simple light bulb hung beside a white sheet to purpose built, mains-powered traps
with mercury vapour bulbs, automatic timer-switches and integral killing jars.
Traps can be set for just a few hours on one night, or remain in situ for trapping
every night over a period of several years. Moths are either identified in the field,
trapped alive for subsequent identification or killed with a chemical such as
tetrachloroethane.
Other insect groups, notably crane flies (Diptera: Tipulidae), may also be captured
in significant numbers in light traps.
A network of permanent moth traps is operated by the Rothamsted Light Trap
Network (Rothamsted Research, Harpenden). This network includes the 12 UK
Environmental Change Network (ECN) sites and the methodology used is
described by Sykes & Lane (1996).
Soil sampling
Soil sampling is frequently used for counts of large soil-dwelling invertebrates such
as earthworms, fly larvae (e.g. Tipulidae) and beetle larvae (Ausden, 1996). A
known volume of soil is either dug with a spade or lifted using a soil corer. Soil
samples collected are broken up by hand to reveal the invertebrate species being
sought, for example earthworms or leatherjackets (larvae of Tipula spp).
Alternatively, leatherjackets may be extracted from soil cores using a ‘Blasdale
extractor’ (or similar equipment), which uses a heat source to force live larvae
downwards into a tray of water (i.e. applying a similar principle to that used by the
Tullgren funnel). This significantly reduces labour inputs when large volumes of
soil have to be sorted. Smaller, more difficult to observe species can be retrieved
from soil samples using a ‘wet sieve’ technique, which involves washing soil
through a 2 mm sieve (Ausden, 1996).
Counts of the number of organisms in a known volume of soil will allow estimation
of the total number in larger soil volumes or per unit area of land on that site e.g.
the number of leatherjackets per hectare of grassland.
Pitfall traps
Pitfall traps are small, smooth-sided traps (usually plastic cups) placed into holes in
the ground for ground-dwelling invertebrates to fall into (Luff, 1975; Ausden,
1996; Sykes & Lane, 1996). Seven to 10 mm diameter cups, usually set out
according to a grid or at intervals along a transect, are placed vertically in the soil,
so that the rim of the cup is approximately level with the soil surface. A small
30
quantity of anti-freeze is placed in the cup, to prevent captured invertebrates from
eating each other and to slow decomposition (Ausden, 1996). Anti-freeze is
preferred to alcohol, because it evaporates much less readily.
Invertebrates such as ground beetles, rove beetles and money spiders (Linyphiidae)
moving across the soil surface fall into the trap and cannot escape. Traps are
commonly left ‘open’ (i.e. without lids) in the field for periods of one or two
weeks.
It is important to note that the numbers of invertebrates captured in pitfall traps
reflect levels of activity, as well as population size. Relatively large catches during
one particular trapping period do not necessarily indicate increased abundance.
Flooding of pitfall traps can be a problem during periods of heavy rainfall. This
problem can be effectively eliminated by positioning a plastic plant pot saucer, on a
simple wire frame, a few centimetres above the trap. Accidental trapping of small
mammals and amphibians can be reduced by positioning wire mesh (e.g. ‘chicken
wire’) over the top of the cup and/or as a cone placed in the bottom of the cup.
Pitfall traps may also act as water traps for flying insects (e.g. Diptera and
Hymenoptera), but should not be used to count numbers of these groups as catches
will be heavily influenced by the colour and visibility of the traps.
Beating
A simple method for the collection of a wide range of invertebrates from the foliage
of plants (Ausden, 1996). It is particularly useful for the collection of relatively
loosely attached phytophagous (plant-eating) and predatory species - including
caterpillars (of moth, butterflies and sawflies), leaf beetles and ladybirds - from
branches of shrubs and small trees. It is less suitable for species that readily fly off
when disturbed, although a variation on the beating method involves striking a
branch and then netting the insects that fly away (Ausden, 1996).
Under the standard method, a branch is struck sharply with a rod or stick, to
dislodge invertebrates. These invertebrates are then collected on a cloth or ‘beating
tray’ positioned on the ground, directly beneath the branch. They are then picked
up using a pooter and transferred to sample bottles. Beating trays, which can either
be purchased or home-made, comprise a cloth-covered tray, sloping towards the
centre (Ausden, 1996).
Standardisation of the methodology used - in particular the number and strength of
strikes on branches, the number of branches hit and the size of the cloth or beating
tray – will allow relative estimates of invertebrate numbers (Ausden, 1996).
Beating is easier in young stands. In dense, older stands, it can be difficult to get a
clear aim at a tree, and hitting thick stems may fail to dislodge insects in the
uppermost foliage. Selecting the number of branches to hit can also be more
difficult when they are growing close together.
31
Flight interception traps
These involve screens of fine black netting (e.g. Dacron or Terylene) stretched
across insect flight paths, to intercept flying insects. Insects that fly into the netting
drop into trays of water (with a few drops of detergent, to reduce surface tension)
placed immediately beneath the netting. Flight interception traps are frequently
placed along woodland rides, woodland edges or hedgerows – where numbers of
flying insects tend to be high.
These traps are conspicuous and, consequently, prone to disturbance. Their size,
and relative complexity, make replication more difficult.
One variation of the flight interception trap is the Malaise trap. In these traps, the
netting is used to channel insects upwards into a collecting bottle. These traps are
more expensive, but are more efficient at catching smaller, more agile flying insects
that are not caught in large numbers by the simpler type of flight interception trap
described above (which catch larger proportions of large beetles, etc).
Sweep netting
This method involves passing a sweep net (which must have a metal rim) through
vegetation, using alternate forehand and backhand strokes (Ausden, 1996).
Vegetation should be at least 15 cm tall, dry and neither trampled nor flattened by
wind or rain.
As with beating, invertebrate groups that attach themselves firmly to foliage, and
those that fly off when disturbed, will be under-represented in samples collected.
The number, length, speed, depth and angle of sweeps through the vegetation will
all influence the size and composition of invertebrate catches. Consequently, all
have to be standardised and, preferably, implemented by the same surveyor if any
comparisons between sites/habitats are to be made. Ausden (1996) suggests a
series of net sweeps approximately 1 m in length, taken every other pace while
walking at a consistent speed through the vegetation.
Sweep netting may be difficult in very dense crops, if there is insufficient space to
freely move the net from side to side.
Suction sampling
Invertebrates in low vegetation may be more effectively sampled using a suctionsampler, such as the D-vac. The D-vac is a large and heavy piece of equipment,
carried on the back of the operator, which uses a vacuum to suck air into the mouth
of the machine – drawing invertebrates into a fine netting or muslin sample bag
secured at the inlet point.
Normally, the collecting nozzle of the sampler will be held down on the vegetation
(or soil surface) for a given length of time and on a pre-determined number of
occasions. As the area of the collecting nozzle is known, total catches might be
used to give estimates of the population of the target species or group per unit area
(e.g. Collembola m-2).
32
After returning from the field, invertebrate samples can either be killed in a killing
jar or stored in a deep freeze, for later sorting and identification.
As for sweep netting, suction sampling is not suitable for use on damp vegetation.
In contrast to sweep netting, however, it is most suitable for use on very low
vegetation (height less that 15 cm) and can even be used effectively on bare soil.
Suction sampling with a D-vac can be very problematic. The machines are heavy
and cumbersome, so carrying them across rough terrain, over longer distances or
for long periods can be very uncomfortable and may be unpopular with staff.
During operation, they are also noisy and cause significant vibration. They must be
regularly refuelled with a petrol/oil mixture. They are prone to mechanical
breakdowns, but replacement machines are expensive.
Ausden (1996) suggests that garden leaf collection machines offer a smaller, lighter
alternative to purpose-built D-vac equipment. No direct comparisons between the
efficiency of the two types of machine have been made.
Suction samplers may under-record large species that can effectively shelter from
the D-vac or which attach themselves firmly to the vegetation and those that live
low down in taller areas of vegetation (Ausden, 1996).
4.3
Invertebrate survey methods applied in studies of biomass crops
Many of the published studies of invertebrates in SRC crops have focussed
primarily on species in the crop canopy – in particular the leaf beetles
(Chrysomelidae), which include some important pest species. For example, Sage &
Tucker (1997, 1998a & 1998b) employed a beating method to knock leaf beetles,
and other invertebrates, from stems and foliage onto a 2 m2 sheet laid out beneath
the target willows. Invertebrates on the sheet were then collected with a pooter for
immediate identification or temporary storage in a sample bottle.
Sage & Tucker (1998a & 1998b) also conducted stem searches, to count colonies of
aphids (Hemiptera: Aphidae) and some other invertebrate species that were likely
to be missed, or under-represented, in samples collected by stem beating.
Pitfall trapping has been extensively used for the study of ground beetles
(Carabidae), rove beetles (Staphylinidae) and spiders in arable fields and many
other habitats. Pitfall traps were used in SRC crops by Baxter (1996), Slater et al.
(1997) and Coates & Say (1999).
Earthworms were sampled by Coates & Say (1999) using an irritant chemical
applied to the soil surface. The irritant caused worms to rise to the surface, where
they were counted, washed and returned. Sage & Tucker (1998a) sampled
earthworms and other, relatively large (>2 mm) soil-dwelling invertebrates, by
collecting 25 soil samples, each approximately 15 x 15 x 12 cm deep (2.7 litre),
from an SRC plantation and two nearby pastures.
33
4.4
Advantages and disadvantages of different methods in relation
to surveys in biomass crops
Most of the standard invertebrate sampling methods described in 4.2 are likely to be
feasible for use in SRC, although there may be some practical difficulties, that
make sampling more time consuming (and more expensive) than in other habitat
types. For example, methods used to sample invertebrates in SRC or Miscanthus
crops should allow for the relative difficulty of movement (for recorders) and the
difficulty of digging in dry soils with dense networks of roots or rhizomes.
The nature of Miscanthus crops will make several standard methods impractical for
use.
Cumbersome equipment, such as a D-vac suction sampler, may not be a desirable
option for use in such dense crops. In SRC, sampling of invertebrates on the stems
and foliage of the crop is probably best achieved by stem beating and, if required,
direct examination of leaf samples. Even these methods have some associated
problems, however. For example, leaves cannot easily be collected from
throughout the canopy when crops are taller than 2-2.5 m (although the flexibility
of stems does allow some sampling at heights above normal reach) – so species
predominant in the upper canopy may be under-represented or missed. In twin-row
SRC plantations, collection sheets positioned on one or both sides of a double row
will not collect all invertebrates dislodged when the plants are beaten – as many
will fall directly downwards into the area between the crop rows. As the size and
positioning of sheets, and the frequency and strength of stem beating, is
standardised at all locations this may not be of great significance, although it is
possible that some species, perhaps the heavier ones, may consequently be underrepresented. Invertebrates on the foliage of ground vegetation can be sampled by
sweep-netting, if the vegetation is of an adequate height and ground cover is
sufficient to justify sampling. In Miscanthus crops, beating is unlikely to be
feasible, because of insufficient bare ground on which to position collection sheets.
Sweep-netting and direct searches of leaf samples may be the only feasible options
for foliar invertebrate sampling in Miscanthus. Beating, sweep-netting and D-vac
sampling are all best performed when vegetation is dry, reducing suitable sampling
opportunities.
Digging soil pits, to extract large soil samples can be severely restricted by dry soils
and dense networks of roots or rhizomes. Even the extraction of relatively large
soil cores may be slightly difficult in established crops. These factors may restrict
the sampling of earthworms, and slow the rate of sampling for other large soil
invertebrates, such as cranefly larvae (Tipulidae) or beetle larvae (Coleoptera).
Digging holes for insertion of pitfall traps may also be more difficult than in most
habitats, but should still be readily achievable. In particularly difficult locations
(e.g. long established willow or poplar crops with many large, woody roots near the
soil surface), one option might be to use relatively small volume cups as pitfall
traps. Traps are less likely to fill with water, the most frequent problem
encountered when smaller volume cups are used in open habitats, because of the
high degree of rainfall interception by the crop canopy. If traps are to be changed
relatively infrequently (e.g. intervals of two weeks or more), trap flooding may still
34
be a problem. In these cases, plant pot saucers supported by wire (as described
earlier) can be used to exclude rainwater. Note that restricted movement through
SRC and Miscanthus will make the carrying of large trays of pitfall traps relatively
difficult. Pitfall traps may be very hard to locate in dense Miscanthus crops, so
careful and appropriate marking of trap locations will be essential.
Water traps should be an appropriate method for sampling true flies (Diptera) and
some other groups of flying insects in biomass crops. Theft and vandalism, which
can be a problem for these highly visible traps (particularly when located in
publicly accessible areas), are likely to occur less often within tall energy crops,
where visibility is reduced. Sticky traps, another method of trapping flying insects,
may be less suitable in these crops, due to the close proximity of vegetation to
which traps may stick during movement on windier days.
Direct searching of stems (as used by Sage & Tucker, 1998a, for counts of aphids
and some other groups) and coppice stools may be a suitable technique for surveys
of invertebrate species that are difficult to sample by other methods.
Methodologies can readily be developed to provide quantifiable data suitable for
statistical analysis.
35
5.
BIRDS
5.1
Bird species diversity in biomass crops
5.1.1 Spring/summer
The most common breeding birds in SRC are those species most commonly
associated with scrub and hedgerows, although the predominant species will be
heavily dependent upon rotation stage/crop height (Sage, 1998). Other species will
predominantly utilise crop edges and the wide grass headlands around SRC crops.
As has been previously demonstrated in traditional coppice stands, different bird
species have strong preferences for different stages of growth (Fuller & Moreton,
1987; Fuller et al., 1989; Fuller, 1992; Fuller & Green, 1998). Fuller & Green
(1998), reporting results of a 10-year study of breeding bird populations in a
coppiced lime (Tilia cordata) plantation in Worcestershire, found that overall
densities of birds were generally highest in recently coppiced plots. Warblers
(Silviidae) were much more abundant in recent coppice, and almost absent from old
coppice – with warbler populations collapsing after 11-12 years of re-growth.
A four-year bird survey at a site in Northern Ireland (Sage & Tucker, 1998a)
recorded a total of 32 species that used the SRC crop, and a further 12 species on
farmland that apparently never used the SRC. Of 22 breeding species at the site,
pheasant (Phasianus colchicus) and reed bunting (Emberiza schoeniclus) always
incorporated SRC within their territories. Dunnock (Prunella modularis), sedge
warbler (Acrocephalus schoenobaenus), garden warbler (Sylvia borin), blackbird
(Turdus merula), song thrush (Turdus philomelos), bullfinch (Pyrrhula pyrrhula)
and lapwing (Vanellus vanellus) all included SRC in almost all territories (85% or
more). The four most common species at the site – willow warbler (Phylloscopus
trochilus), wren (Troglodytes troglodytes), robin (Erithacus rubecula) and
chaffinch (Fringilla coelebs) – all used SRC to a significant, though lesser, extent.
However, three other regularly breeding species never included SRC in their
territories: blackcap (Sylvia atricapilla), chiffchaff (Phylloscopus collybita) and
goldcrest (Regulus regulus).
Coates & Say (1999), in an intensive study of five SRC sites in southern England,
recorded 39 breeding species (predominantly songbirds) whose territories included
SRC. Twenty-seven of these species held territories contained within SRC and the
other 12 had territories that extended into SRC plantations. An additional 23 bird
species were recorded in SRC but did not hold territories that included SRC. As
SRC plantations aged, the species composition changed and density of breeding
birds increased. Coates & Say (1999) recorded six species of conservation concern
(Red List species), most of which were strongly associated with a specific age class
of coppice. The early stages were favoured by corn bunting (Emberiza calandra)
(years 0-1) and skylark (Alauda arvensis) (years 0-2), linnets (Acanthis cannabina)
were most abundant in year 2 and bullfinches in year 5. Song thrushes and reed
buntings appeared to have less clear preferences (years 1-5). It should be noted
most SRC crops would be harvested by year 3, and the absence of any harvest at
most of Coates & Say’s sites during their study should be regarded as atypical.
36
Migrant insectivorous species, such as willow warbler, are particularly common in
two and three-year-old willow SRC (Singleton & Bowes, 1991; Sage et al., 1994;
Sage & Robertson, 1996). Poplar appears to support lower densities of songbirds,
especially warblers and buntings (Sage et al., 1994; Sage & Robertson, 1996).
Slater et al. (1997) recorded several warbler species within willow SRC plots in the
SW Midlands and in Mid-Wales: willow warbler, sedge warbler and garden warbler
at both sites, and chiffchaff at the site in Wales. They also recorded some evidence
of whitethroat (Sylvia communis), dunnock and reed bunting breeding within SRC.
Several other species were regularly observed around the plantation edges,
including wren, robin and chaffinch.
Christian et al. (1997) and Collins et al. (1999) surveyed bird populations in hybrid
poplar plantations (2.7-4.0 ha) and surrounding land, in the USA. All poplars had
been planted as experimental, biomass energy crops, at a spacing of 2.5 x 2.5 m wider than SRC crops in the UK. Surveys were conducted in the breeding season
(June-July) and during the period of autumn migration (August-October). They
found that avian abundance and species richness were consistently higher in poplar
plantations than in surrounding arable fields. However, abundance and diversity
were lower in poplar stands than in forests or non-wooded ‘wildlands’ (e.g. old or
conservation scheme grassland). Poplars appeared to be more attractive to birds in
agricultural landscapes than in forest landscapes.
Similar results were reported by Schiller & Tolbert (1996). They also found that
bird species in poplar biomass plantations in the USA, although lower in number
than in natural forest, were a mixture of ‘openland’ and forest species.
Little is currently known about the use of Miscanthus by breeding birds. However,
the relative scarcity of invertebrates on the crop will mean that it is a less valuable
habitat for foraging insectivorous birds. The similarity of the crop to reeds might,
however, make it a suitable alternative nesting habitat for reed-bed species – if
surrounding habitats provide sufficient food resources. Beyea et al. (1995) found
that switchgrass crops, in Iowa (USA), provided significant habitat for several
declining species of specialist prairie grassland birds.
5.1.2 Winter
Bird surveys undertaken by Sage & Tucker (1998a) in the winter of 1996-97
(November to March), at seven willow and poplar SRC sites in southern England,
recorded 29 species. The total number of species recorded at any one site, in five
visits, ranged from 9 to 19. Those present at all sites were blackbird (Turdus
merula), robin, wren, dunnock, blue tit (Parus caeruleus), chaffinch and pheasant.
Pheasants were the most abundant birds, although there were release pens of
pheasants and partridges in the vicinity of all sites and local distributions of these
species were not considered to be representative of wild populations. Long-tailed
tit (Aegithalos caudatus), the most abundant species after pheasant, and song thrush
(Turdus philomelus) were both recorded at six sites. Although very few species
were recorded in large groups, Sage & Tucker reported that flocks of larks and
finches had been recorded in previous years. The mean density of over-wintering
birds in SRC was 3.9 per ha. Bird densities were significantly lower in larger SRC
37
plantations, indicating the relative importance of plantation edges. The wide
headlands of SRC crops were utilised by species such as yellowhammers (Emberiza
citrinella) and finches. Skylarks and meadow pipits (Anthus pratensis) were most
frequently observed in areas of recently harvested SRC.
Coates & Say (1999), in a five-year study of sites in southern England, recorded 51
species in SRC. The overall density of over-wintering birds was high in the
planting year (year 0), decreased over years 1-3, then increased again in years 4 and
5. At two sites where the crop was harvested four years after planting, bird
densities fell in the following winter and then began to rise again. Numbers of
buntings, finches, skylark and snipe (Gallinago gallinago) declined as the coppice
aged, while numbers of pheasant, blackbird, long-tailed tit and blue tit increased.
Sage & Tucker (1998a) recorded several species of conservation concern, including
the Red List species reed bunting, song thrush, skylark and linnet. Coates & Say
(1999), in their winter surveys, recorded 13 species of conservation concern in
SRC, including grey partridge, snipe, woodlark, song thrush, linnet, bullfinch and
reed bunting.
Radio-tracking of snipe, at a site in Devon, has produced some evidence that SRC
provides suitable day-time roosting sites for this species, which then feed by night
in nearby pastures (Sage & Tucker, 1998a).
5.2
Standard bird survey methods
The various techniques for surveying bird populations have been described by
several authors, including Bibby et al. (1992), Buckland et al. (1993) and Gibbons
et al. (1996). Gibbons et al. (1996) state that there are broadly two types of bird
census – those for species that are evenly distributed across the landscape (i.e.
territorial species) and those for species that are highly clumped during the breeding
season (e.g. grey herons, rooks, black grouse, sea birds). In SRC and Miscanthus,
those methods applicable to clumped species will rarely be useful (as bioenergy
crops are unlikely to provide suitable habitat for these colonial breeders), so only
the main methods applicable to surveys of evenly distributed species will be
considered here.
Territory mapping
The aim of territory mapping is to estimate the boundaries of breeding bird
territories and, thus, the numbers of breeding pairs of each species within the
survey plot. Territory mapping techniques require fairly intensive fieldwork, at
regular intervals throughout the breeding season.
Gibbons et al. (1996) recommend that census plots, which should not be linear,
need to be at least 15-20 ha in temperate woodland, or 60-80 ha in more open
habitats, such as farmland, moorland or grassland. Although open habitats require
larger census plots, as few as five visits per year may be adequate, compared with a
minimum of ten required for areas of woodland with higher densities of breeding
birds (Gibbons et al., 1996).
38
The British Trust for Ornithology (BTO) have operated a national census of
breeding birds that has, until recently, been based on a territory mapping
methodology known as the Common Birds Census (Marchant, 1983; Sykes &
Lane, 1996). This methodology has also been widely used in other surveys. It
requires ten census visits between mid-March and late June. Visits should be fairly
evenly spaced throughout this period, ensuring that early and late breeding species
are recorded. Eight visits are completed in the morning (starting before 09.00 and
finishing before 12.00) and two in the evening. The positions of all birds observed
or heard are recorded on a map, using species codes. Bird activities such as calling,
singing and displaying are also recorded using a standardised notation. After all
visits have been completed, all records for each individual species are combined on
a single map (one per species) – from which the number and position of breeding
territories can be estimated.
Territory mapping is only really suitable for surveys of territorial species during the
breeding season. However, the methodology used to produce maps of breeding
territories does allow a straightforward calculation of bird densities and provides a
good picture of bird distribution across a site at any time of year, which may be
useful for the analysis of bird-habitat associations (Gibbons et al., 1996).
Consequently, a simplified common bird census methodology might also be used
for autumn and winter bird surveys.
Modified territory mapping techniques have been used in many studies, in a range
of habitats, including traditional coppice and woodland. For example, Fuller &
Green (1998), in a study of breeding bird populations in coppiced lime stands,
made three survey visits each year, in mid-May. Bellamy et al. (1996), in a survey
of small woods, visited each site four times between mid-March and early August.
Point counts
This method involves bird observations from a number of fixed positions for a
short, pre-determined period. Point counts are best suited to recording highly
visible or vocal species (e.g. songbirds) in a wide variety of habitats (Gibbons et
al., 1996). Point counts will provide estimates of the relative abundance, or
absolute densities, of different species at any chosen time of year.
Point-count stations are laid out systematically (according to a grid) or randomly.
The sample may be stratified, if appropriate. Gibbons et al. (1996) recommend that
there should be more than 20 stations at any site, and that these should be at least
200 m apart, to minimise the chances of recording the same birds at different
points. They also suggest that the site should be visited at least twice and that the
duration of counts at each station should be between three and ten minutes, with
five minutes probably being adequate in most habitats. Counts should not begin
immediately after the recorder’s arrival at the station, but a few minutes should be
allowed for the birds to settle after the initial disturbance.
Point counts can be made over an unlimited distance or within a pre-determined
band. Donald et al. (1998), for example, recording breeding birds in coniferous and
broadleaved woodland, counted only those birds observed within 50 m of their
three point count stations. Each station was visited twice and each count continued
for 10 minutes.
39
Such a methodology can provide relative indices of abundance, but a slightly more
complex approach is necessary for estimates of density. Density estimates require
birds to be counted in two or more distance bands. The simplest method is to
record those birds in a band up to a fixed distance from the surveyor (e.g. 25 m in
dense woodland and 50 m in more open habitats), and to record all those seen or
heard beyond that distance separately (Gibbons et al., 1996). Standard formulae
are then used to calculate the density of each species.
Counts can be heavily influenced by time of day and weather conditions.
Line transects
This method involves recording birds along, or on either side of, a fixed transect
(normally a straight line). Transects must be of a fixed length and randomly
allocated. They must not be selected to follow footpaths, hedgerows, streams or
other linear features – as this might heavily bias the results. Line transects are best
suited to recording birds in extensive, open and fairly homogeneous habitats e.g.
moorland (Gibbons et al., 1996).
In the UK, the BTO have adopted a line transect methodology to replace the more
labour-intensive territory mapping methodology used until recently. The Breeding
Bird Survey (BTO, 1995; Sykes & Lane, 1996) requires surveyors to walk two
straight line transects across a 1 km square. These transects are walked twice each
year - ‘early’ and ‘late’ in the breeding season.
Methods for estimating density are similar to those described for point counts.
Catch per unit effort (mist netting)
Catch per unit effort methods (mist netting) can be used, by qualified and licensed
ringers only, to monitor changes in bird populations, productivity and survival
(Gibbons et al., 1996). To allow such assessments, there must be standardisation of
mist net locations, net size (height and length), net type, timing and duration of
netting period and weather conditions during netting.
This type of methodology – used in several established, long-term studies,
including the BTO’s Constant Effort Sites (CES) Scheme (Peach et al., 1998) allows more than just the estimation of annual or longer-term changes in bird
populations, but also gives an indication of the extent to which changes may be due
to reduced/increased productivity or survival. Mist netting is usually considered to
be most suitable for recording birds in woodland or scrub, although many of the
CES Scheme sites are also in reed-beds.
Birds that are caught in the mist nets are usually identified to species, aged and
sexed before being ringed (if not already ringed).
Mark-release-recapture (ringing)
Mark and release methods for bird surveys, involves initial capture of birds by
qualified and licensed ringers, and fitting of coloured leg rings (most widely used),
wing tags, neck collars or radio transmitters. The use of highly visible, or
otherwise readily detectable, rings or tags will usually make recapture unnecessary.
40
In practice, mark-release-recapture methods are infrequently used for population
estimates in bird species, and are more commonly used for research on aspects of
migration, survival/longevity, feeding patterns, habitat usage, etc.
5.3
Bird survey methods applied in studies of biomass crops
Bird territory mapping, with repeated visits during the spring and summer, was
used to survey breeding bird populations in SRC by Kavanagh (1990), Slater et al.
(1997), Sage & Tucker (1998a) and by Coates & Say (1999). Similar methods,
without the final analysis of territories, were used by Sage & Tucker (1998a) and
Coates & Say (1999) to map the distribution of winter bird populations.
Sage et al. (1994) and Sage & Robertson (1996) used point count surveys,
recording all birds seen or heard within 50 m semi-circles.
Mist netting was employed by Singleton & Bowes (1991) in their study of birds in
an SRC plantation in North Yorkshire, trapping birds on 10 occasions between June
and October.
The study of US poplar biomass sites by Christian et al. (1997) and Collins et al.
(1999) involved walking line transects through the centre of each poplar plantation
(or other adjacent crops or semi-natural habitats) and around their perimeters.
A novel method for the monitoring of pheasant and partridge movement into and
out of SRC was developed by Baxter et al. (1996). A line of 0.25 m2 (0.5 x 0.5 m)
quadrats filled with sand was laid along one edge of each SRC plantation. Regular
inspections were made to count footprints of the two gamebird species being
studied.
5.4
Advantages and disadvantages of different methods in relation
to surveys in biomass crops
The use of skilled and suitably experienced ornithologists is essential for the
accurate identification of birds in any field survey. Spring and summer surveys
also require personnel capable of identifying bird calls and songs. Although
species identification is much easier if birds are captured, for example by mist
netting, such techniques can only be employed by suitably trained and licensed
staff.
Movement through SRC crops can be difficult, although pathways between pairs of
tree rows can usually be fairly readily walked along at any time. Visibility through
plantations is restricted at all times (other than shortly after planting, cut-back or
harvest), but particularly so when tall crops are in leaf. This makes the need for
highly competent ornithologists, with the skills to identify birds by sight or sound,
even more critical.
41
Sand quadrats are potentially useful for surveying game bird usage of SRC, but
regular inspections, and levelling of sand after rain, are essential.
Despite the difficulties, territory mapping (or adaptations of territory mapping
methodologies for winter counts) or point counts will be suitable in most surveys of
birds in SRC. This will provide some compatibility with previous UK surveys.
Bird surveys in established Miscanthus crops will be more difficult, because of the
greater restrictions on movement and visibility. Territory mapping or point counts
(at the edge of crops) should still provide useful data, but more intensive methods
such as mist netting might be considered.
42
6.
MAMMALS
6.1
Mammal species diversity in biomass crops
Coates & Say (1999) in their study of five SRC sites in southern England, trapped
four species of small mammal: common shrew (Sorex araneus), bank vole
(Clethrionomys glareolus), wood mouse (Apodemus sylvaticus) and harvest mouse
(Micromys minutus). However, only one site had all four species present –
although the method used resulted in small sample sizes, which probably underrepresented the true diversity of small mammals present (see Sections 6.3 and 6.4).
Slater et al. (1997) also trapped only low numbers of small mammals during five
short trapping periods at a newly established willow SRC site in Mid-Wales. Three
species were captured within the SRC: wood mouse (mainly), yellow-necked
mouse (Apodemus flavicollis) and field vole (Microtus agrestis). Field voles,
which were relatively abundant in adjacent rough pasture, were not captured in
SRC until the final trapping period, in early October.
Christian et al. (1997) and Collins et al. (1999) surveyed small mammal (Blarys,
Clethrionomys, Microtus, Peromyscus, Sorex and Zapus spp.) populations in hybrid
poplar plantations and surrounding land, in the USA. They found that small
mammal abundance and diversity in poplar plantations were similar to those of
adjacent or nearby arable crops. Small mammal diversity and abundance were,
however, significantly higher in surrounding forests and scrub than in poplar
plantations. There were no consistent differences between animals’ body mass or
breeding activity in the habitats studied.
Bodnor (1995; cited in Sage & Tucker, 1998a) found that populations of small
mammals, such as the wood mouse, in SRC were affected by management
intensity, with weedy SRC holding larger numbers than relatively weed free
plantations.
Sage & Tucker (1998a) recorded sightings, or other clear evidence, of at least nine
species of mammal in SRC sites across Britain and Ireland, although they did not
undertake any formal mammal survey. Among the most frequently observed were
rabbit (Oryctolagus cuniculus), brown hare (Lepus capensis) and roe deer
(Capreolus capreolus). The other species recorded were common shrew, wood
mouse, bank vole, mole (Talpa europaea), red fox (Vulpes vulpes), badger (Meles
meles) and stoat (Mustela erminea). Sage & Tucker (1998a) also suggested that
additional species (e.g. hedgehog), not observed during their site visits, would be
expected to utilise energy crops to some degree.
Coates & Say (1999) recorded two species of bat in SRC – serotine (Vespertilio
serotinus) and both the 45 kHz and 55 kHz sub-species of pipistrelle (Pipistrellus
pipistrellus) – and hedgehogs, moles, rabbits, brown hares, brown rats, badgers and
roe deer. Between three and 11 mammal species were recorded within the SRC at
any single site. Rabbits were common at all five survey sites. Moles and brown
hares were common at two sites.
43
Christian (1997) recorded tracks of deer and several ‘medium-sized’ mammal
species in small hybrid poplar plantations (2.5 x 2.5 m spacing), in Minnesota,
Wisconsin and South Dakota. From his results, he concluded that most mammal
species normally associated with forests (e.g. squirrels, rabbits and hares) showed
extremely limited use of the poplar plantations. Deer and carnivores (e.g. red foxes
and coyotes) tended to travel through plantations, rather than make concentrated
use of them. He suggested that the poplar plantations were functioning more as
‘openland’ habitats, rather than forest.
6.2
Standard mammal survey methods
The relatively highly conspicuous, larger mammal species can be counted –
although methods must take proper account of the likelihood of missing animals or
double-counting. However, several mammal species that are of conservation
concern are both secretive and occur at low densities. This makes them very
difficult to observe and, therefore, to count. Methodologies for surveys of mammal
species may, consequently, rely on counts of dung or footprints or the recording of
evidence of feeding. Such ‘indirect’ methods usually provide indices of abundance,
rather than measures of density (Sutherland, 1996b).
Total (visual) counts
Areas are divided into blocks and the number of individuals of the target species
recorded in each. This method is particularly useful for large, conspicuous
mammals in open areas. Movement of animals between blocks can result in
double-counting and over-recording.
Total counts can be used for deer or hare surveys. However, visibility restrictions
in established SRC and Miscanthus crops are likely to make accurate counts
difficult. Poor visibility is very likely to lead to underestimates of animal
populations, as some animals present will not be seen.
Counting breeding sites
This method is useful for surveys of mammals that build obvious breeding/nesting
sites, or those that live in underground lairs with relatively large and conspicuous
entrance holes (Sutherland, 1996b).
Counts of breeding sites are potentially useful for surveys of badgers (Meles meles),
rabbits (Oryctolagus cuniculus), foxes (Vulpes vulpes) and squirrels (Sciurus
vulgaris and Neosciurus carolinensis), but breeding sites may be misidentified (e.g.
squirrel dreys mistaken for bird nests, or vice versa) and abandoned sites
erroneously counted. Evidence of recent digging; fresh hairs, dung, bedding or
tracks; and strong animal scents (usually readily detectable from fox earths and
badger setts) all indicate that a breeding site is still in use.
Recording and mapping of breeding sites over a period of several years can give a
good indication of declining or expanding populations. If reliable estimates of the
average number of individuals using each site can be determined, then counts of
breeding sites can also be used for density estimates.
44
This type of survey is generally not invasive and is, therefore, not likely to require a
licence from English Nature – as would be needed for research involving trapping
badgers or disturbing their setts.
Counting and mapping calls
The standard method for assessing the relative abundance of bat species
(Chiroptera) in different habitats/sites is to record their echolocation calls using a
portable bat detector, which converts their ultrasonic sounds into a wavelength that
is audible to the human ear (Sutherland, 1996b). One system commonly employed
in the UK is to set the bat detector to a frequency of 45kHz, and to record the
number and location of ‘bat passes’ using a map (Sutherland, 1996b; Sykes &
Lane, 1996). One variation on this type of ‘fixed frequency’ recording is to locate
two or more detectors at various locations, each pre-tuned to a different frequency
(e.g. Grindal & Brigham, 1998). An alternative is to use a broad-band method,
with an acoustic bat detector tuned to ‘frequency division’ – allowing the
continuous recording of all frequencies (Vaughan et al., 1997).
Different bat species can be identified fairly reliably from the wavelength of their
ultrasonic calls. This can be determined by tuning the bat detector through a range
of frequencies, to detect the wavelength of the strongest signal. The calls of
different bat species also have other characteristic differences that can assist
identification.
Bat detectors can be directly connected to tape recorders, if required. Calls can
then be subsequently identified, with the assistance of pre-recorded tapes or CDs
(e.g. ‘The Bat Detective’). Sound analysis software (e.g. BatSound v3.31) is also
available, for use with modern frequency division and time expansion detectors, to
record and analyse calls and very accurately identify bat species.
It is important to note that the use of these techniques for bat surveys does have
certain limitations, for example:

Detector surveys are biased to loud, abundant species such as pipistrelles
(Pipistrellus pipistrellus). Quieter species, such as the brown long-eared bat
(Plecotus auritus), will be more difficult to detect.

Separation of the calls of many Myotis species (e.g. whiskered bat, Myotis
mystacinus; Brandt’s bat, M. brandtii; Natterer’s bat, M. nattereri; and
Daubenton’s bat, M. daubentonii) can be difficult or impossible using most
tuneable bat detectors.

Seasonality is very important. Surveys can only be conducted between late
spring and early autumn. Ideally, several surveys should be completed
between mid-June and early September.

Timing, over the course of the night, is vital as bats are most active shortly
after sunset and shortly before sunrise – although this varies between species.

Weather conditions are also important. Surveys should not be carried out when
rain is heavier than a light drizzle or when there are strong winds, because bats
may not be flying in these conditions (Sykes & Lane, 1996).
45
More information on bat detectors can be found on various websites e.g. The Bat
Conservation Trust (www.bats.org.uk/batinfo/batdets.htm), the University of Leeds
(www.biology.leeds.ac.uk/staff/dawa/bats/Detector.htm), Stag Electronics Ltd.
(www.batbox.com) and Alana Ecology Ltd. (www.alanaecology.com).
Trapping
Small terrestrial mammals, including mice (Rodentia: Muridae), voles (Rodentia:
Microtidae) and shrews (Insectivora: Soricidae) are generally difficult to observe
but can be quite readily trapped. Consequently, trapping is the preferred method
for surveying most small mammal species.
Traps are usually some form of metal box, with an entrance door at one end which
closes when an animal either moves a lever or presses down on a treddle, whilst
entering the trap. They are then unable to get out. Food is put in the trap, partly as
a lure and partly as a source of food to ensure the survival of the trapped mammal.
In the UK, the most frequently used trapping method for small mammals is the
Longworth trap (Sutherland, 1996b; Fitzgibbon, 1997). These small, two-part
aluminium traps are placed in pairs or groups of three, along transects or on a grid.
Bedding material, usually hay, and food for both herbivorous and insectivorous
mammals is placed in each. Normally, this would be cereal grain for mice and
voles and insect pupae (fly castors from an angling shop) for shrews. Traps are
checked twice a day (morning and late afternoon) over a period of 3-5 days. For
example, traps may be set on a Monday afternoon, then checked twice a day up
until, and including, Friday morning (i.e. over seven trapping periods – three days
and four nights). Any eaten food, wet bedding, etc. is replaced. All trapped
animals are identified to species, sexed, aged (adult or juvenile), weighed and given
a unique mark (by clipping fur). Marking animals allows subsequent new captures
to be distinguished from previously caught animals. This is then a mark-releaserecapture method that can provide an estimate of population size.
Some animals tend to be ‘trap-happy’ and are caught repeatedly, whilst others tend
to avoid traps. Weather conditions will have an influence on numbers of small
mammals trapped, as they tend to be most active on dark, warm and dry nights
(Sutherland, 1996b).
In some studies, researchers have preferred to use ‘snap-traps’, which kill the small
mammals that are attracted to them. These traps are simple mouse or rat traps, as
commonly used to kill rodent pests in and around homes and offices. Snap-traps
are very widely used in small mammal surveys in the USA, but live traps are
preferred in Europe.
Dung counts
Location of the dung/faecal pellets of some mammals may be easier than observing
the animals themselves. Dung counts, therefore, provide a useful technique for
estimating the relative abundance of several species, including rabbits, deer, foxes
and otters. It has been successfully used to assess differential use of a variety of
habitat types within a species’ population range (Doney, 1998a).
46
Frequently used methods involve clearing all dung/droppings from transects of a
pre-determined length and width, then to return and count all new dung deposited
along those transects perhaps 14 days later (e.g. Sykes & Lane, 1996). Doney
(1998a), using a computer model, evaluated four dung sampling methods for the
evaluation of deer densities. He concluded that the methodology recommended by
the UK Forestry Commission (eight 7 m x 7 m square plots laid out on alternate
sides along alternate sides of a ground survey line) had the highest potential
accuracy, and was not significantly slower than the other methods tested. Doney
(1998b), however, has demonstrated that the rate of pellet decay (for fallow deer,
Dama dama) is affected by factors such as air temperature, wind speed and (in
some instances) crop/vegetation cover. Detailed information on deer survey
techniques has been published by Mayle et al. (1999).
The success of this method depends on reliable identification of dung deposited by
different species. Useful information is provided by Bang & Dahlstrom (1974) and
in other field guides.
Feeding signs
Some mammal species, particularly herbivores, leave very clear and recognisable
signs of their feeding activity (Bang & Dahlstrom, 1974). For example, the
presence of dormice in a woodland or hedgerow can be detected from
characteristically holed hazelnut shells.
Other methods
Other methods for surveys of the distribution and abundance of mammal species
include radio-tracking and thermal imaging (Gill et al., 1997).
6.3
Mammal survey methods applied in studies of biomass crops
Small mammals
Coates & Say (1999), in their intensive study of five SRC sites in southern
England, included short periods of small mammal trapping, annually over a fiveyear period. They used plastic, ‘Longworth-type’ traps to capture mice, voles and
shrews. Traps were set during May or June and checked every 2-3 hours, over a
24-hour period. Trapping, at all five sites, was initially undertaken in SRC plots,
adjacent agricultural crops and adjacent non-agricultural habitats (e.g. hedgerows or
scrub). Traps were positioned at 10 m intervals along transects running from near
the edge of the coppice or agricultural field towards the centre. Traps were baited
with cereal grain and either blowfly larvae or a mixture of commercial rodent feed
and dog meat.
Slater et al. (1997) set Longworth traps in willow SRC and surrounding rough
pasture. Traps were set for two-night periods in February (pasture only), April,
June, August and October. Relatively few mammals were caught.
In one US study, Collins et al. (1999) trapped small mammals in hybrid poplar
plantations, and surrounding habitats, using ‘snap-traps’.
47
Bats
Coates & Say (1999) used a tuneable, heterodyne bat detector (Batbox III) at two
SRC sites, walking the perimeter of the coppice and transects through it – between
dusk and 1.00 am. Hedgerows and other nearby habitats were also walked.
Other mammals
Sage & Tucker (1998a) and Coates & Say (1999) made casual observations of
mammals in SRC plots and recorded other evidence of the presence of mammal
species.
Christian (1997) recorded tracks of deer (Odocoileus virginianus), lagomorphs
(rabbits and hares), carnivores (red foxes and coyotes, Canis latrans) and squirrels
crossing 170 m long transects in hybrid poplar plantations and other nearby landuse types. Transects were walked 3-7 days after snowfall.
6.4
Advantages and disadvantages of different methods in relation
to surveys in biomass crops
Longworth traps are generally accepted as the best method of assessing small
mammal populations in a broad range of habitats, and are suitable for use in SRC,
Miscanthus and other biomass crops. Staff should, however, be suitably trained
and approved to operate these traps.
The methodology used by Coates & Say (1999) would, however, need significant
modification to enable a more complete picture of small mammal use of biomass
crops to be obtained. Although sampling over a five-year period should allow for
annual fluctuations in mammal populations, Coates & Say’s survey did not capture
enough individuals to facilitate valid statistical analysis of their data. Sample size
could readily be increased by extending the trapping period from one day to four or
five, and by adding a second trapping period in the autumn (October). Previous
studies have shown that small mammal populations in hedgerows and woodlands
are much higher during autumn than in spring/summer, and this is also likely to be
the case in biomass crops.
Live trapping could also be used for some larger mammals, including Mustelids
such as weasels, stoats and polecats.
‘Snap-traps’ that kill small mammals should not be used. Deliberate killing of
shrews is illegal.
Bat surveys should operate standard methodologies, using tuneable bat detectors.
Transects should be walked along crop margins and, where possible, through the
crop.
Winter weather conditions in the UK would not permit the use of snow-tracking
techniques, as used for mammal surveys in the USA by Christian (1997). Larger
mammal species could be more efficiently surveyed using dung counts and,
perhaps, counts of breeding sites. Total counts of deer or hares should also be
feasible and might be considered, where sufficient numbers of staff are available to
48
‘drive’ animals through the crop, with recorders positioned to count those that
emerge onto the headland at the far side. Total counts obtained from driving
mammals from crops will, however, be difficult in some situations e.g. where
plantations are large, irregularly shaped and/or divided into compartments by
hedgerows and ditches.
More intensive autecological studies of larger mammals, perhaps to quantify their
use of bioenergy crops relative to other habitat types, could be performed using
radio-tracking techniques.
49
7.
REPTILES AND AMPHIBIANS
7.1
Reptile and amphibian species diversity in biomass crops
There have been no formal surveys of reptiles or amphibians in SRC, Miscanthus
or other biomass crops. Sage & Tucker (1998a) did, however, report observations
of common toads (Bufo bufo) and common frogs (Rana temporaria) in areas of
damp grass in the headlands and rides of wetter SRC sites, and newts (Triturus sp.)
at the edge of one willow plot. They also recorded one grass snake within an SRC
crop in southern England. The relatively dry conditions found within most biomass
crops are, however, likely to make them unsuitable habitats for amphibians and,
similarly, the shady conditions will be generally unsuitable for most reptiles (Sage
& Tucker, 1998a).
It is possible, however, that some biomass crop sites (or at least the crop margins
and headlands) might provide suitable habitat for slow worms (Anguis fragilis),
which occurs on woodland edges, hedgerows and scrub (Arnold & Burton, 1978).
Bioenergy crops might even provide potential habitat for smooth snakes (Coronella
austriaca), although this species is currently restricted to southern counties of
England, mainly in dry heathland (with mature heather, dwarf gorse and Molinia
tussocks).
7.2
Standard reptile and amphibian survey methods
Various methods are used for surveys of reptiles and amphibians. Commonly used
methods for surveying amphibians include measurement of area/volumes of frog
spawn, netting or bottle trapping of great-crested newts (Triturus cristatus). More
intensive research projects may employ methods such as ring-fencing ponds, to
direct breeding frogs or newts into large pitfall traps, where they can be regularly
counted. Mainly nocturnal amphibian species can be searched for by torch or lamplight, preferably on nights when soil and vegetation is damp after a period of
rainfall.
Diurnal reptile species can be most readily observed on warm, sunny days –
particularly in the mornings and late afternoons, when they are most active. At
other times they may be found by overturning large rocks or logs. The relative
abundance and distribution of snakes and lizards (including slow worms) can be
assessed by the use of ‘tinning’/‘reptile mats’/refugia trapping. This involves
placing squares (e.g. 75 x 75 cm) of corrugated metal sheeting, wood, plastic,
carpet or other flat or corrugated material across the site, for reptiles to hide under.
These ‘tins’ are best placed in sunny areas, most attractive to reptiles. Tins/refugia
are then regularly checked and animals beneath may, if necessary, be captured by
hand. Catching reptiles may, however, be difficult, potentially damaging
(particularly to legged lizards) and must take account of any possible legal
implications. Refugia traps provide useful comparative data on the relative usage
of different sites or habitat types, but will not normally provide data to facilitate
accurate estimates of population densities.
50
Further details on reptile trapping methods can be found in Foster & Gent (1996),
Foster & Barr (1998) and Gent & Gibson (1998). Further information on
amphibian surveys can be found in Foster & Barr (1998), Gent & Gibson (1998)
and Biota (2003).
Seasonality is particularly important in surveys of reptiles and amphibians. In
Britain, all species hibernate throughout the winter months. Reptiles are most
easily caught, and therefore best surveyed, between late April and late June or
between late August and late September (HGBI, 1998). Amphibians breed
communally, and are most readily observed in ponds during spring. April and May
are usually the preferred months for surveys of breeding amphibians in breeding
ponds (Biota, 2003). However, if frog spawn counts are to be undertaken, these
should be done between late February and late April, with earlier starts in the south.
Great-crested newts, for example, overwinter on land between October/November
and February/March (HGBI, 1998). A proportion of the adults move to the
breeding pond as temperatures begin to rise in February or March, with numbers
peaking there around mid-April. At any one time, less than one third of newts
might be expected to be in the water (HGBI, 1998). Newts also move between
ponds and do not always stay in the water for the whole of the springtime breeding
season. Young newts only occasionally enter the water and may occur up to 500 m
from their pond (HGBI, 1998).
Ring-fencing a pond between early February and mid-May, and placing pitfall traps
in gaps, may result in almost 100% of adult great-crested newts entering the pond
for breeding being caught. Alternatively, this method can be used to catch adults
and juveniles leaving a pond between early June and late October (HGBI, 1998).
Surveys for great-crested newts can only be undertaken by a suitably licensed
worker. Licences are obtained from English Nature, Scottish Natural Heritage or
the Countryside Council for Wales.
Bioenergy crops, however, are not usually adjacent to ponds or other freshwater
habitats. Confirmation of their use by dispersed or migrating amphibians will,
therefore, require alternative methods. Direct searching is possible, beneath
movable objects such as logs and rocks, but amphibians on dry land can be very
difficult to locate – particularly during dry periods (HGBI, 1998). Best results are
likely to be obtained through torch-light searches on wet nights. If a torch-light
search shows that amphibians are present, then more quantitative data can be
obtained by setting pitfall traps, with 5-10 m drift fences to direct animals into the
traps (HGBI, 1998).
7.3
Reptile and amphibian survey methods applied in studies of
biomass crops
No evidence was found of any reptile or amphibian surveys in biomass crops, other
than the observations noted in Section 7.1.
51
7.4
Advantages and disadvantages of different methods in relation
to surveys in biomass crops
Most established amphibian survey methods are only appropriate for use in ponds
or other freshwater habitats. Direct searches, either at night-time using torches or
under rocks during the day, may provide evidence of reptiles and amphibians
within and around biomass crops, but provide no comparable measure of
abundance. Placement of refugia/tins for reptile species, may be appropriate – but
may need to be situated in relatively open, sunny positions to maximise the
probability of their use by the target species. The use of pitfall traps is possible, but
drift fences may be very difficult to erect in dense crops.
52
8.
RECOMMENDATIONS
This comprehensive review of methodologies used in ecological monitoring in
energy crops, and consideration of the applicability and practicality of using
alternative ‘standard’ methods in SRC and Miscanthus, allows recommendations to
be made for a basic framework of techniques for future ecological monitoring in
energy crops.
It is recommended that new UK ecological monitoring projects in energy crops
should be encouraged to adopt a standard suite of protocols, although this should
not preclude any originality or the inclusion of additional species or groups – where
additional funding permits. Standard protocols and survey designs should follow
11 basic principles:
1.
Surveys should, wherever possible, be designed to incorporate monitoring of a
few ‘core groups’ which can be considered to be indicative of overall
biodiversity in the plantation. These groups should include:
 ground flora species;
 ground beetles (Carabidae);
 some arboreal insects (e.g. leaf beetles, Chrysomelidae; and Lepidoptera
larvae);
 breeding birds;
 small mammals (mice, voles and shrews).
The list of core groups should be kept small, to allow for projects with limited
funding; but a secondary list of ‘priority additional groups’ should be published
– for measurement where resources allow. This secondary list should include
representatives of the soil fauna, such as springtails (Collembola). It might
also include butterflies (on and around crop margins), moths, bats and deer.
2.
Surveys must not focus solely on groups for which SRC or energy grasses are
known to provide valuable habitats. An ‘independent’ and wide-ranging
approach is necessary to avoid any accusations of a bias in a survey’s
conclusions. However, it is important to monitor the usage of energy crops by
species of particular regional or national conservation concern e.g. Biodiversity
Action Plan species, such as the brown hare (Lepus capensis) (UK Biodiversity
Steering Group, 1995) and Red Data Book species (e.g. Palmer et al., 1997).
3.
Methodologies should be based upon widely accepted techniques, which are
appropriate for the target organisms in that particular study – preferably those
which have been previously ‘tried and tested’ in surveys of energy crop
plantations. For the groups mentioned in 1 above, plant frequency counts using
nested quadrats, pitfall trapping, stem beating, bird censuses and live-trapping
in Longworth box traps are considered to be suitable.
4.
Methods used must fully consider the practicalities of sampling/recording
within the energy crop concerned. For example, protocols must allow for the
row spacing of SRC crops when specifying the location of quadrats for
vegetation recording, or sampling positions for soil cores (e.g. if sampling for
springtails or mites). Other important practical difficulties would include
53
severely restricted access to Miscanthus crops in summer; limited visibility
within short rotation coppice (SRC) and Miscanthus during spring and summer
and a dense mat of woody roots that inhibit digging (e.g. to extract
earthworms).
5.
Surveys should always be designed to facilitate valid and dependable
statistical analyses of the data recorded. This will involve care in the selection
of sample sites and levels of stratification, and consideration of the use of
‘controls’ - as well as robust field methodologies. Samples should be
representative and take account of the important influences of factors such as
climate, altitude, soil type, adjacent land-uses and plantation size on
biodiversity within plantations. The factors considered in sampling design
will, however, depend upon the specific questions being addressed – rather
than simply being applied in accordance with any fixed suite of variables..
6.
New surveys should make full use of any opportunities for comparisons with
‘baseline data’ – either newly collected (i.e. from monitoring commencing
prior to energy crop establishment) or already available (e.g. from research
sites with biological databases dating from previous studies in the same fields).
7.
Methodologies should, where applicable, be compatible with those used by
national surveys - e.g. BTO breeding bird survey, Butterfly Monitoring
Scheme or Rothamsted light survey (moths) – or national environmental
monitoring networks - e.g. UK Environmental Change Network (Sykes &
Lane, 1996).
8.
Surveys should, wherever possible, allow direct comparisons between the
biodiversity of energy crop plantations and other habitats within the local
landscape – particularly the land-use that it has replaced (e.g. arable or
grassland fields in agricultural landscapes).
9.
Survey plots must incorporate field margins (i.e. crop edges and, the
potentially important, grass headlands) and field boundaries (e.g. hedgerows).
Grassy headlands might, in the longer-term, provide much of the site’s
biodiversity, and one important positive impact of biomass crops may result
from the removal of the negative effects of agrochemical inputs and livestock
grazing from hedgerows and watercourses.
10. Surveys should also consider the wider, landscape-scale impacts of biomass
crops on farmland ecology. Studies should take account of adjoining and
nearby habitats, and consider linkages with semi-natural habitats, and potential
species movements across the landscape.
11. One-off surveys have a value, but it is always desirable to repeat surveys over
at least three years, to allow for temporal changes. Populations of many
species change dramatically from year-to-year, as a result of climatic factors
(e.g. summer rainfall, winter temperatures, and soil moisture), food
availability, predator-prey interactions, etc. Repeat sampling will reduce the
chances of missing a species, or underestimating its mean population size. The
54
precise number of surveys to make should, however, depend upon the
objectives of the study.
9.
ACKNOWLEDGEMENTS
This review was funded by the UK Department for Environment, Food and Rural
Affairs (Defra). Work was completed as part of the project Poplars: a multiple-use
crop for European arable farmers (PAMUCEAF), which was funded by Defra
(Project NF0408) and the European Commission (FAIR6 CT98-4193). Francis
Kirkham, ADAS, provided helpful comments on the draft report.
55
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