Download The potential role of large herbivores in nature

BiologicalJournal of the Linnean Society (lYY5), 56(SuppL): 11-23. With 2 figures
The National Trust and nature conservation: 100 years on
Edited by D.J.
Bullock and H.J. Harvey
The potential role of large herbivores in nature
conservation and extensive land use in Europe
S. E. VAN WIEREN
Department of Nature Conservation, WageningenAgricultural University,
PO Box 8080, 6700 DD, Wageningen, The Netherlands
The concept of ecosystem restoration is gaining momentum in western Europe. This is
necessary because in most managed nature reserves one or more of the following processes,
which are analogous to those that have led to the dramatic loss of biological diversity in
Europe, are still operating: continuing nutrient output, continuing high level of disturbance
and fixing the system in some successional stage. This is partly because most management
activities have been derived from, or copy, former agricultural practices. The study of natural
ecosystems has revealed the key role large herbivores have in maintaining structural diversity
in the vegetation and so biological diversity. Because of this they have been used as tools
in achieving a variety of conservation goals. Here, various effects large herbivores can have
on plant species composition, structural diversity of the vegetation and fauna are briefly
reviewed. Attention is given to pasture-woodlands in southern Europe, which often have a
relatively high biological diversity and share some key features with natural ecosystems: very
low nutrient input, extensive grazing with large herbivores and the presence of natural tree
cover. In a number of European countries attempts are being made to restore normal
functioning multi-(herbivore) species ecosystems.
0 199.5 The Linnean Society of London
ADDITIONAL KEY WORDS:-grazing
woodlands.
-
conservation
-
ecosystem restoration - pasture
-
CONTENTS
Introduction . . . . . . . . . . .
A short history of decline of nature and natural values .
Ecological references . . . . . . . .
Influence of man . . . . . . . . .
Further decline
. . . . . . . . .
Recent developments . . . . . . . .
Lessons for nature conservationists
. . . .
The historic frame of reference of nature conservation .
Effects of large herbivores . . . . . . . .
. . . . . .
Abandonmentoffarmland
Diversity and grazing . . . . . . . .
Facilitatingenergyflow
. . . . . . .
Facilitating other fauna
. . . . . . .
Species differences
. . . . . . . .
Density andvegetation structure . . . . .
Perspectives . . . . . . . . . . .
Acknowledgements
. . . . . . . . .
References
. . . . . . . . . . .
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0 1995 T h e Linnean Society of London
12
S. E. VAN WIEREN
INTRODUCTION
In the past decade ‘restoration ecology’ has become part of theory and
practice in nature conservation (Western & Pearl, 1989). In western Europe
it is now becoming increasingly clear that conservation of the constituent
species in an ecosystem already present by means of well defined management
practices is often untenable and not sufficient (Dudley, 1992). It is therefore
necessary to determine the ecological context in which species evolved, and
from there attempt to restore ecosystems to a more natural state than the
one they are in at present. The study of natural ecosystems is providing
insight into the key role played by large herbivores in the maintenance of
structural diversity in vegetation. This has led to the use of large grazing
herbivores as tools to achieve a variety of conservation goals, including
attempts in Europe to restore ecosystems (Gordon et al., 1990; Gordon &
Duncan, 1988; Van Wieren, 1991).
A SHORT HISTORY OF THE DECLINE OF NATURE AND NATURAL VALUES
Ecological references
In western Europe, true nature (i.e. landscapes with a minimum of human
interference) probably existed until the Atlantic period (6000-3000 BC) when
most of it was covered with forests of varying degrees of openness (Iversen,
1973; Bottema, 1988; Peterken, 1991) depending upon the influence of large
herbivore grazing and browsing. The large herbivore and predator fauna was
still complete: aurochs, Bos primigenius Bojanus, tarpan Equus ferus Boddaert,
wisent Bison bonasus L., red deer Cervus elaphus L., roe deer Capreolus capreolus
L., moose Alces alces L., wild boar (Sus scrofa L., wolf Canis lupus L., brown
bear Ursus arctos L., and lynx Lynx bnx L., were all extant.
InJuence of man
In the Sub-Boreal (3000-800 BC) forest clearance, hunting and the
domestication of animals by Neolithic man brought about large changes in
the landscape (Bogucki, 1988). After the Neolithic, human population density
slowly but steadily increased and by the Middle Ages huge areas of forest
were cut down and open man-made landscapes became dominant. Increases
in grazing pressure, combined with deforestation and other forms of soil
exploitation, led to the development of heathlands and acid grasslands. In
less exploited landscapes, pasture-woodlands developed. Farming became
increasingly specialized with changes from mixed farming to either grassland,
arable land or production forests (Vera, pers. comm.) resulting in the main
landscapes observed by mid 19th century. By that time the aurochs and the
tarpan were extinct, while the ranges of many other mammals (brown bear,
wild boar, wolf, lynx and beaver Castor Jiber L.) had shrunk. The wisent
almost became extinct.
Further decline
The increased importation of fertilizer at the end of the nineteenth century
marked a new stage in agricultural development. The input of fertilizer made
LARGE HERBIVORES AND NATURE CONSERVATION IN EUROPE
13
man independent of the animal dung produced by large herds of sheep and
cattle which foraged over large areas. This resulted in a loss of semi-natural
grazing lands because larger areas could now be used for more intense farming,
leaving little room for most species (Van Wieren, 1991). Mechanization and
other agricultural developments such as the lowering of the water table in
the first half of the twentieth century further accelerated this process as
outlined below.
Recent developments
Intensive farming systems
In the past few decades agricultural development has led to an ever
increasing human impact on the land in north-western Europe, resulting in
an accelerating rate of species loss (Baldock & Beaufoy, 1993; Weinreich &
Musters, 1989). The most important developments have been higher nutrient
inputs, leading to an even lower diversity in nutrient levels; increase in
levelling (of shrubs, hedges etc), thereby destroying edge and other habitats;
improved drainage, leading to losses of wetland species; introduction of
highly intensive farming; increased fragmentation of habitats; earlier and
increased frequency of mowing with a detrimental effect on grassland bird
communities; higher livestock stocking rates and longer grazing periods, with
negative consequences for a number of breeding bird species.
Extensive farming systems
Since the 1960s dramatic changes have occurred in the semi-natural
landscape in many parts of Europe. Details of these changes can be found
in Baldock & Beaufoy (1993) who identified some major developments on
two levels. First, there are changes in farming practices such as increase or
decrease in stocking density, changes in harvesting and mowing dates, and
change from hay-making to silage making. Second, there are fundamental
changes such as conversion to radically different types of agriculture, total
abandonment of land and afforestation of farmland.
Through all these developments the relatively high biological diversity of
many millions of hectares of semi-natural grazing lands in Europe is now
threatened. An example of recent changes in species abundance in The
Netherlands is given in Table 1. Table 2 compares percentage area of
protected nature reserves in the European Community with other regions in
the world and demonstrates clearly that the poorer southern countries have
retained more suitable conditions for wildlife than the richer countries in the
north.
Lessons for nature conservationists
From the short history described above it becomes very clear that there
is a negative relationship between the activities of man the farmer and the
maintenance of biological diversity, and that this relationship runs through
history from Neolithic times to the present day. In the EC agriculture is
driven by economics and politics, currently through the Common Agricultural
Policy (CAP) which determines prices for key products and provides incentives
14
S. E. VAN WIEREN
T.4BI.E 1. Changes in number and abundance of species in The
Netherlands between 1940 and 1980 (after Weinreich & Musters,
1989). '-' indicates no data available
Taxonomic group
No. of
species
in group
in 1940
~~
Fungi
Plants
Butterflies
Fish
Amphibians
Reptiles
Birds
Mammals
Percentage Species
-
Lost
~~
314
1436
70
46
14
7
180
62
-~
~
~
Decreasing
~~
Increasing
~
-
36
8
7-30
54
34
69
64
100
30
30
26
2.3
10
-
1
5
-
30
36
62
TABLE2. Percentage surface area of
protected nature reserves in (A) countries
within the European Community and (B)
regions of the world. The category 'parks'
used here comprises IUCN categories I
(strict nature reserves) and I1 (national
parks). (From Wallis de Vries, 1995)
Percentage of parks
~-
~~
A
Italy
France
Greece
Denmark
Netherlands
Spain
Germany
Portugal
Belgium
Ireland
Luxembourg
UK
0.97
0.50
0.46
0.31
0.26
0.24
0.04
0.04
0
0
0
0
B
European Community
Nordic Countries
East Europe
Asia
North America
South America
Africa
Oceania
0.34
5.07
1.06
2.13
2.30
2.70
2.72
3.73
'LCII'AL WORI.1)
2.42
LARGE HERBIVORES AND NATURE CONSERVATION IN EUROPE
15
for capital investments. The unpredictable whims of policy make it an
unreliable ally to conservation of biological diversity.
The major causes by which natural systems have changed or disappeared
can be summarized as: loss of habitat (area); loss of structural diversity in
the vegetation; increased disturbance; nutrient input; nutrient output through
harvesting plant and animal products; and lowering of the water table.
THE HISTORIC FRAME O F REFERENCE O F NATURE CONSERVATION
At the turn of the century conservation organizations arose in a number
of European countries. Observed habitat loss, combined with significant
declines in some species, was the stimulus and nature reserves were created.
It is not surprising that the main aim of the early conservationists was to
preserve prevailing landscapes with their constituent species. In The
Netherlands many nature reserves were left to themselves without any
management. This frequently allowed a continuing deterioration in the
ecosystem (Van Wieren, 1991) and it was realized that active management
was needed. This has consisted of the continuation of farming practices:
mowing, cutting sods, rotational grazing, coppicing and burning. Nature
conservation thus aimed for the maintenance or redevelopment of historic
agricultural landscapes by means of historic agricultural practices, an aim
which is still current. In the light of the history of man’s impact as described
above, it can be argued that the historic approach has serious shortcomings
with regard to conservation, because it implies, in many cases, promoting a
process that has led to a decline in biological diversity. In many cases the
continuation of former agricultural management practices is no longer feasible.
It is costly and laborious, and it is difficult to find a market for the harvested
products. Furthermore, the desired landscapes have experienced a long history
of decline and are thus already seriously impoverished (we are mainly
concerned with open landscapes such as heathlands, grasslands, moors and
downs, with relatively low structural diversities). The frequently expressed
opinion that man has enriched the landscape and that diversity has even
increased (Green, 1989) is only true in some cases (Western & Pearl, 1989)
and does not take into account local extinctions (some of which went
undetected) of many species. Many of these were believed to be associated
with dead wood, large trees or carcasses of large animals (Dudley, 1992).
When nutrients are extracted from the system by harvesting plant or animal
material, impoverishment continues. Mowing, sod cutting, tree cutting or
intensive grazing are disturbance factors that only benefit a small number of
species. These activities are alien to normally functioning temperate terrestrial
ecosystems and cannot usually be regarded as substitutes for natural processes.
The desired landscapes can only be preserved or developed by active
management because they deviate significantly from some climax stage, and
the ecosystem is kept in some early successional phase. The aim of fixing
the ecosystem is in itself artificial, as it is dynamic by nature, both with
respect to structure and species composition. Management goals are further
complicated by the fact that most desired landscapes have only had a short
history and represent only transitional stages containing rather arbitrary shortlived assemblages of species.
S. E. VAN WIEREN
16
From the above it can be argued that there may be risks involved in
linking conservation of biological diversity to a historical agricultural frame
of reference. The scope is limited and goals unlikely to be achieved. Recent
inventories make it clear that the loss of natural values is continuing at an
accelerating rate, even when including nature reserves (Baldock & Beaufoy,
1993; Bink et al., 1994). It has to be recognized that conservation in western
Europe is unusual in focusing on natural values associated with early
successional stages and hence with landscapes with a high degree of openness.
Many conservationists fear that ecological changes in abandoned areas such
as the development of a monotonous vegetation structure dominated by a
few competitive species will result in the loss of biological diversity. This
may well be the case when the ecosystem is either very young or incomplete
and lacking in keystone species. Among the keystone species, large herbivores
probably played a more important role in the maintenance of open landscapes
in natural situations than has previously been thought (Iversen, 1973; Vera,
pers. comm.).
EFFECTS OF LARGE HERBIVORES
The main processes by which herbivores exert their influence are grazing,
treading, dunging and urination (Fig. 1). These processes occur selectively.
Animals select on different levels-vegetation type, plant species and plant
part-and as a result there are local differences in grazing intensity. Mutatis
Animal characteristics
General attributes
Processes
Ecophysiological niche
Body size
Dunging
Social organization
Social behaviour
Density
I
Vegetation structure
Succession
Species richness and
abundance of animals
and plants
Figure 1. Relationship between animal characteristics, general aspects of the grazing process
and the grazed system.
LARGE HERBIVORES AND NATURE CONSERVATION IN EUROPE
17
mutandis similar differences in spatial variation can be found in treading
pressure and dunging intensity. The various ways in which these processes
work can result in changes both in abiotic conditions and the structural
diversity of vegetation. To these general processes can be added speciesspecific and animal density effects. Together, these factors have a high
potential for causing variation in the diversity of temperate terrestrial
ecosystems some of which has been described previously (e.g. Bakker, 1989;
Gordon et al., 1990; Putman, 1986; Van Wieren, 1991) and only a brief
overview is given here.
Abandonment of farmland
When grazing ceases there is frequently a decline in species richness, and
a thick litter layer forms (Willems, 1983). For example, after 24 years of
abandonment of formerly grazed grasslands, species diversity, equitability and
pattern diversity diminished (Persson, 1984). In contrast, when grazing is
resumed, in a previously abandoned grassland, species diversity can increase
again. Ten years after the reintroduction of grazing, the number of species
in a grazed salt-marsh had doubled (Bakker, 1989).
Diversity and grazing
Some of the high diversity that characterizes many grazed ecosystems can
be explained by processes described above. But species diversity seems to
be highest at intermediate levels of plant biomass (Grime, 1979) and
disturbance (Huston, 1979). Both conditions can be brought about by
adjustment of the grazing level. Moderate grazing seems to have a mediating
effect on competition between plant species, permitting more species to be
sympatric than would be the case in the absence of grazing (Naveh &
Whittaker, 1980). Grazing can also create gaps and a large variety of these
is sometimes considered an important explanation for high biologcal diversity
(Bakker, 1989). When grazed systems are allowed time to develop, structural
variation is increased and favourable conditions for ‘specialists’ arise, so that
species richness can increase even further (Fig. 2).
Facilitating energy Jlow
Grazing and trampling facilitate energy flow through ecosystems because
there is little accumulation of litter whilst at the same time the decomposition
rate is relatively high (Siepel & Van Wieren, 1990). For example, after 4
years of grazing a Deschampsia Jlexuosa (L.) Trim-dominated former heathland,
cattle had reduced the litter layer from 52 to 28 t organic matter ha-’ (Van
Wieren, 1988). In this grassland both the biomass and productivity of D.
jlexuousa were greatly reduced, gaps were created, and Calluna uulgaris (L.)
Hull and Vaccinium uitis-idaea L. started to regenerate. The same mechanism
would work in counteracting the now frequently reported risk of fires in
abandoned lands in the drier parts of southern Europe (Baldock & Beaufoy,
1993).
in
S. E. VAN WIEREN
Grazing intensity
Figure 2. Response surface curve outlining the species richness (S) response to grazing
intensity through evolutionary time. Unbroken lines connect points with the same S. The
ridge (broken line) connects peak S values. (From: Naveh & Whittaker, 1980.)
L=Low s
H = High S
Facilitating other fiuna
A well-known effect of grazing by larger herbivore species is facilitation
of smaller herbivores by which the former create suitable habitat or feeding
sites for the latter. In an abandoned grass-heath grazed with ponies, rabbit
Oryctolagus cuniculus L. density was positively correlated with the grazing
pressure of the ponies (Thalen et al., 1987). Gordon (1988) found that cattle
grazing on the Isle of Rum increased the reproductive performance of red
deer. The effects of grazing on wetland birds are frequently found to be
positive and grazed wetlands are also particularly attractive to geese (Van
Wieren, 1991). However the occurrence of such grazing facilitation is
dependent upon grazing intensity. Negative effects of a very high grazing
pressure have been reported for insects (Rahmann, 1987), small rodents (Hill,
1982) and birds (Beintema & Muskens, 1987).
Species dzferences
The various hoofed herbivore species differ in many respects but notably
in size and ecophysiological niche. With respect to the latter, Hofmann
LARGE HERBIVORES AND NATURE CONSERVATION IN EUROPE
19
(1973, 1982) distinguished three main types of ruminants: concentrate selectors,
bulk and roughage feeders, and intermediate feeders. The distinctions were
based on differences in the anatomical and morphological characteristics of
the digestive tract, which in turn were related to the capability to digest
cellulose. Concentrate selectors are poor cell wall digesters, concentrating on
the cell contents; roughage feeders are specialized grazers with a high capacity
to digest cell walls, while intermediate feeders have a versatile intermediate
strategy. The horse, a non-ruminant, can be classified as a grazer. These
anatomical and physical differences lead to different feeding styles (Thalen,
1984). Browsers feed predominantly on herbs and leaves and twigs of woody
plants while grazers are specialized grass eaters. Intermediate feeders are in
between these two types (Breymeyer & Van Dyne, 1980). In lowland western
Europe the original species assemblage would have been roe deer, red deer,
aurochs, tarpan and wisent. Within this group, all major feeding styles can
be found and it is likely that western European ecosystems function best if
representatives of all feeding styles are present. The concept of restoring
natural ecosystems hence excludes non-native species such as sheep and
goats.
It can be argued that because the aurochs and the tarpan are extinct, and
because they have been identified as keystone species, ecological replacements
for them in the form of hardy domestic breeds of cattle and ponies should
be used. If this occurs, they should be managed, as much as is possible, as
wild animals.
Density and vegetation structure
In the management of ecosystems using large herbivores, density is perhaps
the single most important variable. Increasing density beyond the ecological
carrying capacity, the natural limit of a species population set by resources
in a particular environment, can have a number of effects. Frequently, a
major shift in species composition occurs as the result of preferential foraging.
The regeneration of preferred species can be seriously hampered or even
prevented. Heavy grazing by domestic livestock in Scandinavia resulted in
the large scale elimination of Salix cinerea L., Fraxinus excelsior L. and Ulmus
sp. (Ahlen, 1975). In the New Forest, a very high grazing intensity for a
long period has led to the disappearance of shrub species such as hazel,
Corylus avellanus, and also some of the palatable species in the ground layer
(Tubbs, 1981). In western Europe, succession to broadleaved forest can be
seriously retarded because species of the later successional stages (e.g. oak
Ruercus sp and beech Fagus sylvatica L.) are preferred while pioneer species
like birch Betula sp and Scots pine Pinus sylvestris L. are not preferred
(Mitchell et al., 1977; Van Wieren & Borgesius, 1988). When herbivore
pressure increases even further, forest regeneration is prevented on a large
scale and grasslands and dwarf-shrub heaths spread (Ahlen, 1975; Putman et
al., 1984).
When conditions are suitable, and grazing pressure is not too high, pasturewoodlands can develop. The woodland can be maintained by protecting the
trees or by scrub encroachment. In the case of the latter, broadleaved species
such as oaks can establish within thorny shrubs and so grow out of reach
20
S. E. VAN WIEREN
of large herbivores (Burrichter et al., 1980). If different successional stages
are present and fringe communities develop, pasture woodlands can be very
rich in species (Dierschke, 1974; Alexander, 1995; Kirby et al., 1995).
Pasture-woodlands were once common throughout Europe and they may
well have dominated the landscape for more than 1000 years (Pott & Hiippe,
1991). Today in north-western Europe, only a few remnants of any size
(such as the New Forest) remain, but in the south, large pasture-woodlands
still exist. One of the richest systems is the Dehesa landscape in Spain (Ruiz
& Ruiz 1986). The Dehesas cover about 3-5 million ha of extensive grazing.
The average pasture contains about 150 higher plant species (Fernandez Ales
et al., in Baldock & Beaufoy, 1993). Globally threatened species like the
Spanish imperial eagle (Aquila heliaca Savigny) and the black vulture (Aegypius
monachus L.) occur. There are 40 important bird areas ( B A S ) in Extremadura
alone (Sears, 1991) and a rich mammalian fauna is present including red
deer, boar, lynx and wolf. It can be argued that pasture-woodland comes
closer to natural ecosystems than most other man-modified landscapes in
Europe. This is because of a closer physical resemblance, and some important
characteristics: extensive grazing with large herbivores, low input of fertilizer,
and the presence of natural tree cover.
Of course pasture-woodlands are not the same as the original ecosystem,
and species losses have occurred, but because of important analogies we can
derive some major principles from them with respect to ecosystem restoration.
PERSPECTIVES
This paper is not intended to denigrate all attempts to conserve single
species or communities. These goals can be justified. It is, however, argued,
that in many cases attainment of these goals will be very difficult and that
the scope is limited. Therefore, the concept of ecosystem restoration at the
landscape level is discussed and attempts to establish a series of ‘restoration
experiments’ throughout Europe are noted. The main principles involved in
ecosystem restoration and the conditions under which it can occur can be
summarized as follows:
(1) Large areas are needed. Every species has its own area needed to
sustain a viable population, but it is estimated that at least 10000 ha would
be required for an ecosystem containing viable populations of large mammals.
In Europe it seems likely that more land for conservation will be available
as a result of current developments in agricultural policy (Baldock & Beaufoy,
1993), and that opportunities will arise for the establishment of such largescale nature reserves.
(2) The concept of restoration indicates that only indigenous species are
involved. Knowledge of the ‘reference’ ecosystem is thus a prerequisite.
(3) Because of a frequently impoverished starting point, it is important that
at least some keystone species are reintroduced. Among these should be the
large indigenous herbivores. Domestic breeds of cattle and ponies can be
used as substitutes for the extinct aurochs and tarpan but they should be
treated as wild species. If possible, large predators should also be reintroduced.
(4)Natural systems function without main-induced nutrient input or nutrient
LARGE HERBIVORES AND NATURE CONSERVATION IN EUROPE
21
output and there should be a presumption against extraction of plant or
animal matter.
(5) Structural diversity in the vegetation is the key to biologcal diversity
(Holden, 1987) and so should be encouraged. If an area is large enough
and the keystone species are present, structural diversity in the vegetation
will be maintained. If the ecosystem is incomplete and important processes
are lacking, man can (and may have to) influence the densities of the
herbivores to enhance structural diversity in the vegetation.
(6) Normally functioning ecosystems do so with little involvement of man.
This means that disturbance should be minimized. Disturbance, following
Van Andle & Van den Bergh (1987), is “a change in conditions which
interferes with the normal functioning of a particular biological system”.
The whole concept of ecosystem restoration is based on faith in the
working of natural processes leading to ecosystems that contain a larger array
of species than in man-managed landscapes. The space, time and the presence
of key factors through which natural processes can operate are necessary
prerequisites to successful restoration. This concept is now beginning to be
taken up in several European countries. In The Netherlands, restoration
projects have been initiated along the major rivers and in coastal (marsh)
areas (De Bruin et al., 1987). In Germany, plans are being made to restore
a typical central European ‘ Urecosystem’ in the biosphere reserve of
Schorfheide-Chorin (R. Hofmann, pers. comm). Here the starting conditions
are optimal because large mammals like boar and red deer are already
present, while moose and wolf from Poland are colonizing the area. Examples
of attempts to improve habitats by use of large grazing animals can be found
in The Netherlands, in Britain (Rum) and in France (Camargue). In Killarney
National Park (Ireland) experiments have recently started with reintroducing
the local Kerry cattle on the hills to stop the spread of coarse grasses and
to improve conditions for red deer.
In spite of these attempts there is still a need for a number of case studies
where a variety of scenarios can be studied in an experimental context.
ACKNOWLEDGEMENTS
I would like to thank Frans Vera and Michiel Wallis de Vries for valuable
comments on the manuscript.
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