Download Forest edges and habitat selection in birds: a functional approach

Forest edges and habitat selection in birds: a
functional approach
Duncan McCollin
School of Environmental Science, University College Northampton, Park Campus, Northampton,
NN2 7AL, U.K.
e-mail: [email protected]
Edge effects encompass a complex panoply of biotic and abiotic phenomena across
woodland borders. I identify four main explanations which have been proposed to explain avian
habitat selection with respect to forest edges: (1) individualistic resource and patch use, (2) biotic
interactions; (3) microclimate modification and (4) changes in vegetation structure. (1) relates
nest site location in woodlands relative to the edge to the proximity of food resources. It is
shown that, all other things being equal, birds which are wholely dependent on resources found
within woodlands will tend to avoid forest edges. Woodland species dependent upon resources
found in adjacent habitats will tend to be found near to edges to enable their exploitation. (2)
identifies competition, predation and brood parasitism as factors which have the potential to
influence bird habitat selection near edges. (3) identifies microclimate modification as a
potential influence which may act directly on nesting success or indirectly through its effects on
food supply; (4) relates the activities of birds, such as nesting, feeding or use of song posts, to
vegetation structure and/or floristic composition at the edge. Research on edge effects of birds
in woodland has provided few practical recommendations to conservation managers. Forest edge
management needs to take into account the multiple cause and effects which influence habitat
selection at the edge and to target species of conservation concern.
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Edge effects may be defined as changes which occur at the abrupt transition between
adjacent habitats, resulting from the juxtaposition of contrasting ecosystems on either side of the
discontinuity (Sammalisto 1957). It has long been recognised that edges contain increased
biodiversity since they attract species able to exploit both sides of the discontinuity in addition
to those species characteristic of either side (Leopold 1933, Odum 1971, Clapham 1973, Odum
1983). Recent concern over the biological consequences of habitat fragmentation has led to
a renewed interest in edge effects (Saunders et al. 1991, Bierregaard et al. 1992) yet,
surprisingly, even for well studied taxa such as birds, few studies have attempted to determine
what resource factors are supplied by edges (Luken 1990).
Recent reviews of edge effect research have highlighted two areas of particular concern:
poor research design, and the difficulties in making comparisons between studies due to a lack
of consistency in methodology (Paton 1994, Murcia 1995). Paton (1994) re-examined 21
avian predation and parasitism research papers and found that studies which showed no evidence
of edge effects had often failed to investigate beyond the potential range of edge influence (50
m). Many studies show site-specific trends generating little consensus and apparent
inconsistencies in edge effects can often be attributed to a lack of replicates (Murcia 1995).
Ehrlich (1996) called for a more sophisticated knowledge about temperate forests to be
developed, including edge effects, for which there is a need to formulate and test clear
mechanistic hypotheses (Murcia 1995). Accordingly, the primary aims here are: to distinguish
between the main types of edge effects and to elucidate potential mechanisms which influence
habitat selection at forest edges. The review focuses on birds since they have been the subject
of much edge effect research and represent a group for which habitat requirements are
reasonably well-known.
The influence of edges on habitat selection in birds
The reasons why individuals in a woodland or forest bird population establish territories
in particular locations depends upon an interplay between the ‘niche gestalt’ of individual bird
species and a number of site-specific factors including geographical location, altitude, land
productivity, woodland area and isolation, edge effects, stand structure and floristic composition,
and social and demographic considerations (James 1971, Fuller 1995). Here, I examine bird
habitat selection in relation to forest edges, with an emphasis on breeding populations,
distinguishing between four main primary causal factors which operate at the local-scale: (1)
species-specific differences in resource and patch use, (2) biotic interactions, (3) microclimatic
modification and (4) vegetation structure. These causal factors provide the basis to formulate
primary hypotheses forming the foundation for erecting secondary and tertiary interaction
hypotheses to provide a theoretical framework for investigating avian edge effects. Where
appropriate, areas requiring further attention in addition to potential confounding factors are
highlighted.
Individualistic resource and patch use
Many woodland birds appear to actively use or avoid woodland edges (Galli et al. 1976,
Tomialoj_ et al. 1981, Møller 1987, Fuller and Warren 1991). However, edge species per se
need to be distinguished from those species which occur at the edge in response to specific
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aspects of the environment, e.g., vegetation (Haapenen 1965, Tomialoj_ et al. 1981). Indeed,
it is the distinction between the influence of internal and external factors at the edge which
provides a useful distinction between ‘edge’ and ‘ecotone’ species. Edge species may be
defined as species which occur near to edges due to influences external to the edge whilst
ecotone species occur near to the edge due to internal changes within the stand. Birds
responding this way have also been termed 'forest-periphery' and 'forest-margin' species,
respectively, the latter being species exclusive to the edge whilst the former could occur deep in
the interior of large forest areas, but were found most often at the edge (Gromadzki 1970).
Plotting incidence in small woods versus large woods is a method that has been used to
detect occupancy patterns of breeding bird species with respect to edge (Howe 1984). Using
this method, Taylor et al. (1987) identified several edge species in a British suburban area
including the finches (Fringillidae), yellowhammer, Emberiza citrinella, and green woodpecker
Picus viridis. A summary of the consistency in edge-orientated distribution patterns of bird
species across five investigations undertaken in northern Europe is presented in Table 1. This
summary is divided into three sections: the first categorises species as having a preference for
the edge; the second presents birds consistently regarded as ‘typical’ (i.e., interior) forest species,
and the third presents species having conflicting interior and edge classifications.
Obligate edge species
In highly fragmented European agricultural landscapes species associated with edges
adjacent to open country are largely dependent upon food resources found in farmland (Table
1a). These include rook Corvus frugilegus and carrion crow C. corone corone, which feed
mainly on grain (especially Triticum, Avena, Hordeum), earthworms (Lumbricidae) and their
eggs, and grassland insects (although crow in addition takes live small mammals and carrion
meat)(Holyoak 1968). Similarly, starlings Sturnus vulgaris exploit the upper layers of pasture
soils and their diet comprises 80% leatherjackets (Diptera: Tipula spp.) and 16% earthworms
(Dunnet 1955). Other typical edge species include finches which depend largely on seeds found
in open farmland (Newton 1967) and woodpigeon Columba palumbus, which has its highest
densities in small woods (less than 5 ha in size), and in woods which have the greatest proportion
of edge habitat within 20 m of the wood margin (Inglis et al. 1994). Edge species are often
colonial nesters, a strategy associated with patchily distributed food resources (Perrins and
Birkhead 1983, McCollin 1993). The type of land use adjacent or near to forest edges, and the
food resources available therein, have an important influence upon the incidence of certain bird
species in forest edges. I shall return to the subject of edge context later.
Obligate interior species
Whilst a preference for edges by certain bird species in woodlands is well established
(e.g., Helle and Helle 1982, Kroodsma 1982a, 1982b, 1984, Hansson 1983), Fuller (1990, 1995)
noted that relatively few woodland bird species in Britain show a marked avoidance of the edge.
This may be because interior species have already been lost during fragmentation (Ford 1987),
or simply that they have not been recognised as interior species since they are able to persist in
highly fragmented landscapes by their ability to encompass several small fragments into single
territories or by augmenting woodland territories with non-woodland trees in adjacent hedgerows
(Gromadzki 1970). Many interior forest species on mainland Europe are sedentary hole-nesting
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species associated with mature woodland (Angelstam 1990) (Table 1b). This is in contrast to
North America, which has a much higher proportion of unfragmented forest, where interior
species tend to be ground-nesting, long-distance migrants (Whitcomb et al. 1981).
Specialist forest species will, on average, tend to avoid edges if they exploit resources
optimally within their territories or home ranges (Kuitenen and Mäkinen 1993). I modelled the
effects of nest-site location on bird energy expenditure by generating 100,000 random ‘foraging’
points, following the method of Skalski (1987), and determining the distance travelled from
various nest-site locations along the radius of a circular territory using the Law of Cosines. A
circular territory is assumed since this is the shape predicted by central place foraging theory to
minimise the ratio of circumference to area (Stephens and Krebs 1986).
Mean travel costs incurred increase significantly with nest site placement from the
territory centre (F= 4.4E+4, P<<0.001) (Fig. 1). Hence, on theoretical grounds alone, birds which
are entirely dependent upon woodlands during the breeding season will, other things being equal,
tend to avoid edges. This model is corroborated by field investigation. Kuitenen and Mäkinen
(1993) placed 50 nest boxes arranged along ten transects from the edge into the interior in forest
in southern Finland and found that treecreepers Certhia familiaris favoured nest boxes away from
the forest edge with preferred sites occupied at a distance consistent with predictions from a
knowledge of territory size. Predation could be ruled out as an influence on nest site placement
since there were insignificant differences in predation rates between the edge and interior
(Kuitenen and Helle 1988).
In practice, habitat is often distinctly heterogeneous and both the availability of nest sites
and the location of prey items undoubtedly influence nest site placement. For species, such as
bark gleaners, whose resources are often relatively homogeneously spread throughout the
woodland, interior distributions are predicted. Great spotted woodpecker Dendrocopos major
and nuthatch Sitta europaea, for example, have been shown to have interior distribution patterns
in a British woodland (Fuller 1988). Radio-tracking, such as that undertaken on Great Spotted
Woodpeckers in an Hertfordshire (U.K.) woodland by Smith (1987), may prove an invaluable
technique to follow up this line of research.
Ambiguous/conflicting classifications
There are a number of species which are ambiguous in the way they have been classified
in various studies (Table 1c). For example, both blackcap Sylvia atricapilla and willow warbler
Phylloscopus trochilus were classified as forest species by Gromadzki (1970) and Tomialoj_ et
al. (1981) but Haapenen (1965) classified both as edge species. Ambiguities such as these may
arise due to differences in habitat selection in different parts of the species’ range or due to
differences in vegetation structure in different study areas. The influence of vegetation structure
is considered later.
Interspecific interactions
Ambuel and Temple (1983) suggested that area-dependent changes in biotic interactions
play a part in excluding certain long-distance migrants from small woodlots in North America
(Whitcomb et al. 1981) by effectively lowering reproductive success in small wooded fragments
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compared to larger fragments (Andrén and Angelstam 1988). Increased predation, brood
parasitism and competition results in the loss of bird species from edge habitat due to habitat
fragmentation (Ambuel and Temple 1983, Martin 1981, Brittingham and Temple 1983). For
more detailed discussions of these interactions the reader is referred to recent reviews (e.g.,
Paton 1994, Andrén 1995). These three effects are briefly considered below.
Competition
Competition is an area which has been neglected in studies of edge effects, a reflection
of the inherent difficulties of devising and testing process hypotheses to explain community
patterns in ecology (Wiens 1989). Both, removal (e.g., Garcia 1983) and introduction
experiments (via the use of nest-boxes (e.g., Nilsson et al. 1985)) as well as manipulation of food
supply may be effective although this area will probably remain a minor concern for investigation
compared to other more readily testable factors.
Predation
Predation risk has been identified as a major organizing factor for farmland bird
communities (Suhonen et al. 1994) and nest predation is undoubtedly an important cause of
mortality irrespective of nest-site location with respect to edge (Perrins 1979). Predation has
been implicated as a major factor in habitat selection and in the evolution of life history strategies
of birds as well as being the predominant focus of much edge effect research (Martin 1993a).
In woodland, the risk of nest predation in both natural and artificial nests has been shown to be
high in edges for open and cup nests as well as in natural nest cavities (Sandström 1991). The
relative importance of nest predation in edges depends on a number of factors including
concealment (Møller 1989a), nest density (Nilsson et al. 1985), height above ground (Martin
1993b, Nour et al. 1993) and distance from the edge (Paton 1994).
The reasons why predators and their prey are attracted to edges and the consequences of
their interactions were summarised by Andrén (1995) and Marini et al. (1995). Predation at the
edge may be the result of (1) an incidental function of the preferred use of edges as travelling
lines by predators (Vickery et al. 1992), (2) active searching for prey as a predator response to
the high density of prey found at the edge, (3) higher predator abundance in edges and (4) a more
diverse predator community (Nour et al. 1993, Andrén 1995). For prey, one consequence is an
avoidance of nesting near to the edge although this also depends on nest-type since species with
partially covered are found closer to woodland edges compared to open-nesting species (Møller
1989a).
If raised predation rates in edges are largely a result of passive incidental events then
predation rates should be density independent. Conversely, if predation is the result of active prey
searching behaviour, then predation rates are predicted to be density dependent. Although
subject to criticism (see Andrén 1995, Møller 1989b, Major and Kendal 1996 for discussion) the
provision of artificial nests has already proved an invaluable tool for investigating predation,
especially when combined with techniques for identifying predators. Therefore, the use of
artificial nests to manipulate potential prey density provides a useful technique to distinguish
between active and passive predation. The reader is referred to both Paton (1994) and Andrén
(1995) for more detailed discussion of these issues.
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Brood parasitism
Concern for the effects of brood parasitism have been most acute in North America where
brown-headed cowbirds Molothrus ater, in particular, have been implicated in causing severe
population declines of forest birds including kirtland's warbler Dendroica kirtlandii (Mayfield
1978, Probst 1986), golden-cheeked warbler D. chrysoparia and bell's vireo Vireo belli (Wiens
1989, Askins 1995). Cowbirds have increased in numbers considerably during the period of
European settlement of North America and parasitism has also been shown to be related to
distance from the edge (Gates and Gysel 1978, Brittingham and Temple 1983, Paton 1994).
Observation involving nest searches is the most direct method of quantifying brood
parasitism. This is a time consuming procedure which necessitates a high level of skill in finding
nests and is fraught with potential biases with respect to nest detectability. Behavioural studies
using mounts (Mark and Stutchberry 1994) and the introduction of dummy eggs to nests (Paton
1994) have already proved useful.
Microclimate modification
In modern agricultural landscapes, woodland edges often have abrupt boundaries with
adjacent land use and climatic conditions in woodland edges are intermediate between the
relatively undisturbed environment of the woodland interior and wider climatic fluctuations more
characteristic of canopy openings (Geiger 1936 but, see Chen et al. 1993). A forest canopy
significantly modifies the trunk space climate resulting in diminished net radiation, reduced wind
speed, precipitation and relative humidity; air temperatures near to the ground are reduced
during daytime but heat loss at night is slow, helping to maintain an elevated ambient vapour
pressure and a higher relative humidity (Geiger 1966, Lee 1978). In general, microclimatic
gradients from edges into forest interiors are typified by decreasing wind speed and insolation,
diminished air temperature, increased relative humidity and vapour pressure deficit (Table 2).
Such gradients show a reasonable degree of consistency even between studies carried out in
different climatic regions.
As a rough rule-of-thumb, microclimate modification extends up to three-times the
canopy height in from the forest edge (Fritschen et al. 1971, Harris 1984, McNaughton 1989)
although gradients extending 10 - 60 m into forests have been reported (Table 2). Microclimatic
modification near the edge may also be influenced by: season (Young and Mitchell 1994),
structure of the vegetation (Geiger 1966, Fritschen 1985) especially in terms of successional
status (Matlack 1994), edge aspect in relation to direction of sunlight (Geiger 1966, Matlack
1993, Young and Mitchell 1994), prevailing winds (Lovejoy et al. 1984, Matlack 1994) and
topography (Lee 1978, Giles 1981). Wind has a greater impact where it has an unimpeded fetch
across open country (Peterken 1996).
To my knowledge, there are no published papers on the direct effects of edge
microclimates on breeding birds, hence discussion of its role is somewhat speculative. Hildén
(1965) noted the potential role of shelter in bird habitat selection but suggested that this had been
largely dismissed by previous workers on the assumption that birds are homeothermic and thus
comparatively independent of their physical environments. However, climatic factors have been
implicated in determining the northern limits of wintering bird distribution due to physiological
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limits on metabolic rates (Root 1988) hence it seems reasonable for breeding success to be
affected by microclimatic changes in edges and in fragmented woodlots, especially near
geographic boundaries of species’ distribution ranges.
Temperature has been shown to be a factor influencing the timing of breeding seasons
(Perrins 1965) and ambient air temperature influences both the heat applied by a female Great
Tit Parus major incubating eggs at the nest (Kendeigh 1963) as well as the daily energy
expenditure of a female Great Tit tending nestlings (Tinbergen and Dietz 1994). Prevailing
environmental conditions have also been implicated as a factor influencing the growth of nestling
Great Tits (Gebhardhenrich and Vannoordwijk 1994). Wind stress has also been shown to affect
foraging site competition between Crested Tits Parus cristatus and Willow Tits P. montanus in
Belgium (Lens 1996). Thus, habitat quality for certain bird species may be reduced at the edge
due to the direct effects of temperature and other light-related microclimatic factors.
A second, indirect way in which climate potentially affects forest birds is by its effects
on the timing and predictability of food resources (Saunders 1982). Birds time their breeding to
have young in the nest when food is most abundant (Lack 1954, Perrins and Birkhead 1983),
hence a lack of synchronization between timing of breeding and the larval hatching of
invertebrate food sources leads to reduced fitness, and might go some way to explain the delayed
natal dispersal observed in crested tit P. cristatus populations in small woodlots in Belgium
(Lens and Dhondt 1994). This effect may be due to asynchrony further down the food chain, e.g.,
a corresponding lack of synchronization between invertebrate larval hatching and oak (Quercus
spp.) tree bud burst in small woodlots, as shown by van Dongen et al. (1994). However, although
first leafing dates of oak are strongly linked to spring temperatures, these are mainly correlated
with those months before leaf flush (Sparks and Carey 1995). It therefore seems unlikely that
asynchrony could be directly linked to the effects of a gradient in air temperature across the forest
edge.
In the temperate zone, the effects of microclimatic modification on birds may be revealed
by contrasting north and south-facing edges. The use of single species, even-aged stands would
effectively control for the potentially confounding effects of vegetation structure. The structural
contrast between forest and adjacent land use may be an important influence on microclimate at
the forest edge. Further work is needed to investigate this. If microclimate modification acts
through disruption of phenology, studies need to be undertaken to compare timing of biological
events in small versus large woods.
Vegetation modification
Increased exposure at the edge leads to elevated soil temperatures (Miller 1975, Chen
et al. 1993), drying of soil (Oosting and Kramer 1946, Kapos 1989) and leaf litter (Ranney et
al. 1981, Matlack 1993) and changes to leaf relative water content (Kapos 1989) and plant
water use efficiency (Kapos et al. 1993). Vegetation modification due to habitat fragmentation,
succession and management may change the availability of nest-sites or affect food availability
for birds at the edge. Few studies have tested either of these factors but an understanding of the
successional dynamics in edges is beginning to emerge.
Clearance renders newly-created forest-edges open to climatic extremes with elevated
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tree mortality being reported largely due to uprooting by severe winds (Williams-Linera 1990,
Esseen 1994). In the tropics, canopy and sub-canopy damage has been reported within 150 m
of forest edges with increased leaf-fall and reproductive asynchrony (Lovejoy et al. 1983,
Lovejoy et al. 1984, Laurance 1991) also being reported. Increased insolation favours
germination of pioneer species from the seed bank (Ng 1983) and, within a few years, promotes
the development of adventitious limbs on trees at the edge, a high density of saplings and greater
shrub cover.
Within five years of clearance of tropical forest a dense wall of vegetation develops at the
edge effectively insulating the forest interior from the extremes of edge climates although in
temperate forests the rate of side canopy closure is slower and may not be fully complete even
within half a century (Williams-Linera 1990, Matlack 1994). Edges opened by clearance set
in motion successional dynamics similar to gap-phase regeneration in natural canopy gaps (see
Brokaw 1985 and Hubbell and Foster 1986 for reviews) in which the degree of microclimatic
modification is related to gap size. Most canopy gaps tend to be small (Foster and Boose 1992),
although edges open to climatic extremes will be generated by large magnitude, albeit
infrequent, disturbance events such as hurricanes. Within a few years edges are soon enveloped
except where the process is halted or modified, for example, by land use change and/or by the
invasion by domesticated animals, capable of removing the forest understorey (Howe et al.
1981, Levenson 1981, Whitney and Somerlot 1985), affecting seed predation (Burkey 1993) and
the dynamics of recolonisation (Janzen 1983).
Vegetation effects on bird habitat selection at the edge
Vegetation potentially influences the distribution of birds at the woodland edge in the
provision of nest-sites, food, and song-posts. The effects of vegetation structure on bird species
in woodlands is lucidly demonstrated by studies of coppice management, in which periodic
felling of trees promotes the development of even-aged stands, each having distinctive structural
characteristics. Bird species richness and density in coppice reaches a peak during thicket stage
immediately before canopy closure (Fuller and Henderson 1992) and the densities of bird species
associated with particular structural phases show clear trends which parallel maturity-related
changes in high forest stands (Ferry and Frochot 1970, Moskát and Waliczky 1992).
In Britain, early stages after coppicing are characterised by the presence of tree pipit
Anthus trivialis and whitethroat Sylvia communis; thicket stages are characterised by the
presence of migrants such as willow warbler, chiffchaff Phylloscopus collybita, blackcap,
garden warbler Sylvia borin and nightingale Luscinia megarhynchos, whilst residents, such as
robin Erithacus rubecula, are more abundant after canopy closure and maturation (Fuller 1992).
Depending on location, these may be joined by other species such as dunnock Prunella
modularis and yellowhammer in the establishment phase and blackbird Turdus merula,
chaffinch Fringilla coelebs, blue tit Parus caeruleus and great tit P. major, after canopy closure
(Fuller and Moreton 1987). Species associated with thicket stage coppice are those most often
found at edges close to open farmland (Fuller and Warren 1991). This could be attributed to the
structure of the vegetation at the edge (Fuller and Whittington 1987) and the dependency of such
species on vegetation structure, rather than edge per se, possibly explains the inconsistencies in
edge classification in Table 1c. Variations in vegetation structure between studies leads to a
highly variable classification of ecotone species.
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Whilst the dominant tree species in woodlands may influence bird community
composition (Opdam et al. 1985, Fuller 1992, Fuller 1995), few attempts have been made to
resolve the degree to which birds in temperate forests are influenced by tree specificity or by the
overall structure of woody vegetation (but, see Bersier and Meyer 1994). Specific elements of
the vegetation may be important for some bird species (Blake and Karr 1987, Fuller 1995) hence
associations between particular trees and shrubs with edges deserves further consideration.
However, whilst understorey plant species have been shown to have edge distribution patterns
which correlate closely with light-related environmental gradients, little evidence has emerged
so far that trees show similar patterns except, perhaps, for sapling densities (Brothers 1993,
Matlack 1994, Fraver 1994, Young and Mitchell 1994).
Vegetation structure can be manipulated experimentally by the removal of vegetation
layers (e.g., Slagsvold 1977, Howlett and Stutchbury 1996). Managed coppice woodlands
comprise ideal stands for such experimental manipulation and the use of single species evenaged coppice could control effectively for floristic differences in vegetation. A direct effect of
vegetation structure on bird densities predicts that a removal of vegetation would result in a
decrease in bird density. If vegetation structure is the main determinant controlling distributions
of particular bird species, vegetation in the woodland interior should contain similar
species/densities as that at the edge and vegetation removal should produce similar trends in
both the woodland interior and edge.
Discussion
The boundaries between the four causal factors are artificial in the sense that they may
be difficult to disentangle, for example, the degree to which vegetation dynamics can be
considered in isolation from microclimatic modification (see later). In attempting to test the
primary hypotheses, difficulties may arise through the inability to control for the various
interactions between causal factors. In addition, edge effects may be additive such that a point
in a small woodland might be subject to a cumulatively larger edge effect than a point at the same
distance from the edge in a larger woodland (Malcolm 1994). Notwithstanding these
reservations, the ways in which these primary factors potentially interact may provide insight into
the complexities of edge phenomena.
Secondary interactions
Each of the primary causal factors potentially interact to generate a number of secondary
interaction hypotheses. Each of the major hypotheses may be regarded as primary or secondary
factors. Each of the four primary factors are arranged along the sides of the Table 3. For each
possible interaction the primary factor is along the top whilst the factor which is acted upon is
along the side. In the table itself, the primary hypotheses are arranged along the diagonal, e.g.,
vegetation structure is a primary factor affecting bird species through the provision of nest sites,
food, cover and song-posts. In each case the predicted outcome in terms of bird densities at the
edge is given.
An example of a secondary interaction hypothesis is the influence of habitat structure at
the edge on the magnitude of predation due to its role in nest concealment (Redmond et al. 1982,
Yahner and Wright 1985, Yahner and Cypher 1987, Ratti and Reese 1988), in providing perch
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sites for nest predators (Yahner and Wright 1985, Ratti and Reese 1988, Møller 1989a) and
through its influence on food resources - attracting a higher density of potential prey. Birds
might thus be attracted to the edge but suffer diminished breeding success as a result of
increased predation. Gates and Gysel (1978) termed this an ‘ecological trap’. This hypothesis
predicts that differences in the composition and density of the avifauna in edge versus interior
should reflect differences in food supply. Manipulation of invertebrate abundance by
experimental defaunation could prove to be informative here.
Increased cover at the edge is predicted to affect only visual predators; it should not
inhibit olfactory or auditory predators (such as rodents or reptiles). Support for this prediction
is provided in a review by Clark and Nudds (1991), who considered the importance of
concealment on nesting success. Concealment was by far the most important determinant of
nesting success in cases where birds were the major predators, but nesting success was
considerably diminished in cases where mammalian predators were present.
Nest predation is the main cause of reproductive loss for many bird species and whilst
there may be phylogenetic constraints, birds may attempt to reduce predation through
behavioural mechanisms or through nest-site choice. Götmark et al. (1995) suggested that nestsite selection may represent a trade-off between concealment and the need for visibility. They
found that song thrushes Turdus philomelos did not maximize concealment within trees as
expected but selected intermediate levels of cover. Risk of predation for artificial nests was
found to be inversely related to cover although they were unable to detect similar trends predation
rates for natural nests.
Experimental manipulation of vegetation structure needs to be combined with nest
predation studies such as the use of artificial nests. Assuming a relationship between bird density
and vegetation structure, experimental manipulation of vegetation structure at the edge may be
used to manipulate prey density to test the effects on predation rates. Again, coppice woodlands
would appear to be ideally suited to this task. Thicket stage coppice may be thinned to reduce
the density of vegetation in order to compare the effects of vegetation density, and hence
concealment, on predation rates.
Comparisons of successional pathways, recruitment and vegetative development in
stands contrasting in aspect would be useful to test for the influence of microclimatic gradients
across the edge. Microclimatic modification as a result of coppicing is well established (Ash and
Barkham 1976, Mitchell 1992) although comparative studies of microclimate in coppice panels
at the edge compared to those in the interior would be informative. Vegetation development at
the edge effectively seals the trunk space from external influences and diminishes the edge effect.
Hence, microclimates need to be monitored over the complete cycle of a coppice rotation in
order to establish the feedback between vegetation and microclimate. Again, contrasting studies
between north and south-facing edges may be instructive.
Higher-order interactions
A tertiary hypothesis, involving the interaction of three primary factors was suggested by
Fuller (1990) to explain edge-orientated distribution patterns of birds in coppice woodlands. The
high density of the shrub layer at the external edge may result in increased availability of food
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resources for birds since woodland edges have been shown to contain increased diversity of
arthropods (Helle and Muona 1985, Bedford and Usher 1994). In turn, both of these factors may
be related to a third factor, the increased insolation at the edge. The densities of birds in upland
spruce plantations was shown to be higher within 10 m of edges compared to the interior. This
pattern was indirectly linked to higher productivity at the edge, as measured by counts of fallen
cones (Patterson et al. 1995).
Confounding factors
Social and demographic considerations
The close proximity of thicket stage vegetation was implicated in an edge-related gradient
in migrant songbird density in old coppice by Fuller et al. (1989). Similarly, Patterson et al.
(1995) found that densities of meadow pipits Anthus pratensis and willow warblers were
influenced by the presence of trees in the 0 - 8 year age-class outside plots. This edge effect, like
the one described below for birds in coniferous plantations in Wales, could arise due to
demographic considerations; in years when population levels are high, birds may occupy a wider
range of habitats, including sub-optimal habitats with individuals being forced out of the
preferred habitat into adjacent less-preferred areas (O’Connor and Fuller 1985).
Where woodland edges are contiguous with structurally similar stands, species may nest
near to the edge simply due to a spillover of individuals from nearby preferred habitat.
Individuals in such populations may thus represent sub-dominant members of a larger population
and show body size characteristics consistent with this such as those detected in work on
blackbird populations in small woodlots (Møller 1991, 1995). Density and spatial configuration
of nests are also attributes of populations dynamics and both have been shown to have an
influence on predation rates at the edge (e.g., Hogstad 1995). At a broader scale, population
models may need to incorporate terms for habitat quality (Pulliam 1988, Harrison 1991, Dias
1996).
Landscape context
The structure, constitution and geographical position of a landscape influences the
composition of its species-pool and thus the potential interactions between its constituent bird
species. The presence of particular bird species in a regional species pool is dependent not only
on landscape and habitat structure but also on the history of interactions between species
according to the trajectory of the fragmentation process. In this sense, each landscape is unique
rendering it difficult to devise general laws.
Assuming an edge-zone of set width, a simple model which illustrates the change in the
proportion of edge to interior in relation to the size of individual wooded patches was described
by Levenson (1981). Small patches may be viewed as 'edge' habitat whereas larger patches
contain a disproportionate amount of 'interior'. Thus, as mean patch size decreases due to
habitat fragmentation the balance is tipped over in favour of edge habitat. Within a range of edge
widths between 10 m to 50 m, woodlands over 100 ha in size are predominantly interior (Fig.
2). In England, woodlands of conservation interest over 100 ha in size comprise only 1.9% (by
number) of sites in England and Wales but account for 23.6% of the area. Many remaining
11
fragments lie in the size range 1 - 5 ha (44.0% by number, 9.6% by area) and therefore constitute
'edge' habitat (Spencer and Kirby 1992).
The total forest edge habitat in a landscape depends not only on cover but also on the
grain of the landscape (Gardner et al. 1987, Lavorel et al. 1993). Fragmentation results in an
increase in the amount of edge in a landscape with a maximum occurring when half the landscape
is occupied by forest (Franklin and Forman 1987). However, when two landscapes with the
same coverage of forest are compared, a coarse-grained landscape with a few large forest blocks
will have considerably less edge habitat than a fine-grained landscape where there are many small
forest patches. Thus, differences in the grain of a landscape result in different proportions of
edge to interior for the same amounts of cover (Gustafson and Parker 1992).
The presence of particular predators in forest bird communities is linked to the amount
of forest cover and farmland in a landscape (Andrén 1995). Edge-related increases in predation
are more common in forest-dominated landscapes and changes in landscape composition due
to fragmentation results in a shift in the balance from specialist nest predators of forest (e.g., jay
Garrulus glandarius and raven Corvus corax) to generalist nest predators associated with
agricultural land (e.g., hooded crow C. c. cornix, jackdaw C. monedula and magpie Pica pica)
(Andrén et al. 1985, Angelstam 1986, Andrén 1992, Santos and Telleria 1992).
Edge context
Few investigations have considered the influence of adjacent land uses on bird densities
in woodland. In Wales, Bibby et al. (1985) analysed the effects of adjacent land use on densities
of birds at the edge (measured at 30 - 40 m) compared to densities in the interior (> 40 m) of
coniferous plantations. No significant differences in bird densities were found where plots
abutted broadleaf woodland, grass, heather or clear-felled forest, although bird densities were
significantly lower at the edge than in the interior where plots were adjacent thicket stage
conifers. The authors recognised, however, that with no plots less than 30 m from the edge, the
study was not well designed to further explore this effect further.
Edges of clear-cuts in extensive forested landscapes provide a rather different set of
resources when compared to agricultural landscapes (Huhta et al. 1996). In a study of edges
between mature conifer forest and clearcuts in Sweden, Hansson (1983) found edge effects to be
most pronounced in tree-gleaners, tits Parus spp., willow warbler and chaffinch, which he
attributed to the rich supply of insects found there. A number of species, including
yellowhammer, tree pipit, red-backed shrike Lanius collurio, woodlark Lullula arborea and
whinchat Saxicola rubetra were confined to the clearcuts. The presence of yellowhammer and
tree pipit shows some similarity to the effects of coppicing in lowland Britain, as described
above, although the latter species are not widespread in parts of Britain where coppicing is still
practised. Similarly, the tree-gleaners associated with edges include the same species associated
with thicket stages, and edges, of coppice in Britain. This suggests that in differing contexts, bird
species are reacting in a predictable way according to the localised availability of resources.
It may be useful to distinguish between different types of forest edges, depending upon
whether they are located between or within woodlands, and to take into account differing
12
adjacent land use types. Three main types of edge may be recognised depending upon their
structural characteristics and locality of the interface: (i) the external edges of woodland adjacent
to open country; (ii) internal edges between stands and rides (trackways) or other canopy
openings, and (iii) edges between stands of different ages (Fuller 1990) or tree communities.
Each type differs in the degree the trunk space beneath the canopy is exposed related to the
amount shrub cover present. When compared to internal edges, such as those created by rides or
trackways within woodlands, the external edge has a greater influence on edge-orientated avian
distribution patterns in wooded patches due to the high density of the shrub layer at the edge
(Fuller and Whittington 1987, Fuller 1990).
Conclusions: implications for management
In general, small forest remnants constitute edge habitat which differ from the interior of
larger forests in microclimate (Kapos 1989, Matlack 1993, Young and Mitchell 1994),
vegetation structure and tree species composition (Chen et al. 1992, Fraver 1994, Matlack 1994,
Young and Mitchell 1994). For birds there are also differences in rates of nest predation, brood
parasitism and competition (Martin 1981, Brittingham and Temple 1983, Andrén et al. 1985,
Wilcove 1985). As a consequence, forest fragmentation results in the replacement of specialist
forest bird species, which depend on conditions found in larger patches, by edge species,
ecological generalists, capable of exploiting non-woodland habitats adjacent to small woodlands
(Galli et al. 1976, Ambuel and Temple 1983).
From a conservation perspective the primary goal must be: first, to identify which extant
species are of conservation interest; then to undertake management practices to the benefit (or,
at least, not to the disadvantage) of the target species. The framework presented here provides
a conceptual model which highlights potential secondary and tertiary knock-on effects of habitat
or landscape manipulation. Forest and landscape management prescriptions must not overlook
or over-simplify the potential repercussions arising from the complex array of biotic and abiotic
phenomena encompassed by edge effects.
Where management of edges is bird-orientated it should operate with clear objectives
since management for certain ‘interest groups’ may be directly at variance with others, e.g.,
predators and their prey. In Britain, where only a few woodland bird species attract
conservation concern, species associated with other habitats, e.g., merlin Falco columbarius of
high moorland, have benefitted from the introduction of large-scale forestry in previously nonwooded areas due to the increased opportunities for nesting in addition to availability of prey
along forest edges (Little et al. 1995). Such an increase would be unfortunate if their prey were
also of high conservation interest (Lavers and Haines-Young 1997).
Acknowledgements - I thank Shelley Hinsley, Henrik Andrén, Jeff Ollerton and an anonymous
referee for their constructive comments.
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20
Table 1. Edge- and interior-related groupings of bird species noted in five studies of woodland
bird in Europe grouped in terms of their consistency of classification. See notes below for
explanation of symbols.
21
Species
Study 1
2
3
4
5
a. species with consistent edge preferences in woodlands/forests across studies
Columba palumbus
EF-P Fg
E
Streptopelia turtur
Ea
EF-P Fg
Upupa epops
EF-M Fg
Lanius collurio
Ec
EF-M
d
Sylvia curruca
E
E
EF-M
Emberiza citrinella
E
Ec,d
EF-M Fg
E
Passer montanus
E
EF-M
E
c
g
Sturnus vulgaris
h
E
EF-P F
E
Carduelis (Chloris) chloris
E
EF-M Fg
E
g
C. carduelis
E
EF-M F
E
Pica pica
Ea
E
Corvus monedula
h
Ec,e
C. corone cornix
E
E
EF-P
b. species with consistent interior distribution patterns in woodlands/forests across studies
Dendrocopos major
F
F
F
L
b
D. minor
F
F
L
Phoenicurus phoenicurus
Fb
F
F
Turdus philomelos
F
F
L
Parus palustris
F
F
F
L
P. montanus
F
F
P. cristatus
F
F
P. ater
F
F
P. caeruleus
F
F
F
Phylloscopus collybita
F
F
L
b
P. sibilatrix
F
Ficedula hypoleuca
Fb
F
Sitta europaea
F
F
F
L
Certhia familiaris
F
Ff
F
b
Loxia curvirostra
F
Oriolus oriolus
F
F
c. species with conflicting interior and edge classifications.
Athene noctua
T
F
Cuculus canorus
Ea
F
Jynx torquilla
E
F
Fg
b
Anthus trivialis
EF-M F
c
Hippolais icterina
E
EF-P F
Sylvia borin
Ec
EF-P F
c
S. atricapilla
E
F
F
L
S. communis
Ec,d
EF-M F
L
22
Muscicapa striata
P. major
Phylloscopus trochilus
Turdus merula
Fringilla coelebs
Pyrrhula pyrrhula
Emberiza hortulana
Garrulus glandarius
Fb
F
E
c,e
E
Ed
EF-P
Ec
Ee
EF-M
F
F
F
EF-P
F
F
F
F
F
F
F
F
L
E
E
L
Notes
1.
Lack and Venables (1939) E = edge-species, F = hole-nesting species, 'truly woodland birds' (Lack and
2.
3.
4.
5.
Venables, p.52), h = hole-nesting species which regularly nest outside woodlands, T = species nesting in
trees and feeding outside woods, a found in woods but primarily birds of open country, b characteristic of
'open' woods.
Haapenen (1965) E = edge-species, c main distribution in Finnish forests in cultivated areas, d also found
in open and closed brush or pine seedling stands, e also found in high coniferous stands
Gromadzki (1970) EF-M = forest-margin species, occurring only at the edge of the forest, EF-P = forestperiphery species, 'occurring deep in the forest as well as on its periphery' (Gromadzki, p. 14), F = forest
species, 'living deep in the forest in preference to forest periphery, but not avoiding the latter' (Gromadzki,
p. 14), f refers to C. brachydactyla
Tomialoj_ et al. (1981) F = species breeding in forest in Bialowieza National Park, g = species foraging
outside of the forest.
Taylor et al. (1987) E = species found predominantly in small woodlands (<2.0 ha in size), L = species
found in predominantly in a larger woodland (39.4 ha).
23
24
Table 2. Empirical evidence for microclimate modification at the forest edge. Where stated, the maximum widths of
microclimate modification are given.
________________________________________________________________________________________________
Microclimate
Max. width of
Max. width of
Gradient
Temperate forest
gradient (m)
Tropical forest gradient (m)
________________________________________________________________________________________________
Air temperature
Miller 1975
60
Kapos 1989
40
Chen et al. 1993
Williams-Linera 1990
Matlack 1993
24
Young and Mitchell 1994
30 - 50
Photosynthetically active
Young and Mitchell 1994
10 - 20
Kapos 1989
20
radiation (PAR)
Light intensity
Chen et al. 1993
Matlack 1993
44
Litter moisture
Ranney et al. 1981
Matlack 1993
50
Plant water use
Kapos et al. 1993
efficiency
Relative humidity
Miller 1975
Kapos 1989
Chen et al. 1993
Matlack 1993
50
Soil moisture
Oosting and Kramer 1946
Kapos 1989
Soil temperature
Chen et al. 1993
Vapour pressure deficit
Miller 1975
60
Kapos 1989
20
Matlack 1993
50
Young and Mitchell 1994
20 - 50
Water vapour pressure
Kapos 1989
Wind speed
Fritschen 1985
Chen et al. 1993
________________________________________________________________________________________________
25
Table 3. Primary and secondary interaction hypotheses. The table is arranged with the causative factors along the horizontal and factors which are
acted upon along the vertical. The items along the diagonal form the primary hypotheses whilst the remaining items are the secondary interaction
hypotheses. The predicted bird densities near the edges are indicated for each hypothesis: negative (-ve); postive (+ve). See text for further
explanation.
___________________________________________________________________________________________
PRIMARY FACTORS
_______________________________________________________________________________
Patch use in
Interspecific
Microclimatic
Vegetation
relation to location interactions
modification
structure
of resources
___________________________________________________________________________________________
Patch use
in relation
to location
of resources
HYPOTHESIS I
Interior (-ve) and
edge species (+ve)
Interspecific
interactions
--
Edge avoidance
due to threat of
predation (-ve)
--
HYPOTHESIS II
(-ve)
--
SECONDARY
FACTORS
--
Ecological
trap hypothesis
(+ve)
Microclimate
modification
--
Vegetation
Structure
--
26
--
--
HYPOTHESIS III
i.) direct effects (-ve)
ii.) indirect effects (-ve)
Effects of vegetation
structure on microclimate
Effects of microclimate modification
on vegetation structure
HYPOTHESIS IV
Availability of:
ii.) Song-posts (+ve)
iii.) nest-sites (+ve)
iii.) food (+ve)
___________________________________________________________________________________________
27
Fig. 1. The effects of nest-site placement, along a radius of a circular territory, on the mean
distance travelled (± 1 S.D.) to 100,000 random foraging points. Mean travel costs incurred
increase significantly with nest site placement from the territory centre (F= 4.4E+4, P<<0.001).
Fig. 2. The effect of edge width on the proportion of interior (left vertical axis) to edge (right
vertical axis) area in square woodlands across a range of sizes not atypical of fragmented
woodlands in Britain.
28
Fig. 2
120
100
% interior
80
20 m
60
50 m
100 m
40
20
0
0.01
0.1
1
10
100
Area (ha)
29
1000
10000