Download Consequences of forest fragmentation for the dynamics of bird

Biological Journal q f f h Linnean .So&&
(1991), 42: 149-163. With 3 figures
Consequences of forest fragmentation for the
dynamics of bird populations: conceptual
issues and the evidence
J 0 R U N D ROLSTAD
Norwegian Forest Research Institute, P. 0 . Box 61, N-1432 ~ s - N L HNorway
,
'I'his paper reviews the consequences of forest fragmentation for the dynamirs of bird populations.
Owing to high mobility and large home ranges, birds usually perceive fragmented forests in a finegrained manlier, i.e. embrare several forest fragments in funrtional home ranges. O n a regional
scale, however, coarse-grained clusters of fine-grained fragments (hierarchical fragmentation may
sub-divide bird populations into isolated demes, which enter a domain of metapopulation dynamics.
Distinctions are made between pure distance-area or population-level effects and more indirect
community-level effects due to changes in landscape composition. Distance-area effects, such as
insularization and decreasing fragment size, directly prevent dispersal and reduce population size.
Landscape effects, surh as reduced fragment-matrix and interior-edge ratios, increase the pressure
from surrounding predators, competitors, parasites and disease. In short, forest fragmentation can be
viewed as a two-step proress. Initially, fine-grained fragmentation triggers distance-area and
landscapr effects on a local scale, which in turn, results in a range retraction of a population to nonfragmmted or less fragmrnted parts of a region. At a certain point, non-fragmented areas heromr so
widely spaced out that regional distance-area effects come into operation, giving rise to
metapopulation dynamics. Although few bird metapopulations have yet been documented,
metapopulation dynamics probably is a rommon chararteristic of bird populations confined to
'hierarchical' fragmented forests.
KEY WORDS-Forest fragmentation
dynamics - conservation biology.
-
bird populations
-
spatial scale
-
metapopulation
CONI'ENTS
Introdurtion . . . . . . . . . . . . . . . . . . .
Conceptual issues . . . . . . . . . . . . . . . . . .
Fine-grained versus coarse-grained patterns: the role of 'hierarchical' fragmentation .
Local versus regional scale: the role of 'mainland' populations . . . . . .
Consequences for population dynamics . . . . . . . . . . . . .
Distance-area or population-level effects . . . . . . . . . . .
Landscape or community-level effects . . . . . . . . . . . .
Evidence for bird metapopulations . . . . . . . . . . . . . .
Acknowlrdgements
. . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . .
References
Appendix . . . . . . . . . . . . . . . . . . . .
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INTRODUCI'ION
Fragmentation of continuous, natural landscapes is one of the most important
factors contributing to the ever-increasing loss of biological diversity (Wilcox &
Murphy, 1985). Sub-division and loss of suitable habitat pushes populations into
a size range where stochastic events are likely to terminate them (Gilpin &
Soult, 1986). Not surprisingly, the consequences of habitat fragmentation have
become a key issue in conservation biology (e.g. Norse et al., 1986; Soul6, 1986).
A fragment is defined as a detached, isolated or incomplete part broken away
OO'LC4066/91/010149+ 15 $03.00/0
149
0 1991 The Linncan Society of London
150
J. ROLSTAD
from a whole. Habitat fragmentation, therefore, is the sub-division of a certain
habitat into several isolated patches. However, whereas it is perfectly possible to
crush a mirror into frngments with the same total area, this is impossible with
natural habitats. Fragmentation of a particular habitat inevitably implies a
reduction of the total area of this habitat and a simultaneous increase in the
areas of other habitats. (Note, however, that the opposite, to reduce a habitat
area without fragmenting it, is fully possible.) Hence, habitat fragmentation
implies both sub-division and loss of the habitat and a corresponding increase in
other habitats. In this respect, organisms confined to fragmented habitats have
to cope with two basic problems: (1) are the remaining fragments large enough
and close enough to each other to provide living space and opportunities for
dispersal and (2) what is the impact of the surroundings, the external threat
(Janzen, 1986)? Man-made habitats support predators, competitors, parasites
and diseases which can spread into the fragments and interact with their
inhabitants.
Habitat fragmentation is a pervasive phenomenon at all levels of spatial scales
and it applies to all living organisms (Lord & Norton, 1990). In this paper I
restrict myself to one taxonomic group confined to one type of habitat; birds
(Aves) in forest habitats. My objectives are: first, to present some conceptual
issues related to spatial scale; second, to give an overview of the consequences of
forest fragmentation for the dynamics of bird populations, and finally, to
evaluate whether birds are likely to possess metapopulation dynamics in
fragmented forests.
Bird populations in forest ecosystems provide good opportunities to study the
consequences of habitat fragmentation for two reasons: first, birds are easy to
observe and their habitat affinities are mostly well known; second, the ongoing
fragmentation of forest ecosystems can be viewed as large-scale natural
experiments (Myers, 1988). Forest areas that covered approximately onequarter of the Earth’s land area in 1960 may be halved by 2020 (Council of
Environmental Quality, 1980: 117). As a taxonomic group, birds are among the
most mobile organisms, which places them in a particular position regarding
fragmentation. On the one hand they may be particularly sensitive because they
have large home ranges and hence are area-demanding. On the other hand the
capability to fly make them able to cope with alien habitats much more easily
than most other taxonomic groups.
CONCEPTUAL ISSUES
Fragmentation has become more or less of a buzzword in conservation
biology, and the term has often been used uncritically to describe all types of
man-made habitat alteration. One of the conceptual problems is how to
distinguish between habitat loss and fragmentation, as habitat fragmentation
also gives rise to a concurrent habitat loss. It should be clear that the primary
impact of fragmentation is through loss of habitat continuity, i.e. insularization.
This is because it is possible to reduce a habitat tract without disturbing its
continuity; that is, habitat loss per se. Nevertheless, habitat loss is an important
by-product of fragmentation, and in this perspective should be taken into
account.
This habitat fragmentation should be viewed as a complex process invol\,irig
many interrelated components, including fragment size, habitat heterogeneity
BIRD POPULATIONS IN FRAGMENTED FORESTS
151
'I'ABLE 1. Different components of h a b i t a t fragmentation and examples of how they m a y influence
population dynamics.
Main romponent
Habitat change
Distance-area or
Reduced connectivity, insularization,
population-level effects increased interfragment distance
Reduced fragment size, reduced total area
Landscape or
Reduced interior-edge ratio
community-level effects
Consequences for population
dynamics
Directly affecting dispersal, reduces
the immigration rate
Directly affecting population size,
increases the extinction rate
Indirectly affecting mortality and production through increased pressure
from predators, competitors, parasites
and disease
Reduced fragment-matrix ratio
Reduced habitat heterogeneity within
fragments
Indirectly affecting population size
through reduced carrying capacity
within the fragment
Increased habitat heterogeneity in
surrounding matrix
Indirectly affecting mortality and production through increased carrying
capacity of predators, competitors,
etc. in the surrounding matrix
Loss of keystone species from the habitat
Indirect effect through disruption of
mutualistic guilds or food webs
within fragments, surrounding habitat (matrix) and edge-effects (Wilcox &
Murphy, 1985; Wilcove, McLellan & Dobson, 1986). In this paper I use habitat
fragmentation as a collective term to describe a wide range of habitat alterations,
all of which are the consequences of the initial sub-division of a continuous
habitat (Table 1).
Another conceptual problem with habitat fragmentation is related to spatial
scale. In the next two sections I will address two dichotomies (1); between finegrained and coarse-grained mosaic patterns (e.g. Pielou, 1974: 149), and (2)
between local and regional scale.
Fine-grained versus coarse-grained patterns: the role of 'hierarchical' fragmentation
It is convenient to define fine-grained fragmentation as the subdivision of a
habitat into patches smaller than the individual territories or home ranges of a n
organism (cf. Rolstad & Wegge, 1989a, and references therein). If the habitat
patches are larger than a single territory, and especially if each patch contains
several individuals, the habitat is fragmented in a coarse-grained pattern.
Depending on the degree of isolation, populations in coarse-grained landscapes
are divided into more or less independent local populations, which may give rise
to metapopulation dynamics. Although not explicitly stated, many island
biogeography studies of bird populations assume coarse-grained fragmentation
(e.g. Fritz, 1979; Wilcove et al., 1986: 246; Van Dorp & Opdam, 1987; Moller,
1987).
Relative to many other taxa, birds have large territories. Tetrao urogallus in
south-central Scandinavia has spring ranges of 10-50 ha, and seasonal
movements up to 10 km (Wegge & Rolstad, 1986; Rolstad, Wegge & Larsen,
1988). Small, medium-sized, and large European woodpeckers have feeding
J. ROLSTAD
152
A
B
C
Figure 1. Three different fragmentation patterns. A, Fine-grained. B, Coarse-grained and C,
Hierarchical. Metapopulation dynamics may occur in both the coarse-grained and the hierarchical
mosaic pattern. The hierarchical pattern probably is the most common type offorest fragmentation.
territories of 20-30, 120-250 and 250-3000 ha, respectively (Blume, 1962;
Petterson, 1984). Scandinavian Accipiler gentilis uses ranges of about 3000 ha
during the breeding season (Widen, 1985a), and Thiollay & Meyburg (1988)
report raptor ranges on Java between 500 and 5000 ha. Some passerine birds
also have large territories, e.g. 15 ha in Parus cinctus (Virkkala, 1987), 20 ha in
Certhia familiaris (Kuitunen & Helle, 1988), and as much as 300 ha in Helmilheros
vermivorus (Robbins, 1979; Hayden, Faaborg & Clawson, 1985). These
observations show that forest fragments of a few hectares or even tens of hectares
(e.g. Galli, Leck & Forman, 1976; MacClintock, Whitcomb & Whitcomb, 1977)
do not contain local populations. Rather, such fragments are patches in a finegrained forest archipelago, which supports an ensemble of bird territories (cf.
Haila, 1986; Haila, Hanski & Raivio, 1987).
Although from a bird's viewpoint forests are seldom fragmented in a coarsegrained manner this does not necessarily mean that metapopulation dynamics is
a rare phenomenon among birds. This is because the grain concept can be
extended to a hierarchical system of patch mosaics (Fig. 1). Assuming that, on a
local scale, small forest patches are clustered so that individual birds may easily
move between them, a local population may then persist in this fine-grained
archipelago of fragments. If this archipelago is surrounded by an area of widely
dispersed fragments, small enough and far enough apart to prevent
establishment of territories, then, on a regional scale, a fragment cluster may
function as one large 'patch' containing a local bird population (Fig. 1C). A
landscape with several fragment clusters, embedded in a matrix containing
scattered fragments, may function as a coarse-grained regional mosaic of finegrained local mosaics. Such a 'hierarchical' system of fragments is probably a
common feature of many forest landscapes, and may give rise to a
metapopulation structure in the same way as 'true' coarse-grained mosaics
(Fig. 1B). Strix occidentalis caurina, in the Pacific Northwest of North America,
appears to have such a metapopulation structure in a hierarchy of old-growth
fragments (Forsman, Meslow & Wight, 1984; Shaffer, 1985; Gilpin, 1987).
Local versus regional scale: the role of 'mainland' populations
Extinction of local populations is often preceded by isolation from large
regional populations. This is illustrated in Pettersson's (1985) study of the last
BIRD POPULATIONS IN FRAGMENTED FORESI’S
I53
Swedish population of Dendrocopus medius, which became extinct in 1983. O n a
regional scale this population became isolated from the central European
‘mainland’ population during the first half of this century. Isolation eliminated
the possibility of recolonization. O n a local scale, reduced fecundity due to a
skewed sex-ratio and inbreeding depression, and elevated mortality due to severe
winter conditions probably contributed to the final extirpation. Similarly, antfollowing birds at the Barro Colorado Island became isolated from the mainland
population during the building of the Panama Canal in 1910-14. By 1974 more
than 40 species had become extinct (Willis, 1974) due to elevated nest predation
(Loiselle & Hoppes, 1983) and loss of microhabitats essential for reproduction
and year-round survival (Willis, 1974; Karr, 1982a, b). Recent reintroductions
(Morton, 1978) have shown that some of the species, e.g. Cyphorhinus arada would
probably have persisted if the island had received a few mainland immigrants.
Thus, the persistence of local populations depends on colonizers from large
‘mainland’ populations. What constitutes a ‘mainland’ for forest birds? How
large must forest tracts be to ensure viable populations with the ability to rescue
(sensu Brown & Kodric-Brown, 1977) isolated subpopulations? Long-term bird
censuses have shed some light on this question. In northern Finland, an isolated
100-ha spruce forest did not retain its original bird assemblage after 60 years of
fragmentation in the surrounding forests (Vaisanen,Jarvinen & Rauhala, 1986).
Helle (1986) reported that even a 7000-ha reserve of virgin forest did not remain
unaffected by the surrounding fragmentation. In northern Finland, old forest
tracts must apparently exceed 20 000 ha to sustain viable populations of resident
passerine birds (Virkkala, 1987).
None of the aforementioned studies evaluated non-passerines in detail. Forest
grouse have decreased markedly in northern Finland during recent decades
(Lindkn & Rajala, 1981), more than expected from the reduction in virgin forest
area (Jarvinen, Kuusela & Vaisanen, 1977; Official Statistics of Finland, 1987).
This is probably because large intact forest areas have been fragmented and no
longer support ‘mainland’ populations. Based on birds’ home ranges, seasonal
movements and demography (e.g. W i d h , 198513; Wegge & Larsen, 1987),
isolated boreal forest tracts should probably exceed hundreds of thousands or
even millions of hectares to ensure long-term viable populations of nonpasserines. Although most of these species presumably tolerate a certain degree
of fine-grained fragmentation within their range, many forest regions in
Fennoscandia may be too fragmented, both at the local and the regional scale
(Punkari, 1984a, b), to serve as population ‘mainlands’.
North American studies also emphasize regional processes in the dynamics of
local bird populations. A 23-ha mixed forest in Connecticut lost five old-forest
species from 1953 to 1976, although forest area and habitat composition
remained fairly constant during that period (Butcher el al., 1981). The
disappearance of forest-interior species was most probably due to fragmentation
in the surrounding area. Whitcomb, Whitcomb & Bystrak (1977) reported local
extinctions of two forest-interior species, Mniolilta varia and Helmitheros vermivorus,
from a 16-ha virgin forest in Maryland which had remained undisturbed for 28
years. Regional range retraction, due to fragmentation in the surrounding areas,
was the most likely explanation of the extinctions.
When interpreting the consequences of forest fragmentation it becomes crucial
to take into account the scale. O n a local scale individual birds and the
154
J. ROLSTAD
minimum area requirement for territories is the key issue. O n a regional scale, a
continuous population may retreat to less fragmented parts of a region and
become patchily distributed. At a certain point, between-population movements
become constrained by a hostile intervening matrix, which in turn results in
complete loss and isolation of local populations. The population may enter a
domain of metapopulation dynamics and, if the isolating process continues,
finally becomes extinct.
CONSEQUENCES FOR POPULATION DYNAMICS
Habitat fragmentation affects populations in two different ways. Sub-division
of continuous habitat increases insularization and reduces the total area. This
‘distance-area’ component of the fragmentation influences dispersal and
population size directly by reducing the immigration rate and increasing the
extinction rate; thus, it can be considered as population-level phenomenon
(Wilcox & Murphy, 1985) (Table 1 ) . However, reduced fragment area and
changed configuration inevitably also affect the habitat composition both within
fragments and in the surrounding matrix (Franklin & Forman, 1987). This
‘landscape’ component of the fragmentation includes fragment-matrix and
interior-edge ratios as well as habitat heterogeneity both within fragments and in
the surroundings (Forman & Godron, 1986). This indirectly influences mortality
and production through increased pressure from the surroundings, be it biotic
factors such as predators, competitors and parasites, or abiotic edge-effects such
as changed wind and light conditions. These effects go beyond the sole
population level, and can be considered as community-level phenomenon
(Wilcox & Murphy, 1985).
Distance-area or population-level efects
A local population may become extinct if fragment size decreases below a
threshold value set by the minimum territory size of the species. Hayden el al.
(1985) list estimates of minimum area requirements for Missouri birds, based on
the smallest fragment in which the species occurred (1.2-341 ha), and the size
class of forest fragments in which the species reached 100% occurrence
(42-340 ha). Whitcomb et al. (1981) compared minimum fragment size with
minimum territory size in 12 New Jersey piedmont birds. They found a
relatively good agreement except for two species, Mniotilta varia and Seiurus
aurocapillus, in which minimum fragment sizes for presence were 6 and 13 times
greater, respectively, than the size of defended territories. In general, forestfragment size is the best single predictor of species number, probability of
occurrence and population density of forest-interior or forest-dependent species
(Forman, Galli & Leck, 1976; Ambuel & Temple, 1983; Howe, 1984; Lynch &
Whigham, 1984; Blake & Karr, 1984, 1987; Opdam, Rijsdijk & Hustings, 1985;
Freemark & Merriam, 1986; Van Dorp & Opdam, 1987; Mdler, 1987; Askins,
Philbrick & Sugeno, 1987). However, fragment size is often correlated with other
factors which may not be taken into account.
Many birds combine several discrete patches into a functional home range.
Tetrao urogallus males in south-central Scandinavia responded to experimental
fragmentation of their territories by increasing the territory size to include
BIRD POPULATIONS IN FRAGMENTED FORESTS
155
several patches of old forest (Rolstad & Wegge, 198913). However, following the
death of these males, the fragmented areas were not recolonized by young males.
Whitcomb et al. (1981), using Breeding Bird Atlas data from Maryland, noted
that three forest-interior species, Dryocopus pileatus, Picoides villosus and Sitta
carolinensis, were fairly common, although the forest landscape was severely
fragmented. These species have large territories and presumably combine several
forest fragments as living space.
At a regional scale, forest fragmentation, be it true coarse-grained or
hierarchical, may delimit local populations. T o assess minimum area
requirements for territorial birds at this scale a simple multiplication of the
minimum territory size with the minimum size of viable populations may suffice.
However, many birds have elaborate social organizations and mating systems.
For instance, old-forest patches of 100 ha seem necessary to ensure persistence of
Fennoscandian Tetrao uroguallus lek populations (Angelstam, 1983; Rolstad &
Wegge, 1987). Other species, such as Seiurus aurocapillus and Wilsonia citrina, tend
to occur in loose colonies (Hann, 1937; Whitcomb et al., 1975), and minimum
areas of continuous forest necessary to sustain viable populations have been
estimated at around 100 ha (Robbins, 1980). Dendrocopus borealis live in social
groups called ‘clans’ (Jackson & Thompson, 1971), with home ranges of 80 ha
or more (Skorupa & McFarlane, 1976). A clan may include up to ten
individuals, but there is never more than one breeding pair per clan (Jackson,
1978). Many jay species have colonial nesting habits or elaborate cooperative
breeding systems (e.g. Brown, 1970, 1974), which increase considerably the
minimum area requirements.
In the same way that individuals can combine several fragments in functional
territories, a regional population may persist as local sub-populations connected
by dispersal, i.e. a metapopulation. The question is how far apart can forest
fragments (in coarse-grained mosaics) or clusters of small fragments (in
hierarchical mosaics) be to still support a viable metapopulation? Critical
parameters here are natal dispersal distance and juvenile mortality. In Strix
occidentalis caurina the juvenile dispersal distance averaged 30 km (Thomas et al.,
1990, and references therein). Tetrao urogallus juvenile females disperse 2-25 km,
whereas males disperse only 0.5-5 km (Koivisto, 1963). Juvenile mortality is
high in both species, estimated at 0.82 in S. occidentalis caurina.
Evidence for isolation effects is suggested by Opdam and coworkers, who
reported that interpatch distance, patch density and the density of the corridor
network influenced the probability of occurrence of old deciduous forest species
like Sitta europaea, Parus palustris and Picus viridis in severely fragmented areas of
The Netherlands (Opdam, Van Dorp & ter Braak, 1984; Van Dorp & Opdam,
1987). Jarvinen & Haila (1984) also attributed isolation effects to explain why
the same assembly of species were absent from the Aland archipelago in southern
Finland. These birds are all characterized by strong side-tenacity and low
dispersal capacity.
Landscape or community-level efects
Forest fragmentation creates non-forest habitat. This habitat, be it clearcuts,
agricultural fields or urban areas, may increase the carrying capacity of
generalist predators, open field competitors or nest parasites that may interact
av.Lsiox .f
9s 1
BIRD POPULAl’IONS IN FRAGMENTED FORESTS
157
A well-known example of edge effect relates to brood parasitism in North
American forest songbirds. The endangered Dendroica kirtlandii, which nests in
the jack pine barrens in Michigan, was nearly extinct in the late 1960s. This was
most probably due to high nest parasitism by Molothrus ater, which inhabits the
edges and open habitats in fragmented forests (Mayfield, 1978, and references
therein; Bri ttingham & Temple, 1983). Similarly, parasitism from M . bonariensis
has been identified as a primary cause of the decline of Agelaius xanthomus (Post &
Wiley, 1976).
In hole-nesting birds inhabiting forest interior, competition for suitable nest
holes with edge species may be detrimental. Interference from Sturnus vulgaris
may have excluded Dendrocopus medius in certain European habitats (Buhler,
1976). In North America, woodpeckers such as Melanerpes carolinus and
M . eythrocephalus may exclude less aggressive woodpeckers such as Dendrocopus
borealis (Jackson, 1978).
The absence of bird species from small forest fragments is often attributed to
reduced habitat heterogeneity (e.g. Boecklen, 1986). Because most species
require two or more habitat types (Root, 1967), fragmentation may restrict
between-habitat movements. Seiurus motacilla of eastern North America, which
nests and forages near open water, is seldom encountered in forest fragments
without these microhabitats (Wilcove et al., 1986). Willis (1974) and Karr
(1982a) attributed several extinctions of landbirds on the Barro Colorado Island,
Panama, to this mechanism. Johnson (1975), in a North American study of
boreal birds on mountaintops, explained 91yo of the variation in total number of
bird species by an index of habitat heterogeneity.
Secondary extinctions or ripple effects, due to loss of keystone species,
indirectly affect populations through disruption of mutualistic relationships or
food webs (Wilcox & Murphy, 1985; Wilcove el al., 1986). This is the case for the
ant-following birds in tropical rain forests. O n the Barro Colorado Island these
birds suffer from a high nest loss. Terborgh & Winter (1980) suggested that this
is due to the extinction of top predators which controlled the populations of
smaller mamnialian predators. After the island was isolated, small and mediumsized nest-predators have become remarkably abundant, a process called
‘mesopredator release’ (Soult et al., 1988). Experimental studies in the
Amazonian rain forest (Lovejoy, 1980; Lovejoy et al., 1986) have shown that a
100-ha fragment did not support ant-following birds, probably due to the
extirpation of their food source, ant-colonies. Ecosystem decay (Lovejoy et al.,
1984), ecosystem degeneration (Kushlan, 1979) and faunal collapse (Soult,
Wilcox & Holtby, 1979) are all appropriate terms for such community-level
effects of fragmentation.
In conclusion, both ‘distance-area’ and ‘landscape’ effects have been invoked
as explanations of spatial and temporal changes in bird populations in
fragmented forests. Whereas landscape effects, such as nest predation and
parasitism, are well documented, distance-area effects are often suggested by
indirect evidence only. The reason for this may be found in the high mobility of
birds; i.e. that forests are usually fragmented in a fine-grained pattern.
Unfortunately, very little is known about critical dispersal distances and
sufficient dispersal rates for birds in fragmented forests; that is, population-level
phenomenon at the regional scale.
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J. ROLSTAD
EVIDENCE FOR BIRD ME’I‘APOPULAI’IONS
The only explicit attempt to evaluate the impact of forest fragmentation on
birds, on the whole range of spatial scales, is found in the development of a
conservation strategy for Strix occidentalis caurina, in the Pacific Northwest of
North America (Gutitrrez & Carey, 1985; Thomas et al., 1990, and references
therein). Owing to a strong affinity to old-growth forest (Forsman, Meslow &
Strub, 1977), patches of old-growth surrounded by second-growth forest are
perceived as habitat fragments of the owl. However, relative to the home range
size, averaging 100-200 ha, the old-growth forest is fragmented in a fine-grained
pattern by 10-20-ha clearcuts at the local scale. Individual pairs are apparently
able to live in areas with as little as 20-30y0 of old-growth because the
archipelago of old-growth patches serves as living space. Regional removal and
fragmentation of old-growth habitat, however, render large areas unsuitable for
the owl. On a regional scale, the owl population presumably is sub-divided into
more or less isolated local demes with little or no exchange of individuals
(Gutitrrez & Carey, 1985).
To ensure a viable owl population and to lessen the negative effects of
fragmentation, the conservation strategy is to distribute suitable habitat across
the landscape: so-called ‘Habitat Conservation Areas’ (HCAs). These should be
large enough to reduce demographic stochasticity, and spaced closely enough to
facilitate dispersal of owls among them. A recent strategy (Thomas et al., 1990)
advocates a network of HCAs, each capable of containing up to 50 pairs
(6-700 km‘), spaced out a maximum of 20 km apart, which theoretically ensures
that c. two-thirds of dispersing juveniles will reach neighbouring HCAs. Thus,
the plan incorporates three key considerations: ( 1) to provide multiple, extensive
and continuous areas of suitable habitat; (2) to distribute these areas across the
landscape at distances that encourage demographic interaction among them,
and (3) to provide adequate connectivity in the form of surrounding landscape
features to facilitate that demographic interaction. This design feature should
ensure the persistence of a metapopulation through interacting demographic
units.
Fritz ( 1979, 1985) documents metapopulation dynamics in a Dendragupus
canadensis population in the Adirondack Mountains of New York. Living near
the southern periphery of the species’ boreal range, the metapopulation was
distributed within an archipelago of 32 conifer forest patches embedded in
deciduous forest. Patch size varied from 20 to 591 ha, and 25 (78%) of the
patches were occupied during the 2-year study. The pattern of occupancy was
explained in terms of local extinctions due to demographic stochasticity and
recolonizations from nearby patches. Whereas 95% of the patches larger than
100 ha were occupied, the smaller ones experienced only 60% occupancy.
Furthermore, 92% of the patches within 10 km from a colonizing source were
occupied, compared with only 30% for the more distant ones. Although Fritz
(1979, 1985) demonstrated a clear distance-area effect, he did not explain how
far the metapopulation was situated from the ‘mainland’ range in boreal
Canada, neither did he provide data on migration distances. Thus, the question
of whether the entire metapopulation was maintained by immigrants from the
boreal source population remains unanswered.
Forest-dwelling raptors on the island of Java appear to exhibit a
BIRD POPULATIONS IN FRAGMENTED FORESTS
159
Figure 3. The distribution range of Tetra0 urogallus in central Europe strongly suggests a rnetapopulation structure. Redrawn from Klaus el al. (1986).
metapopulation structure as a result of coarse-grained fragmentation of the
tropical rain forest (Thiollay & Meyburg, 1988). Forest reserves range in size
from 15000 to 76000 ha and they are spaced at 13 to 670 km intervals. The
total population of the endemic Spizaetus bartelsi is only about 30-36 pairs,
distributed in three forest reserves. If the local populations become small enough
to be strongly influenced by demographic stochasticity, the whole
metapopulation may become extinct.
A Palaearctic candidate for metapopulation dynamics is Tetrao urogallus, a
forest grouse associated with the older stages of boreal forests (Rolstad & Wegge,
1989a). In central Europe T. urogallus has been extirpated from a major part of
its original range (e.g. Muller, 1982). Key factors involved in the population
decline are elevated nest predation and chick mortality due to fragmentation on
a local scale. The present distribution of T. urogallus in central Europe strongly
suggests that the regional population is distributed as a metapopulation (Fig. 3).
Although many forest ecosystems are severely fragmented, few studies have
documented metapopulation dynamics in forest-dwelling birds. This is probably
because most studies have been conducted on a local scale where the
fragmentation pattern usually appears as fine-grained relative to birds’ home
ranges. However, considering the hierarchical pattern of fragmentation, many
bird populations may experience metapopulation dynamics. To fill in the gaps in
our knowledge, future studies urgently should incorporate population-level
phenomena on the regional scale.
ACKNOWLEDGEMENTS
M. E. Gilpin, I. Gjerde, I. Hanski, 0.Jarvinen and an anonymous referee
provided helpful comments on the manuscript.
I60
J. ROLSTAD
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APPENDIX
Common names of species mentioned in the lext
Birds:
Accipiter gentilis
Agelaius xanthomw
Certhia familiaris
Cyphorhinus arada
Dendragapus canadensis
Dcndrocopus borealis
Dendrocopus medius
Dcndroica kirtlandii
Diyocopus pileatus
Helmitheros uermiuorus
Melanerpes carolinus
Melanerpes erythrocephalus
Mniotilta uaria
Molothrus ater
Molothrus bonariensis
Parus cinctus
Parw palwtris
Picoides villosus
Picus uiridis
Seiurw aurocapillus
Seiurus motacilla
Sitta carolinensis
Sitta europaea
Spizaetus bartelsi
Strix occidentalis caurina
Sturnus uulgaris
Tetra0 urogallus
Wilsonia citrina
Mammals:
Procyon lotor
Goshawk
Yellow-shouldered blackbird
Common treecreeper
Song wren
Spruce grouse
Red-cockaded woodpecker
Middle spotted woodpecker
Kirtland’s warbler
Pileated woodpecker
Worm-eating warbler
Red-bellied woodpecker
Red-headed woodpecker
Black-and-white warbler
Brown-headed cowbird
Shining cowbird
Siberian tit
Marsh tit
Hairy woodpecker
Green woodpecker
Ovenbird
Louisiana waterthrush
White-breasted nuthatch
Common nuthatch
Java hawk eagle
Northern spotted owl
Starling
Capercaillie
Hooded wabler
Raccoon