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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 . . . . . . . . . . . . . . . . . . . . 149 150 151 152 I54 154 155 158 159 160 163 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. I58 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. 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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