Effects of Selection Cutting and Landscape-Scale Harvesting on the Reproductive Success of Two Neotropical Migrant Bird Species

Abstract: We measured the effects of forest harvesting at two spatial scales on the productivity of two Neotropical migrant bird species, the Black-throated Blue Warbler (   Dendroica caerulescens ) and the Ovenbird (  Seiurus aurocapillus ). We selected two forest landscapes representing extremes in harvesting intensity for northwestern New Brunswick, Canada. Within each landscape, we selected study plots located either in recent (<5 years) selection-cut stands (30% removal every 20 years) or in stands that were lightly harvested at least 30 years ago. Black-throated Blue Warblers reached higher densities in selection cuts, but their reproductive performance ( pairing and fledging success) did not differ significantly between stand types (uncut vs. selection cut) or between landscape types (intensively vs. moderately harvested). In 1998, nonetheless, 61% of the estimated number of fledglings produced in our plots were born in selection cuts. In contrast, this proportion was only 23% for Ovenbirds in 1998 and 1999 combined. Ovenbirds had lower densities, and their reproductive performance was significantly lower in selection cuts than in uncut plots. These results indicate that the effects of selection cutting on demography are species-specific and that Ovenbird persistence in selection cuts may be compromised unless the intensity or frequency of cutting is decreased.     

Landscape Context Moderates Edge Effects: Nesting Success of Wood Thrushes in Central New York

Abstract:  Despite two decades of research into the effects of habitat fragmentation and edges on nesting birds, critical information about how edges affect the success of natural nests of Neotropical migratory songbirds breeding in heterogeneous landscapes is still missing. We studied abundance and nesting success in Wood Thrushes ( Hylocichla mustelina ) breeding across a heterogeneous landscape in central New York from 1998 to 2000 to test the hypothesis that edge effects on nesting passerines are stronger in fragmented than contiguous landscapes. We monitored nests to estimate nesting success in edge and interior habitats in both fragmented and contiguously forested landscapes. In contiguous landscapes, daily survival rate did not differ between edge nests (0.963) and interior nests (0.968) (χ²= 0.19, p = 0.66). In contrast, in fragmented landscapes, daily survival estimates were higher in interior (0.971) than edge (0.953) nests (χ²= 3.1, p = 0.08). Our study supports the hypothesis that landscape composition moderates edge effects on actual nests of birds but does not determine the mechanisms causing these patterns.     

Nesting Success of a Neotropical Migrant in a Multiple-Use, Forested Landscape

Abstract: We studied the nesting success of an individually marked population of Kentucky Warblers ( Oporornis formosus ), a species that nests in disturbed and undisturbed forests, in a heterogeneous, managed forest site in the Shawnee National Forest in southern Illinois from 1992 to 1995. We examined the effects of forest stand type (clearcuts of various ages, tree plantations, and older forest) and distance from habitat edges on rates of nest predation and brood parasitism by Brown-headed Cowbirds (   Molothrus ater ). Brood parasitism levels gradually decreased from 60% to 3% ( nests) over a distance of 2 km from an agricultural edge proximal to a known cowbird foraging site (a pig feedlot), but they did not vary with distance from any other kinds of edges or with forest stand type. Rates of nest predation ( nests) did not vary with distance from any edges, but they were significantly lower in older forest than within even-aged clearcuts, a tree plantation, and in successional vegetation adjacent to a residential facility. These results suggest that, even in fragmented landscapes with high overall levels of parasitism and nest predation, management practices within and immediately adjacent to forest tracts can affect the nesting success of some species, but not necessarily as a simple function of distance from edge. For the Kentucky Warbler, our results suggest that a management strategy that avoids even-age silviculture and leaves core stands of older forest far from cowbird feeding areas can increase nesting success to levels similar to those measured in more forested landscapes.     

Forest Fragmentation Increases Nest Predation in the Eurasian Treecreeper

Abstract:  We used long-term breeding data to monitor the influences of fragmentation and habitat composition at different spatial scales on the reproductive success of Eurasian Treecreepers ( Certhia familiaris ) breeding in nest boxes. We collected data from the same forest patches (2.7–65.1 ha in size) during seven breeding seasons. Nest predation varied considerably over the years and was the primary cause of nesting failure (mean annual rate of 21.6 ± 12.8%). Nest predation explained most of the variation in fledgling production during the study period. Landscape-level fragmentation (radius of 500 m from territory center) affected nest predation more than did fragmentation on the territory scale (radius of 200 m from territory center). In general, nest loss due to predation in fragmented landscapes (32.4%) was almost threefold that of less fragmented (12.0%) landscapes. Of the habitat variables, predation rate correlated positively with the density of edges between forest and open land and with the proportion of sapling stands on the spatial scale of 500 m around a nest. In the core area of a territory (radius of 30 m from territory center), a high density of trees increased the frequency of nest predation. Further, a high proportion of agricultural land close to a nest site increased nest losses of treecreepers, probably because of a high degree of mustelid predation. Our results showed that the spatial scale on which we examined nest predation is important and that even within moderately fragmented landscapes it is possible to detect fragmentation-related nest predation.     

Progress in Restoration of the Mauritius Kestrel

In the 1970s, the Mauritius Kestrel ( Falco punctatus ) was the most endangered bird of prey in the world, at one time with only two known pairs surviving in the remnant native forest of the Black River Gorges (ca. 4,000 ha). At the end of the 1991–1992 breeding season, a minimum of 30 nesting pairs and more than 170 individuals were distributed in four separate forested areas, thanks mainly to manipulation of the reproductive potential of the wild pairs, to captive propagation, and to reintroduction (restocking). Since 1984, 139 young have been reared from 618 eggs laid by captive kestrels, and 147 from 265 wild eggs incubated and hatched in the laboratory; 235 young kestrels have been released on Mauritius by hacking and fostering. Adjustments in feeding and nesting habits of kestrels hacked and released outside the Black River Gorges in areas dominated by exotic vegetation and agriculture have allowed these kestrels to survive and reproduce in an array of previously unused habitats. Now that the kestrels have been released from dependence on the remnant and dying native forest, a viable population of more than 100 nesting pairs should be achievable in a few more years.     

Nesting Success of Neotropical Migrant Songbirds in a Highly Fragmented Landscape

Abstract: Small forests in agricultural landscapes are generally thought to be population sinks where the nesting success of Neotropical migrant songbirds is too low to sustain populations. In 1996 and 1997, we assessed the nesting success and productivity of Wood Thrushes (  Hylocichla mustelina ) and Rose-breasted Grosbeaks (  Pheucticus ludovicianus ) in the Region of Waterloo, a highly fragmented agricultural landscape with 14% forest cover in southwestern Ontario. We located 154 Wood Thrush nests and 63 Rose-breasted Grosbeak nests in 26 small forest fragments (3–14 ha) and 15 large fragments (26–140 ha). We used the Mayfield method to determine nesting success. Across all sites nesting success was 51% for Wood Thrushes and 46% for Rose-breasted Grosbeaks. Nesting success and productivity rates were used in a simple population growth model that suggested that the Wood Thrushes but not the Rose-breasted Grosbeaks were a self-sustaining population. Forest size and distance of nests to the nearest forest edge (measured in five distance classes ranging from 0 to over 100 m) did not significantly affect the nesting success or productivity of either species. Brown-headed Cowbirds (   Molothrus ater ) parasitized 47% of all Wood Thrush nests and 10% of all Rose-breasted Grosbeak nests; parasitism significantly reduced Wood Thrush productivity but not that of Rose-breasted Grosbeaks. Our findings challenge the prevailing notion that small woodlots in a farmed landscape are invariably habitat sinks and offer additional incentive for habitat protection in settled landscapes where small forest fragments are often all that remain for conservation purposes.     

The Effect of Edge on Avian Nest Success: How Strong Is the Evidence?

Wildlife biologists historically considered the edge between adjacent habitat types highly productive and beneficial to wildlife. A current dogma is that edges adversely affect a wide range of avian species by increasing depredation and parasitism rates of nests. I critically evaluated existing empirical evidence to test whether there was a gradation in nest success as a function of distance from an edge. Researchers investigating this question have been inconsistent in their experimental designs, making generalizations about edge-effect patterns difficult. The majority of studies I examined found nest success varied near edges, with both depredation rates (10 of 14 artificial nest studies, and 4 of 7 natural nest studies) and parasitism rates (3 of 5 studies) increasing near edges. In addition, there was apositive relationship between nest success and patch size (8 of 8 studies). The most conclusive studies suggest that edge effects usually occur within 50 m of an edge, whereas studies proposing that increased depredation rates extend farther than 50 m from an edge are less convincing. Prior research has probably focused on distances too far from an edge to detect threshold values, and future research should emphasize smaller scales. 100–200 m from an edge at 20–25 m increments. Researchers often use relatively arbitrary habitat characteristics to define an edge. Therefore, I propose that only openings in the forest canopy with a diameter three times or more the height of the adjacent trees should be included in edge analyses. This review suggests that fragmentation of eastern North American temperate forests could lead to increased nest predation and parasitism, and there is need to determine if similar processes occur in other forested regions of North America.     

Response of Red-Backed Voles to Recent Patch Cutting in Subalpine Forest

We examined the response of southern red-backed voles ( Clethrionomys gapperi ) to patch cutting in a forested landscape in which 23% of the forest cover had been removed by timber harvest. We live trapped voles in and around 18 patchcuts in one watershed of southern Wyoming. Although we found a significant difference in capture rates between patchcut interior and forest habitats in 1 of 2 years, voles did not strongly avoid the interior of patchcuts. This result contrasts with results from most studies of voles in clearcut ecosystems, which report that red-backed voles are generally rare or absent from clearcuts. Capture rates were highest on both sides of the patchcut edge, which also contrasts with studies of voles at the edges of forest remnants. The use of patchcut interior and edge habitats could not be explained as a consequence of juvenile voles dispersing to those habitats or males moving through the habitat in search of mates. We suggest that, despite similar physiognomy in patchcut and clearcut sites, the differences in landscape structure in perforated versus fragmented landscapes lead to very different patterns of vole movement. Understanding the scales at which voles perceive landscapes as coarse- or fine-grained will be key to developing predictive models to aid managers in designing timber sales that maintain high vole populations. Our results emphasize the importance of the spatial pattern and scale of disturbance in determining the response of vertebrates to landscape change and the need for more refined investigations of the consequences of deforestation.     

Fencerows, Edges, and Implications of Changing Connectivity Illustrated by Two Contiguous Ohio Landscapes

We evaluated the amounts and implications of changes in habitat connectivity on rural landscapes by modeling the colonization success and subsequent habitat colonization of a model edge organism within real landscapes. We first inventoried the changes in the fencerow and forest-edge network of two contiguous Ohio (U.S.A.) landscapes, an agriculturally dominated till plain and a more diverse and dynamic moraine landscape, from 1940 to 1988. On the moraine the number of fencerows changed little from 1940 to 1971. The number increased by 86 during 1971–1988 as marginal farms were subdivided. The total length of fencerows on the moraine increased 2.5-fold over 48 years. On the till plain the number and total length of fencerows remained relatively constant through the study period. The sum of fencerows and forest edges was used as a measure of total ecotonal edge. On the moraine total edge increased through the study period, whereas on the till plain it decreased. We selected two levels of landscape connectivity, low and high, to model animal habitat colonization success. As connectivity increased the earliest successful colonists preempted an increasingly large proportion of the total suitable habitat, and the probability of successful colonization by later-arriving individuals decreased. The changes in connectivity that resulted from changes in both the fencerow network and the proportion of forested land have resulted in contiguous landscapes that present very different colonization potentials to organisms with long-distance dispersal capability. Given the current uncertainty of the effects of corridors on species-preservation efforts, we suggest that further modeling of this type prior to field testing will add useful insights, especially if conducted using specific species and landscape types.     

Edge Effects in the Great Tit: Analyses of Long-term Data with GIS Techniques

Abstract:  In contemporary fragmented landscapes, edges are commonplace, and understanding the effects of edge environments is thus essential for the conservation of forest communities. The reproductive output of forest passerines is often reduced close to forest edges. Possible explanations include overcrowding by conspecifics, elevated rates of predation, and the occurrence of lower-quality habitat and/or individuals at forest edges. We attempted to separate these processes by examining edge effects in the absence of nest predation and by effectively controlling for differences in breeding density and the quality of habitats and individuals. We used an edge distance index (EDI), which accounts for the number and distribution of edges in close proximity to a breeding location, to help explain variation in breeding density, nesting success, and reproductive traits of 8308 pairs of Great Tits ( Parus major ) breeding between 1965 and 2005, in Wytham, near Oxford, United Kingdom. Results from linear mixed modeling confirmed higher breeding density and a higher proportion of immigrant individuals at forest edges. Nevertheless, independently of these effects, we also found that birds laying later, with smaller clutches but larger eggs, were typical of edge environments. The number of offspring recruited to the breeding offspring per breeding attempt was also reduced at edges, both directly and mediated through changes in clutch size and laying date. Edge effects on life histories were detectable within individual females and up to 500 m from the woodland edge. Woodland edges are increasingly common in contemporary fragmented landscapes. Therefore these results, which suggest a pervasive effect of edges on reproduction, are of considerable importance to the management and conservation of forest communities.     

Effects of Restoring Oak Savannas on Bird Communities and Populations

Abstract:  Efforts to restore and maintain oak savannas in North America, with emphasis on the use of prescribed fire, have become common. Little is known, however, about how restoration affects animal populations, especially those of birds. I compared the breeding densities, community structure, and reproductive success of birds in oak savannas maintained by prescribed fire (12 sites) with those in closed-canopy forests (13 sites). All sampling was conducted in Illinois (U.S.A.). Of the 31 bird species analyzed, 12 were more common in savannas, 14 were not affected by habitat structure, and 5 were more common in forest habitat. The species favored by disturbance and restoration included Northern Bobwhites ( Colinus virginianus ), Mourning Doves (  Zenaida macroura ), Red-headed Woodpeckers (  Melanerpes erythrocephalus ), Indigo Buntings (  Passerina cyanea ), and Baltimore Orioles ( Icterus galbula ). Those more common in closed-canopy forest included Ovenbirds ( Seiurus aurocapilla ) and Wood Thrushes (  Hylocichla mustelina ). Few species were unique to one type of habitat, but overall avian community structure in oak savannas and closed-canopy forests was generally distinctive. Estimates of nesting success (derived from 785 nests) revealed that 6 of the 13 species considered experienced greater productivity in the savanna habitat. Rates of brood parasitism were unaffected by restoration and habitat structure. Within savannas, tract size had little effect on breeding abundances and reproductive success. My results illustrate that restoration techniques can significantly affect the ecology of constituent animal populations and communities and have key implications regarding avian conservation and the management of forest habitat in fragmented landscapes. Small patches of forest habitat that regularly function as population sinks may offer far better prospects for birds if they are subjected to disturbance and ecosystem restoration.     

Wood Thrush Populations Are Not All Sinks in the Agricultural Midwestern United States

Abstract: Conservation strategies for forest-breeding, Neotropical migratory birds focus on the identification and preservation of source habitats. In sources, populations maintain relatively high levels of reproductive success, whereas in sinks, reproductive success is consistently below net replacement rate. In agricultural regions of the midwestern United States, forest patches of various sizes appear to act as sinks for Neotropical migrants. In 1994–1996 I monitored Wood Thrush (   Hylocichla mustelina ) nests in four forest patches in an agricultural landscape in northern Indiana. Given the prevailing estimates of adult and juvenile survival, I report productivity data that show substantial annual variation and that are sufficient to suggest the maintenance of temporal sources. Indeed, some Wood Thrush populations may act as source habitats in regions with relatively high levels of Brown-headed Cowbird (   Molothrys ater ) parasitism and nest predation because pairs produce multiple clutches and have the ability to fledge cowbirds and their own young from parasitized nests. My results and those from other studies of Wood Thrushes in landscapes with fragmented forests illustrate the variable ways in which source-sink dynamics are affected by habitat fragmentation. In some agricultural regions, populations in small forest patches may act as sources for Wood Thrushes and may be a legitimate focus of conservation efforts aimed at some migratory songbirds.     

The Early Development of Forest Fragmentation Effects on Birds

The early development of forest fragmentation effects on forest organisms is poorly understood partly because most fragmentation studies have been done in agricultural or suburban landscapes, long after the onset of fragmentation. We develop a temporal model of forest fragmentation effects on densities of forest-breeding birds and provide data from an active industrial forest landscape to test the model. The model and our empirical data indicate that densities of several forest-dwelling bird species can increase within a forest stand soon after the onset of fragmentation as a result of displaced individuals packing into remaining habitat. Along with higher densities in the newly formed fragments, pairing success in one species, the Ovenbird ( Seiurus aurocapillus ), was lower in fragments than nonfragments, possibly due to behavioral dysfunction resulting from high densities. Thus, density was inversely related to productivity. The duration and extent of increased densities following onset of fragmentation depends on many factors, including the sensitivity of a species to edge and area effects, the duration and rate of habitat loss and fragmentation, and the proximity of a forest stand to the disturbance. Incipient forest fragmentation may affect populations differently from later stages of fragmentation when the geometry of the landscape has reached a more stable configuration. Our model and data indicate, for reasons unrelated to traditional edge effects, that large tracts of forest can be important because they are relatively free from the variety of plant and animal population dynamics that might take place near new edges, including the encroachment of individuals displaced by habitat loss.     

Independent effects of fragmentation on forest songbirds: an organism-based approach.

The degree to which spatial patterns influence the dynamics and distribution of populations is a central question in ecology. This question is even more pressing in the context of rapid habitat loss and fragmentation, which threaten global biodiversity. However, the relative influence of habitat loss and landscape fragmentation, the spatial patterning of remaining habitat, remains unclear. If landscape pattern affects population size, managers may be able to design landscapes that mitigate habitat loss. We present the results of a mensurative experiment designed to test four habitat loss vs. fragmentation hypotheses. Unlike previous studies, we measured landscape structure using quantitative, spatially explicit habitat distribution models previously developed for two species: Blackburnian Warbler (Dendroica fusca) and Ovenbird (Seiurus aurocapilla). We used a stratified sampling design that reduced the confounding of habitat amount and fragmentation variables. Occurrence and reoccurrence of both species were strongly influenced by characteristics at scales greater than the individual territory, indicating little support for the random-sample hypothesis. However, the type and spatial extent of landscape influence differed. Both occurrence and reoccurrence of Blackburnian Warblers were influenced by the amount of poor-quality matrix at 300- and 2000-m spatial extents. The occurrence and reoccurrence of Ovenbirds depended on a landscape pattern variable, patch size, but only in cases when patches were isolated. These results support the hypothesis that landscape pattern is important for some species only when the amount of suitable habitat is low. Although theoretical models have predicted such an interaction between landscape fragmentation and composition, to our knowledge this is the first study to report empirical evidence of such nonlinear fragmentation effects. Defining landscapes quantitatively from an organism-based perspective may increase power to detect fragmentation effects, particularly in forest mosaics where boundaries between patches and matrix are ambiguous. Our results indicate that manipulating landscape pattern may reduce negative impacts of habitat loss for Ovenbird, but not Blackburnian Warbler. We emphasize that most variance in the occurrence of both species was explained by local scale or landscape composition variables rather than variables reflecting landscape pattern.     

Effects of Pioneer Tree Species Hyperabundance on Forest Fragments in Northeastern Brazil

Abstract: Despite many studies on fragmentation of tropical forests, the extent to which plant and animal communities are altered in small, isolated forest fragments remains obscure if not controversial. We examined the hypothesis that fragmentation alters the relative abundance of tree species with different vegetative and reproductive traits. In a fragmented landscape (670 km²) of the Atlantic Forest of northeastern Brazil, we categorized 4056 trees of 182 species by leafing pattern, reproductive phenology, and morphology of seeds and fruit. We calculated relative abundance of traits in 50 1‐ha plots in three types of forest configurations: forest edges, small forest fragments (3.4–83.6 ha), and interior of the largest forest fragment (3500 ha, old growth). Although evergreen species were the most abundant across all configurations, forest edges and small fragments had more deciduous and semideciduous species than interior forest. Edges lacked supra‐annual flowering and fruiting species and had more species and stems with drupes and small seeds than small forest fragments and forest interior areas. In an ordination of species similarity and life‐history traits, the three types of configurations formed clearly segregated clusters. Furthermore, the differences in the taxonomic and functional (i.e., trait‐based) composition of tree assemblages we documented were driven primarily by the higher abundance of pioneer species in the forest edge and small forest fragments. Our work provides strong evidence that long‐term transitions in phenology and seed and fruit morphology of tree functional groups are occurring in fragmented tropical forests. Our results also suggest that edge‐induced shifts in tree assemblages of tropical forests can be larger than previously documented.     

Persistence of Forest Birds in the Costa Rican Agricultural Countryside

Abstract:  Understanding the persistence mechanisms of tropical forest species in human-dominated landscapes is a fundamental challenge of tropical ecology and conservation. Many species, including more than half of Costa Rica's native land birds, use mostly deforested agricultural countryside, but how they do so is poorly known. Do they commute regularly to forest or can some species survive in this human-dominated landscape year-round? Using radiotelemetry, we detailed the habitat use, movement, foraging, and nesting patterns of three bird species, Catharus aurantiirostris , Tangara icterocephala , and Turdus assimilis , by obtaining 8101 locations from 156 individuals. We chose forest birds that varied in their vulnerability to deforestation and were representative of the species found both in forest and human-dominated landscapes. Our study species did not commute from extensive forest; rather, they fed and bred in the agricultural countryside. Nevertheless, T. icterocephala and T. assimilis , which are more habitat sensitive, were highly dependent on the remaining trees. Although trees constituted only 11% of land cover, these birds spent 69% to 85% of their time in them. Breeding success of C. aurantiirostris and T. icterocephala in deforested habitats was not different than in forest remnants, where T. assimilis experienced reduced breeding success. Although this suggests an ecological trap for T. assimilis , higher fledgling survival in forest remnants may make up for lower productivity. Tropical countryside has high potential conservation value, which can be enhanced with even modest increases in tree cover. Our findings have applicability to many human-dominated tropical areas that have the potential to conserve substantial biodiversity if appropriate restoration measures are taken.     

Forest Edges as Nutrient and Pollutant Concentrators: Potential Synergisms between Fragmentation, Forest Canopies, and the Atmosphere

Abstract: Forest fragmentation leads to a dramatic increase in forest edge, and these edges may function as traps and concentrators for wind-borne nutrients and pollutants. We assessed the influence of forest edges on atmospheric deposition and subsequent inputs to the forest floor in deciduous-forest fragments in the eastern United States. To quantify these inputs, we collected throughfall—water that has passed through the forest canopy—from edge and interior zones of forests adjacent to open fields. During the 1995 growing season, atmospheric input (wet and dry deposition) of sulfur to forest edge zones was elevated compared with input to forest interiors. Throughfall fluxes of dissolved inorganic nitrogen and calcium were also greater at edges than interiors. The mean edge increases ranged from 17% to 56% for the nutrients and pollutants we measured. When we manipulated the structure of forest edges by removing all vegetation below half the canopy height, throughfall flux in the edge zone declined sharply and was less than that of the respective interior zone. Changing the vegetation structure of the edge also shifted the zone of highest throughfall flux farther into the interior of the forest. Our data suggest that forest edges can function both as significant traps for airborne nutrients and pollutants from adjoining agricultural or urban landscapes and effective concentrators of below-canopy chemical fluxes. These enhanced fluxes may have cascading effects on soil-nutrient cycling, microbial activity, seedling dominance, and other ecological processes near forest edges.     

Influence of Landscape Elements in Riparian Buffers on the Conservation of Semiaquatic Amphibians

Abstract:  Studies on riparian buffers have usually focused on the amount of land needed as habitat for the terrestrial life stages of semiaquatic species. Nevertheless, the landscape surrounding wetlands is also important for other key processes, such as dispersal and the dynamics of metapopulations. Multiple elements that influence these processes should therefore be considered in the delineation of buffers. We analyzed landscape elements (forest cover, density of roads, and hydrographic network) in concentric buffers to evaluate the scale at which they influence stream amphibians in 77 distinct landscapes. To evaluate whether our results could be generalized to other contexts, we determined whether they were consistent across the study areas. Amphibians required buffers of 100–400 m of suitable terrestrial habitat, but interspecific differences in the amount of habitat were large. The presence of amphibians was related to roads and the hydrographic network at larger spatial scales (300–1500 m), which suggests that wider buffers are needed with these elements. This pattern probably arose because these elements influence dispersal and metapopulation persistence, processes that occur at large spatial scales. Furthermore, in some cases, analyses performed on different sets of landscapes provided different results, which suggests caution should be used when conservation recommendations are applied to disparate areas. Establishment of riparian buffers should not be focused only on riparian habitat, but should take a landscape perspective because semiaquatic species use multiple elements for different functions. This approach can be complex because different landscape elements require different spatial extents. Nevertheless, a shift of attention toward the management of different elements at multiple spatial scales is necessary for the long-term persistence of populations.     

Vegetation Responses along Edge-to-Interior Gradients in the Mixed Hardwood Forests of the Roanoke River Basin, North Carolina

Compared with forest interiors, forest edges typically have a different plant species composition and community structure, a phenomenon known as "edge effect." Edge effects make the functional interior area of a forest smaller than its actual area. The objective of this study was to estimate how far the effects of agriculturally maintained edges penetrate the mixed hardwood forests of the Roanoke River Basin, North Carolina. I determined percentage cover for all vascular plant species in 10-by-100-meter belt transects on north-facing or south-facing edges of four relatively undisturbed forests. Changes in the percentage cover of individual species, the relative cover of exotic species, and species richness all indicated that edge effects penetrate deeper on south-facing edges (to 60 meters) than on north-facing edges (to 20 meters). Analyses of species responses to the edge showed a number of species to be edge oriented, but no species was found to be interior oriented. The results of multivariate analyses (ordination and cluster analysis) suggested that edge effects could be detected to 50 meters on south-facing edges and 10–30 meters on north-facing edges. These results allow us to better understand the difference between a forest's actual area and its functional interior area.     

The Effect of Fragment Shape and Species' Sensitivity to Habitat Edges on Animal Population Size

Abstract:  Habitat fragmentation causes extinction of local animal populations by decreasing the amount of viable "core" habitat area and increasing edge effects. It is widely accepted that larger fragments make better nature reserves because core-dwelling species have a larger amount of suitable habitat. Nevertheless, fragments in real landscapes have complex, irregular shapes. We modeled the population sizes of species that have a representative range of preferences for or aversions to habitat edges at five spatial scales (within 10, 32, 100, 320, and 1000 m of an edge) in a nation-wide analysis of forest remnants in New Zealand. We hypothesized that the irregular shapes of fragments in real landscapes should generate statistically significant correlations between population density and fragment area, purely as a "geometric" effect of varying species responses to the distribution of edge habitat. Irregularly shaped fragments consistently reduced the population size of core-dwelling species by 10–100%, depending on the scale over which species responded to habitat edges. Moreover, core populations within individual fragments were spatially discontinuous, containing multiple, disjunct populations that inhabited small spatial areas and had reduced population size. The geometric effect was highly nonlinear and depended on the range of fragment sizes sampled and the scale at which species responded to habitat edges. Fragment shape played a strong role in determining population size in fragmented landscapes; thus, habitat restoration efforts may be more effective if they focus on connecting disjunct cores rather than isolated fragments.