■ Abstract The literature on effects of habitat fragmentation on biodiversity is huge. It is also very diverse, with different authors measuring fragmentation in different ways and, as a consequence, drawing different conclusions regarding both the magnitude and direction of its effects. Habitat fragmentation is usually defined as a landscape-scale process involving both habitat loss and the breaking apart of habitat. Results of empirical studies of habitat fragmentation are often difficult to interpret because (a) many researchers measure fragmentation at the patch scale, not the landscape scale and (b) most researchers measure fragmentation in ways that do not distinguish between habitat loss and habitat fragmentation per se, i.e., the breaking apart of habitat after controlling for habitat loss. Empirical studies to date suggest that habitat loss has large, consistently negative effects on biodiversity. Habitat fragmentation per se has much weaker effects on biodiversity that are at least as likely to be positive as negative. Therefore, to correctly interpret the influence of habitat fragmentation on biodiversity, the effects of these two components of fragmentation must be measured independently. More studies of the independent effects of habitat loss and fragmentation per se are needed to determine the factors that lead to positive versus negative effects of fragmentation per se. I suggest that the term “fragmentation” should be reserved for the breaking apart of habitat, independent of habitat loss.

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Effects of Habitat Fragmentation on Biodiversity

The concept of the ecological trap, a low-quality habitat that animals prefer over other available habitats of higher quality, has appeared in the ecological literature irregularly for over 30 years, but the topic has received relatively little attention, and evidence for traps remains largely anecdotal. Recently, however, the ecological trap concept has been the subject of a flurry of theoretical activity that is likely to raise its profile substantially, particularly in conservation biology. Ecological trap theory suggests that, under most circumstances, the presence of a trap in a landscape will drive a local population to extinction. A number of empirical studies, almost all of birds, suggest the existence of traps and demonstrate the difficulties of recognizing them in the field. Evidence for ecological traps has primarily been found in habitats modified by human activities, either directly (e.g., through the mowing of grassland birds' nests) or indirectly (e.g., via human-mediated invasion of exotic species), but some studies suggest that traps may occur even in relatively pristine areas. Taken together, these theoretical and empirical results suggest that traps may be relatively common in rapidly changing landscapes. It is therefore important for conservation biologists to be able to identify traps and differentiate them from sinks. Commonly employed approaches for population modeling, which tend to assume a source-sink framework and do not consider habitat selection explicitly, may introduce faulty assumptions that mask the effects of ecological traps and lead to overly optimistic predictions about population persistence. Given the potentially dire consequences of ecological traps and the accumulating evidence for their existence, greater attention from the community of conservation biologists is warranted. In particular, it is important for conservation biologists and managers to incorporate into conservation planning an explicit understanding of the relationship between habitat selection and habitat quality.

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When Good Animals Love Bad Habitats: Ecological Traps and the Conservation of Animal Populations

2004 IUCN red list of threatened species : a global species assessment

Recognition and analysis of spatial autocorrelation has defined a new par- adigm in ecology. Attention to spatial pattern can lead to insights that would have been otherwise overlooked, while ignoring space may lead to false conclusions about ecological relationships. We used Gaussian spatial autoregressive models, fit with widely available software, to examine breeding habitat relationships for three common Neotropical migrant songbirds in the southern Appalachian Mountains of North Carolina and Tennessee, USA. In preliminary models that ignored space, the abundance of all three species was cor- related with both local- and landscape-scale habitat variables. These models were then modified to account for broadscale spatial trend (via trend surface analysis) and fine-scale autocorrelation (via an autoregressive spatial covariance matrix). Residuals from ordinary least squares regression models were autocorrelated, indicating that the assumption of independent errors was violated. In contrast, residuals from autoregressive models showed little spatial pattern, suggesting that these models were appropriate. The magnitude of habitat effects tended to decrease, and the relative importance of different habitat variables shifted when we incorporated broadscale and then fine-scale space into the analysis. The degree to which habitat effects changed when space was added to the models was roughly correlated with the amount of spatial structure in the habitat variables. Spatial pattern in the residuals from ordinary least squares models may result from failure to include or adequately measure autocorrelated habitat variables. In addition, con- tagious processes, such as conspecific attraction, may generate spatial patterns in species abundance that cannot be explained by habitat models. For our study species, spatial patterns in the ordinary least squares residuals suggest that a scale of 500-1000 m would be ap- propriate for investigating possible contagious processes.

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Spatial autocorrelation and autoregressive models in ecology

Ecological processes neat" habitat edges often differ from processes away fromedges. Yet, the generality of "edge effects" has been hotly debated because results vary tremendously. To understand the factors responsible for this variation, we described nest predation and cowbird distribution patterns in forest edge and forest core habitats on 36 randomly selected plots in three states in the midwestern United States. We tested four hypotheses that may explain the variation and mechanisms responsible for edge effects - among the 36 plots: (1) the landscape context, (2) the local predator community, (3) the local bird (host-prey) community, and (4) the nest site microhabitat structure. We used artificial nests baited with quail and clay eggs to determine nest predation patterns and " predators and used point count surveys to determine cowbird and host abundance in forest edge and forest core habitats. Raccoons, Opossums, canids, and birds accounted for most predation of artificial nests. Neither local host abundance nor mean nest concealment of artificial nests significantly •influenced nest predation rates in habitat edge or in habitat core. Nest predation was sig- nificantly greater in highly fragmented landscapes than in unfragmented landscapes and was significantly higher in edge habitats than in core habitats. However, detection of edge effects varied, depending upon landscape type. Higher predation rates in edge habitats were detected in highly and moderately fragmented landscapes, but not in unfragmented land- , , s.capes. Both mammalian and avian predator groups contributed to higher predation rates along edges in highly and moderately fragmented landscapes. ,Cowbird abundance was significantly related to host abundance, but the effect of hosts varied depending upon habitat type. In edge habitats, cowbird abundance was negatively associated with host abundance in all three landscapes studied. By contrast, cowbird abun- dance was positively associated with host abundance in core habitats. Once the effects of host abundance were removed, cowbird abundance in core habitat was greater in highly fragmented landscapes than in moderately and unfragmented landscapes, but did not differ between the latter two. In edge habitat, cowbird abundance did not differ between land- scapes, but abundance in edges tended to be highest in the highly fragmented landscape and lowest in the unfragmented landscape. Cowbird abundance did not vary between edge and core habitat in any of the landscapes studied. We suggest that the first approximation to predicting the impact of agricultural or permanently managed edges on forest songbird reproductive success is to assess habitat characteristics at the landscape scale. Given geographic location, local factors such as host abundance and predator composition should be assessed.

https://www.nrs.fs.fed.us/pubs/jrnl/1997/nc_1997_Donovan_001.pdf

Variation in Local-Scale Edge Effects: Mechanisms and landscape Context

Fragmentation of breeding habitat in North America has been implicated in the decline of forest-nesting, Neotropical migrant birds. We used a comparative approach to examine the effects of fragmentation on three forest-nesting migrants: Ovenbird (Seiurus aurocapillus), Red-eyed Vireo (Vireo ofivaceus), and Wood Thrush (Hylocichla mustelina). We surveyed birds and monitored reproductive success on 28 study plots in fragmented and contiguous forests in two midwestern regions. Distribution of individuals between fragmented and contiguous forests appeared to vary among species and regions, but total nest failure was significantly higher in fragments than contiguous forests in both regions for all species (p = 0.053). We attributed greater nest failure to increased nest predation (p = O. 093) and increased brood parasitism by the Brownheaded Cowbird (Molothrus ater, p = 0. 009). In addition to greater total nest failure, partial nest failure due to cowbird parasitism led to a reduction in the number of host fledglings. Although the causes of nest mortality appeared to be species specific, total nest failure and partial nest failure acted in concert to reduce the number of offspring per adult for all three species on fragments. We used simple population growth models to assess the viabaity of the three species in fragmented and contiguous habitats in both regions. In general, populations on fragments appeared to be population sinks and populations on continguous forests appeared to be population sources. Assuming constant mortality during winter, projected growth indicated that without immigration Ovenbird and Red-eyed Vireo populations should become extinct on fragments in both regions and Wood Thrush populations should be maintained or slightly decline on fragments. Populations of all three species should increase in contiguous habitats in both regions without emigration. We suggest that habitat fragmentation reduces local reproduction and may have ramifications for the entire population. A clear understanding of population demography depends on examination of demographic dynamics within and among sources and sinks. We emphasize that the long-term viability of these species depends on maintaining large tracts of forest throughout the breeding range until the spatial scale at which source and sink populations interact can be determined.

https://www.nrs.fs.fed.us/pubs/jrnl/1995/nc_1995_Donovan_001.pdf

Reproductive Success of Migratory Birds in Habitat Sources and Sinks

The population of male Kirtland's warbler (Dendroica kirtlandii) in the breeding season has averaged 206 from 1971 to 1987. The Kirtland's warbler occupies dense jack pine (Pinus banksiana) barrens from 5 to 23 years old and from 1.4 to 5.0 m high, formerly of wildfire origin. In 1984, 73% of the males censused were found in habitat naturally regenerated from wildfire or prescribed burning. The rest were in plantations (11%) or in harvested, unburned jack pine stands stocked by natural regeneration (16%). Twenty-two percent (630 of 2,886) of the Kirtland's warbler males counted in the annual censuses from 1971 through 1984 were found in 26 stands that were unburned and naturally regenerated following harvest. From 1982 to 1987, suitable regenerating areas were barely sufficient to replace currently occupied maturing stands, so population growth was impeded. Ecosystems of suitable size and regeneration characteristics (wildfire and plantation) doubled in area by 1989. In response, the population of Kirtland's warblers increased from 167 to 398 males between 1987 and 1992, but they withdrew almost entirely from the unburned, unplanted barrens by 1989 when the area of more suitable regeneration types increased. Minimum (368 males) and maximum (542 males) population estimates for 1996 were calculated based on 1984 average density (1.9 males per 40 ha) and peak population in burns (2.8 males per 40 ha).

https://www.nrs.fs.fed.us/pubs/jrnl/1993/nc_1993_probst_001.pdf

Relating Kirtland's warbler population to changing landscape composition and structure

Previous discussion about factors limiting the population of Kirtland's warbler has centered about nesting success and adult mortality in winter. I present a review and synthesis of published and unpublished reports on Kirtland's warbler demography which suggests that this species may also be regulated by habitat maturation and fragmentation, pairing success, fledgling mortality and breeding dispersal. Only 85 % of male Kirtland's warblers may pair successfully, though this low pairing success is offset somewhat by polygyny. Estimates of fledgling mortality (30%) and pairing success are combined with published data on Kirtland's warbler productivity (3.1 young/pair) and adult survivorship (75 %) to calculate annual recruitment of yearlings (28%) assuming a static population. Kirtland's warblers are concentrated into a few large breeding areas, each of which provides suitable habitat for only 10-14 years. The growth and decline of these "colonies" is described from a comprehensive, 13-year population count. For the short term, regenerating habitat may not be sufficient to replace currently occupied maturing stands, and a habitat shortfall is predicted for 1986-1987. INTRODUCTION The known nesting range of the Kirtland's warbler (Dendroica kirtlandii) is restricted to an area ca. 120 by 160 km in northern Lower Michigan. All nests have been found within 13 counties. But migrants and stray summer males have been collected and observed across a much broader range from Missouri to the SW, Minnesota to the NW and Virginia to the E (Tilghman, 1979). The quantity of suitable breeding habitat available to the Kirtland's warbler (KW) has decreased in recent decades (Ryel, 1981b). Typically, the species occupies dense 1.7 to 5.0-m-tall jack pine (Pinus banksiana) stands of wildfire origin. Plantations are also used, including a few red pine (P resinosa) stands, but logged, unburned jack pine stands stocked by natural regeneration from nonserotinous cones are usually not dense enough for breeding warblers. In 1951, Harold Mayfield organized the first census of the entire known population of the species (Mayfield, 1953). This census counted 432 males. The second census in 1961 discovered 502 male Kirtland's warblers (Mayfield, 1962). However, the third count taken in 1971 showed a 60% decrease to 201 males (Mayfield, 1972). The principal reason for this decline appeared to be nest parasitism by the brown-headed cowbird (Molothrus ater). Mayfield (1960) estimated the parasitism rate to be 55% and Walkinshaw and Faust (1974) estimated it to be 69%. They found that less than one Kirtland's warbler fledged per nest. In response to the dramatic population decline from 1961 to 1971, a Kirtland's warbler Recovery Team was appointed under authority of the Endangered Species Act of 1973. This team instituted the following steps to help the Kirtland's warbler: (a) annual population census: (b) cowbird control; (c) closure of breeding areas during the nesting season, and (d) an expanded habitat management program. I will address four major topics in this paper: (1) a synthesis of published and unpublished demographic data that postulates lower reproductive potential and higher rates of breeding dispersal and fledgling mortality for Kirtland's warblers than have been assumed previously; (2) the present concentration of Kirtland's warblers into only a few breeding areas, and the growth and decline of such colonies; (3) the relation between habitat quantity, decline of individual nesting areas and the total Kirtland's warbler population; (4) an overview

A Review of Factors Limiting the Kirtland's Warbler on its Breeding Grounds

We compared pairing success of male Kirtland's Warblers (Dendroica kirtlandii) in different habitats to test the hypothesis that a lower proportion of males in marginal habitat are mated. Fewer than 60% of the males in marginal habitat were paired, but 95% of the males in suitable habitat were paired. We estimated the overall pairing success of the known breeding population at 85%. We could not estimate the number of females because the adult sex ratio is unknown, and an unknown proportion of Kirtland's Warblers are polygynous. The Kirtland's Warbler population was fairly constant from 1971 to 1983, despite markedly improved nest success resulting from cowbird control measures. If there are more males than females, or if many females fail to breed or must accept mated males or marginal habitat, population growth could be impeded. We combined reduced pairing success with an estimate of fledgling mortality, and revised the estimated number of fall immatures to between 369 and 471 birds--about 36% lower than the uncorrected estimate. Lower annual productivity of a static population implies higher annual survivorship of adults, yearlings, or both.

https://www.nrs.fs.fed.us/pubs/jrnl/1987/nc_1987_Probst_001.pdf

Pairing success of Kirtland's warblers in marginal vs. suitable habitat

We investigated how environmental gradients measured along the St. Croix River in Minnesota and Wisconsin, U.S.A., influenced the turtle assemblage. Among seven species, the five most common species were generalists and had wide distributions throughout the study area. However, patterns in assemblage structure were related to environmental gradients along the river. Sex ratios were male-dominated for the five most common species, and few or no juveniles were captured during the study. The first two canonical axes of a canonical correspondence analysis accounted for 92.7% of the variation in species-environment gradients. Most of the variation in distribution and abundance was attributed to gradients in channel morphology and physical characteristics along the river channel. Abundances of common snapping (Chelydra serpentinal), false map (Graptemys pseudogeographica), and painted (Chrysemys picta bellii) turtles were associated with muck substrates and the number of basking sites (i.e., snags, rocks), which increased farther downstream. Abundance of spiny softshell turtles was closely related to increased water velocity and depth, which veere related to hydraulic control points in the river. Abundance of common map turtles was associated with the presence of open sandy areas, uniform channel bottom, and gravel substrates. Geomorphic changes along the St. Croix River clearly influence the turtle assemblage and these specific relations should be considered in efforts to preserve and restore components of the assemblage.

https://www.nrs.fs.fed.us/pubs/jrnl/1999/nc_1999_DonnerWright_001.pdf

John R. Probst

Papers

Reproductive Success of Migratory Birds in Habitat Sources and Sinks

Relating Kirtland's warbler population to changing landscape composition and structure

A Review of Factors Limiting the Kirtland's Warbler on its Breeding Grounds

Pairing success of Kirtland's warblers in marginal vs. suitable habitat

Responses of turtle assemblage to environmental gradients in the St. Croix River in Minnesota and Wisconsin, U.S.A.