Independent Effects Of Fragmentation On Forest Songbirds: An Organism-Based Approach

Abstract

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.

Destruction of the Forest Habitat in the Tatra National Park, Slovakia

“…The most frequently studied fragmentation consequences include assessment of the effects on birds. Betts et al (2006), who investigated dependence of two bird species on fragmentation of their natural biotopes, confirms the hypothesis that landscape configuration is important for selected species only in case of a too small range area and isolated occurrence of suitable biotopes. Faaborg et al (1993) pointed to the main fragmentation consequences for the neotropical migrating birds and simultaneously presented his proposals how to minimise the negative effects of fragmentation in landscape management.…”

Riparian corridors enhance movement of a forest specialist bird in fragmented tropical forest

“…A second general implication of our results is to address the ongoing debate about the relative importance of habitat configuration and composition for conservation (16). Although much of this literature has focused on predicting occurrence or population size (43)(44)(45), movement is the domain most pertinent to configuration. Our measure of configuration (treatment) was included in models much more often than our composition variable (proportion tree cover).…”

Environmental correlates of avian diversity in lowland Panama rain forests

“…Subregions held more forest bird species when forest was represented by one or two large fragments rather than several smaller fragments. However, a fragmentation effect on species persistence may be present only in subregions that have experienced a certain amount of forest loss (Belts et al, 2006;G. Rompre, unpublished data).…”