Divided we stand: Exploring the positive effects of habitat fragmentation

Background/Question/Methods

The historic perspective on habitat fragmentation has been overwhelmingly negative. This perspective is largely attributable to the countless studies correlating declines in species richness, distribution, and abundance with features of fragmented landscapes, including smaller habitat patches and higher edge-to-interior ratios. But a growing body of evidence suggests that the effects of habitat fragmentation are weak relative to the effects of habitat loss. Indeed, recent syntheses suggest that most terrestrial fragmentation studies have confounded the two landscape-level processes, and this confounding has led to the negative correlations observed. Building on a largely ignored body of research documenting the positive effects of habitat fragmentation, this study presents data for a skink species (Lampropholis guichenoti) that increased in abundance three-fold following the experimental fragmentation of its Australian forest habitat. We explore mechanisms that could have contributed to this unexpected positive response. 

Coupling 25 years of pitfall records with morphometric analyses, invertebrate surveys, and microclimate measures, we tested four hypotheses for the positive effect of fragmentation on this species: 1) release from predation pressure (top down control), 2) release from competitive pressure (bottom up control), 3) creation of preferred thermal microclimates (energy niche theory), and 4) skewed sex ratios in the population due to altered incubation conditions.

Results/Conclusions

Our results refute the hypothesis that fragmentation reduced predation pressure but suggest that fragmentation reduced competitive pressure by increasing prey resource availability. In addition, site-specific microclimate data confirm that thermal niche space varies dramatically between forest types (fragmented Eucalyptus forest, unfragmented Eucalyptus forest, and pine matrix). This suggests that changes in the availability of energy may also be responsible for the increase in abundance. Initial analyses of sex ratio data remain inconclusive as to whether or not this species has temperature-dependent sex determination (TSD). Further studies will be needed to confirm TSD before we can test the hypothesis that warmer incubation temperatures led to female-biased clutches and, thus, higher population growth. This study aims to expand our understanding of the effects of habitat fragmentation, both positive and negative, while also providing a comprehensive approach to fragmentation research. Measuring the impacts of landscape structure in terms of its effects on biological processes should bring us closer to a mechanistic understanding of biodiversity declines in the face of habitat loss and fragmentation.