Species are not equally affected by habitat fragmentation, with impacts depending on traits such as diet breadth, trophic rank and dispersal ability. Disproportionate reductions of dietary specialists are consistently observed in insect herbivore communities in fragmented landscapes. Causal mechanisms for this pattern are poorly understood, including whether these causes are via bottom-up, or top-down pathways. Theory predicts that fragmentation-mediated changes in host plant communities would cause a loss of specialization through bottom-up effects, because specialists are more dependent on specific hosts. Additionally, top-down effects of altered predator and parasitoid communities could affect specialists and generalists differently. Finally, large browsers, such as deer, could indirectly increase specialization by inducing plant defenses, or decrease specialization by preferentially browsing plant species that support specialized insect herbivores.
We compare effects of fragmentation on generalist and specialist caterpillar abundance across temperate forest fragments of varying area. By sampling both the caterpillar community in forest fragments, and their host plants, we ask whether bottom-up effects of host plant availability explain changes in specialist caterpillar abundance, relative to generalists. Additionally, we measure relative amounts of deer browsing to elucidate whether indirect top-down effects can account for differential changes in specialists and generalists as fragmentation increases.
Results/Conclusions
Effects of fragmentation on the relative abundance of specialist and generalist caterpillars varied across study years. In one year, generalist abundance decreased in small fragments, while specialist abundance increased. In other years, patterns matched other studies, with specialists disproportionately affected by fragmentation. Similarly, the effect of deer browsing on specialist and generalist abundance varied by year, with both specialists and generalists exhibiting negative effects of browsing.There was no evidence that fragmentation-mediated changes in host plant communities facilitated changes in the relative abundance of specialists and generalists.
One possible explanation for the variation among years may be an outbreak of the generalist Gypsy Moth (Lymantria dispar). During this outbreak, L. dispar formed a larger proportion of generalists, and responses to fragmentation and deer browsing for this species differed from other generalists. Additionally, specialist numbers were reduced in the outbreak year, suggesting direct competition for better-defended host plants, exacerbated by decreased growth efficiency of specialists in the same year. Our results suggest that interspecific competition and top-down effects indirectly mediated by large browsers play a stronger role in shaping the abundance of both specialists and generalists in fragmented landscapes than bottom-up effects of host plant availability.