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Abstract Growing evidence suggests that organisms with narrow niche requirements are particularly disadvantaged in small habitat patches, typical of fragmented landscapes. However, the mechanisms behind this relationship remain unclear. Dietary specialists may be particularly constrained by the availability of their food resources as habitat area shrinks. For herbivorous insects, host plants may be filtered out of small habitat fragments by neutral sampling processes and deterministic plant community shifts due to altered microclimates, edge effects and browsing by ungulates.We examined the relationship between forest fragment area and the abundance of dietary‐specialist and dietary‐generalist larval Lepidoptera (caterpillars) and their host plants in the northeastern USA. We surveyed caterpillars and their host plants over 3 years in equal‐sized plots within 32 forest fragments varying in area between 3 and 1014 ha. We tested whether the abundances and species richness of dietary specialists increased more than those of dietary generalists with increasing fragment area and, if so, whether the difference could be explained by reduced host plant availability or increased browsing by white‐tailed deer (Odocoileus virginianus).The overall abundance of dietary specialists was positively related to fragment area; the relationship was substantially weaker for dietary generalists. There was notable variation among species within diet breadth groups, however. There was no effect of fragment area on the diversity of dietary‐specialist or dietary‐generalist caterpillars. Deer activity was not related to the abundances of either dietary‐generalist or dietary‐specialist caterpillars.Plant community composition was strongly associated with fragment area. Larger fragments were more likely to include host plants for both dietary‐specialist and dietary‐generalist caterpillars. Deer activity was correlated with decreased host plant availability for both groups, with a slightly stronger impact on host plants of dietary specialists. Although dietary specialists were more likely to lack host plants in fragments, the relationship between fragment area and host availability did not depend on caterpillar diet breadth.This study provides further evidence that decreasing patch area disproportionately impacts specialist consumers. Because this relationship was derived from equal‐sized plots, it is robust to some criticisms levelled at fragmentation research. The mechanisms for specialist consumer declines, however, remain elusive. 

Reduced ecological specialization is an emerging, general pattern of ecological networks in fragmented landscapes. In plant–herbivore interactions, reductions in dietary specialization of herbivore communities are consistently associated with fragmented landscapes, but the causes remain poorly understood. We propose several hypothetical bottom–up and top–down mechanisms that may reduce the specificity of plant–herbivore interactions. These include empirically plausible applications and extensions of theory based on reduced habitat patch size and isolation (considered jointly), and habitat edge effects. Bottom–up effects in small, isolated habitat patches may limit availability of suitable hostplants, a constraint that increases with dietary specialization. Poor hostplant quality due to inbreeding in such fragments may especially disadvantage dietary specialist herbivores even when their hostplants are present. Size and isolation of habitat patches may change patterns of predation of herbivores, but whether such putative changes are associated with herbivore dietary specialization should depend on the mobility, size, and diet breadth of predators. Bottom–up edge effects may favor dietary generalist herbivores, yet top–down edge effects may favor dietary specialists owing to reduced predation. An increasingly supported edge effect is trophic ricochets generated by large grazers/browsers, which remove key hostplant species of specialist herbivores. We present empirical evidence that greater deer browsing in small forest fragments disproportionately reduces specialist abundances in lepidopteran assemblages in northeastern USA. Despite indirect evidence for these mechanisms, they have received scant direct testing with experimental approaches at a landscape scale. Identifying their relative contribu