Data from: Underrepresentation of dietary specialist larval Lepidoptera in small forest fragments: Testing alternative mechanisms

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 three 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.