Does variation in patch use among butterfly species contribute to nestedness at fine spatial scales?

Abstract

A nested pattern occurs whenever the species observed in depauperate habitat patches are a subset of those found in more species‐rich patches. Ecologists have documented many instances of nestedness caused by population‐level processes such as colonization and extinction at biogeographic scales. However, few researchers have examined whether nestedness may exist at fine scales due to the ways in which individual organisms discriminate among potential habitat patches. In 1999, we experimentally fragmented an old‐field habitat into patches of varying size to test whether nestedness could exist on a fine spatial scale. Five treatments of differing patch size were replicated five times in a Latin square design by selectively mowing 15×15 m2 plots within an old‐field (patch areas: 225, 180, 135, 90, and 45 m2). Specifically, we tested whether butterflies foraging within a network of patches differing in area conformed to a nested subset structure. We also classified species according to (1) their flight height while foraging (high or low), and (2) their adult habitat breadth (ubiquitous, general, or restricted) to determine whether nestedness could be explained by difference in species’ tendency to discriminate among patches differing in area. We found significant evidence that a community of foraging Lepidoptera conformed to a nested subset structure based on the difference between the observed nestedness within the butterfly community and the nestedness obtained from randomly generated species presence/absence matrices. Poisson regression analyses demonstrated that high‐flying, habitat‐restricted species avoided the smallest patches (90 and 45 m2) in favor of larger remnants, whereas low‐flying, habitat generalists used all patch sizes. Thus, our study is one of the first to demonstrate that nestedness among species subsets can be observed at fine spatial scales (within a single 1.5 hectare field) and may be maintained by species behavioral differences: discriminating species (i.e. high‐flying, habitat restricted) avoided the smallest patches, and less discriminating species (i.e. low‐flying, ubiquitous) were distributed throughout the field without regard to patch size. Our results also suggest that nestedness should be viewed as yet another scalar pattern in ecology, generated by variation in patch use by individuals at fine‐scales as well as the more traditionally invoked processes of extinction and colonization of species at broad‐scales.