Patchiness in resource distribution mitigates habitat loss: insights from high-shore grazers

Abstract

Theory predicts that coarse-grained heterogeneity in the spatial distribution of resources benefits the abundances of exploiter populations that are vulnerable to habitat loss, in contrast to fine-grained habitat fragmentation. This generic principle has never been tested empirically, despite its potential application across scales of exploitation from foraging ranges to metapopulations. We designed a field experiment to measure population responses to the distributional pattern of limiting resources independently of resource richness. Populations of the rocky-shore snail Melarhaphe neritoides were monitored for responses in density to controlled manipulations of the pattern and abundance of refuges. On two shores, one with naturally high snail density and one with naturally low density, snail refuges were made by drilling holes into each of three 0.5-m2 plots in each of two categories of distribution pattern: patchy or even, at each of four richness levels: 4, 16, 36, or 64 holes per plot. We found that snail densities over the subsequent 2 yrs remained higher in patchy than even plots at the low-density shore, and higher in even than patchy plots at the high-density shore. Grazed areas were larger in patchy than even plots at low refuge richness, switching to larger in even than in patchy at high refuge richness. Both scales of pattern-by-richness interaction were explained by density-dependent overlap of foraging ranges around refuges. The novel implication for wildlife management is that vulnerable populations (often species of conservation concern) may benefit from clumping their resources, whereas robust populations (often pest species) may benefit from even distributions.