Critical patch size and connectivity: Species, interactions, and community structure

A key concept in spatial ecology is that of critical patch size, namely the minimum habitat area below which a species cannot persist.  Whether species generally exhibit critical thresholds for patch size depends on their ecological traits and the connectivity among habitat patches.  Here, we review the empirical evidence for critical thresholds of patch size at the species level and outline mechanisms by which habitat patchiness can impact consumer-resource pairs, food web modules, and other community subunits.  For example, although several studies of plant-pollinator interactions and rates of parasitism have demonstrated patch size effects on persistence, other types of interactions have received little attention. Furthermore, few studies to date have investigated how the number of trophic levels, guild membership, and other measures of community structure are altered on habitat fragments.  Using a salt marsh arthropod food web as a case study, we demonstrate that species’ responses to gradients in patch size and connectivity are largely predictable based on ecological traits (trophic position, diet breadth, dispersal capability) and that the presence of specific sets of interacting species is dependent on spatial context.  Notably, parasitism rates and the occurrence of food web modules involving intraguild predators strongly depend on patch size, leading to significantly altered metacommunity structure across the patch size spectrum.  Understanding when and how local species interactions and food web structure exhibit threshold responses to patch size is a key to predicting the consequences of landscape structure on higher levels of biological organization.