Two of the most important factors determining community structure and diversity within habitat patches are patch size and patch quality. Existing paradigms in island biogeography, metapopulation biology, landscape ecology, and metacommunity ecology emphasize patch size, along with patch isolation, as critical characteristics affecting diversity. The effects of patch characteristics on colonization and post-colonization community structure have largely been limited to studies concerning patch size and isolation, with only recent attention being given to patch quality. And despite growing conservation concerns with habitat fragmentation, there has been little investigation into how patch size and isolation interact with patch quality. Using freshwater habitats as model patches, we examined how patch size and patch quality interact to affect perceived patch quality by colonizing aquatic beetles. We examined colonization in an assemblage of dytiscid and hydrophilid beetles in response to patch quality (presence/absence of caged fish) and patch size. We constructed 6 rectangular mesocosm arrays (blocks) of 6 pools each (N=36), crossing 3 patch sizes (1.2m, 1.8m and 2.7m diameter) with the presence/absence of fish, green sunfish (Lepomis cyanellus) and golden shiners (Notemigonus chrysoleucus) in a large, old field at the UM Field Station. Pools were of the same material, color, and shape (round).
Results/Conclusions
Our experiment was colonized by > 5500 beetles of 51 species over the two months of the experiment. Both patch size and patch quality affected beetle colonization, and for some species showed a significant interaction. Effect of fish were typically strong, but showed considerable variation among species. Surprisingly, beetle abundance and species richness was highest in small fishless patches, while dytiscids overall preferred larger patches and hydrophilids smaller patches. Species within both families showed an array of patterns, from strong preferences for one size extreme, to no preference with regard to size, and variation in response to predation risk. Patch size affects colonization rate via passive capture, but we suggest that patch size functions as another component of patch quality for some organisms, and interacts with other determinants of patch quality, such as predation risk. In this context patch size itself becomes a niche dimension across which species may behaviorally sort in natural systems. Because of strong, shared avoidance of fish, patch size may be a critical factor in species sorting and processes of community assembly in freshwater habitats, allowing species to behaviorally segregate along gradients of patch size in preferred fishless ponds.