Professor of Biology East Carolina University, United States
Abstract: The biodiversity of a region (gamma diversity) depends on how many species occur in an average habitat patch within the region (alpha diversity) and how much variation there is in the identities of species present in patches within the region (beta diversity). Models suggest that both generalist and specialist predators should reduce alpha diversity, but generalist predators would enhance beta diversity while specialist predators would reduce beta diversity. Such differences are expected as generalist predators enhance vulnerabilities to extinction in all species by reducing their population numbers while specialist predators enhance vulnerabilities of their preferred prey. Consequently, changes in gamma diversity depend on the magnitude of change in beta diversity relative to alpha diversity and the direction of change in beta diversity. Few studies have compared the effects of predators that differ in their degree of prey specialization on prey diversity at multiple spatial scales. We conducted an experiment in artificial ponds to assess the effects of one specialist predator (sunfish) and two generalist predators (newts and Anax dragonfly nymphs) on the alpha, beta and gamma diversity of aquatic insects present in ponds. We assigned ponds to one of four types of regions (no predators present, sunfish present, newts present, or Anax present) and assessed what invertebrates were present following natural colonization at the end of summer 2021 and summer 2022. We found 41 species during the first year of sampling. Sunfish presence reduced alpha diversity by an average of 2.2 species and reduced gamma diversity by an average of 4.4 species compared to ponds that contained no predators. The presence of either newts or Anax did not affect either alpha or gamma diversity compared to predator free ponds. Species composition based on presence/absence did not vary substantially among treatments, however ponds with fish did have a different collection of species within them than ponds representing other treatments. There was only weak evidence to suggest that the amount of beta diversity varied among treatments. The trend was for beta diversity to be lower among ponds with either sunfish or Anax and greater for ponds with newts or no predators. These results indicate that the effect of predators on prey diversity varies with scale and type of predator present but the particular patterns of change that we observed are not always consistent with that predicted by theory.
