Consumptive effects of crayfish limit snail populations

One of the best studied freshwater predator-prey interactions is arguably that between crayfish and gastropods. In many ecosystems, crayfish limit populations of thin-shelled snails and induce changes in snail traits. However, the connection between these 2 phenomena and, therefore, the mechanism by which crayfish have population-dynamic effects on snails, has been little studied. I examined the population-level effects of 2 procambarid crayfish (Procambarus alleni and Procambarus fallax) on a 2-species assemblage of snails (physids: Hatia cubensis, planorbids: Planorbella duryi) in experimental mesocosms. Observations early in the experiment suggested that both snail species were responding behaviorally to the crayfish, especially to P. alleni. At the end of the study (8 wk), both species of crayfish had reduced filamentous periphyton by 50% and snail density by >95%. The 2 crayfish species had quantitatively similar effects on both trophic levels. Physids laid eggs early in the study, but they had no size refuge and were almost completely eliminated from the crayfish treatments by the end of the study. Large planorbids had a size refuge and persisted with crayfish, but almost no recruitment occurred in the presence of crayfish. Per-capita egg-laying rates on bottom substrates at the end of the experiment were equal between treatments, results indicating that planorbids persisted in laying eggs and that crayfish continued to consume the hatchlings. The observations from this experiment suggest that crayfish limit snail populations primarily by direct consumptive effects.