Diel changes in resource demand: competition and predation in species replacement among crayfishes
With laboratory experiments and field surveys, we examine community—structuring mechanisms in a system undergoing invasion. Orconectes rusticus, an invading crayfish species, is replacing O. propinquus and O. Virilis in northern Wisconsin lakes. We test the hypothesis that periodicity of species interactions influences invasion success. Specifically, we test whether the intensity of interspecific competition varies on a diel cycle as a function of predation risk from visually feeding fishes. We tested for competitive exclusion in experimental pools (1.4 m in diameter) by determining habitat use of the three crayfishes in single—0 and three—species treatments. The pools contained equal area habitats of cobble, open sand, macrophytes in organic sediments (muck), and macrophytes in sand. In single—species treatments, all three species preferred cobble habitat during the day with a secondary preference for muck—macrophyte habitat. In three—species relative to single—species pools, O. rusticus increased daytime use of cobble habitat while O. virilis and O. propinquus reduced use of cobble and increased use of muck—macrophyte and sand—macrophyte habitats, respectively. Crayfish were evenly distributed among habitats at night. All three crayfishes altered habitat use between night and day, consistent with the need to forage at night and seek refuge from predation during the day. Results of a laboratory experiment giving crayfish the choice of feeding in the open or hiding in clean cobble in the presence and absence of a predator showed that crayfish chose habitat on the basis of food availability when predation risk was low and on the basis of shelter availability when risk was high. Data from experiments were consistent with habitat use observed in single— and mixed—species lakes, supporting our hypothesis that the interaction of competition with diel fluctuations in predation risk contributes to the ongoing O. rusticus invasion.