| The introduction or invasion of exotic species constitutes one of the most important ecological problems worldwide. In particular, invasions of exotic species can have large effects on stream communities reducing the abundance or even eliminating native taxa. The red swamp crayfish, Procambarus clarkii, an omnivorous species native to northeastern Mexico and the south central United States, was introduced into many lakes and streams throughout other parts of the U.S., including California and Hawaii, in the early part of the twentieth century.; The first part of my research consisted of two separate field experiments delineating the effects of a P. clarkii density gradient on benthic invertebrate biomass and diversity and algae, detritus, and sediment levels in two different Californian streams. Habitat conditions, invertebrate community composition, and dominant trophic linkages influenced crayfish effects in both streams, ranging from effects mainly on large invertebrates in the Santa Ynez River to effects on algae, detritus, and sediment levels with repercussions for small consumers in the Ventura River.; The second part of my research encompassed two field experiments which determined the individual and combined effects of P. clarkii and two native consumers, snails (Physella gyrina) and atyid shrimp (Atyoida bisulcata), on the biotas of Californian and Hawaiian streams. In these experiments, P. clarkii had complex and different effects on different systems, probably as mediated through multiple trophic interactions involving benthic invertebrates, periphyton, and leaf litter, and through non-trophic crayfish effects on sedimentation and snail behavior.; The third part of my research used laboratory experiments to examine the effects of confined or free-ranging fed or starved crayfish and crushed snails on snail microdistributions. In general, cues produced by predators (e.g., chemical, mechanical) caused prey (i.e., snails) to move into refugia above the water line or under cover, but exact prey responses depended on the intensity and nature of cues. |
