Invasion success and community effects of signal crayfish (Pacifastacus leniusculus) in eastern Sierra Nevada streams

Alien crayfish threaten freshwater communities in many parts of the world, and are a novel element in many California stream systems. I conducted a five-year field study in eastern Sierra Nevada streams of non-native signal crayfish (Pacifastacus leniusculus) distribution and habitat relationships, community effects, and interactions with native Paiute sculpin (Cottus beldingi, a small benthic fish). My first dissertation chapter examines the distribution of crayfish and sculpin, their associations with reservoirs and stream regulation, and abundance changes over time associated with natural hydrologic disturbance. Crayfish and sculpin densities were positively correlated at scales from local to regional, probably because of the strong associations of both species with low gradients and rocky cover. Crayfish were most abundant in stream segments close to reservoirs, and declined in years following intense and prolonged winter floods. Sculpin were less abundant than expected in regulated reaches, and their recruitment increased in flood years. The second chapter reports the results of field enclosure experiments investigating competition between crayfish and sculpin and top-down effects of both species on benthic invertebrates and algae. These experiments indicated strong, reciprocal interspecific competition, as measured by growth rates, and strong intraspecific competition among crayfish but no evidence of competition among sculpin at natural densities. Crayfish reduced total invertebrate abundance and abundance of larger invertebrates but had no effect on algal biomass. Sculpin had neutral or marginally positive effects on invertebrates and positive effects on algae. The final chapter considers behavioral mechanisms for the competitive effects of crayfish on sculpin, evaluated experimentally in a stream observation facility of the Sagehen Creek Field Station. Sculpin reduced their use of shelters and pools, shifted into higher-velocity microhabitats, and spent more time fleeing in the presence of crayfish. Crayfish used shelters, pools, and low-velocity habitats more than sculpin. Under natural conditions, potential refuges (unembedded rocks) were closely associated with total crayfish and sculpin densities, suggesting that competition for cover may occur in Sagehen Creek. I conclude that although crayfish may alter communities in reservoir-influenced streams, and can invade far upstream during prolonged droughts, their effects are probably minor or temporary in most unregulated stream reaches.