Dynamics in species composition of stream fish assemblages: Environmental variability and nested subsets

Stream landscapes are highly variable in space and time and, like terrestri al landscapes, the resources they contain are patchily distributed. Organis ms may disperse among patches to fulfill life-history requirements, but bio tic and abiotic factors may limit patch or locality occupancy. Thus, the dy namics of immigration and extinction determine, in part, the local structur e of assemblages. We sampled fishes and stream habitat at 12 localities for two years (96 samples) to examine the deterministic nature of immigration and extinction processes in stream fish assemblages. Mean immigration rates for assemblages were highest at large stream localities. where the pool of potential immigrants was largest. Mean extinction rates were highest where variability in the flow regime was high, though local refugia appeared to modify the extinction process at one locality. Significant nested subset pa tterns in species composition occurred over time for 7 of the 12 localities . The strength of the nesting was associated with mean immigration and exti nction rates. Higher extinction rates corresponded to stronger nestedness, whereas higher immigration rates were associated with weaker nestedness. Ac ross all species, both immigration and extinction rates were strongly assoc iated with mean abundance. Species with high local abundances had higher im migration rates and lower extinction rates than did species with low local abundances. There were no significant associations between trophic guild or body size and immigration and extinction rate. This work supports the hypo thesis that immigration and extinction rates for assemblages are predictabl e along environmental gradients, and that species are less prone to local e xtinction and more prone to colonize areas when they maintain high local ab undances. The extinction process in local assemblages can be a highly order ed event leading to strong nested subset patterns. but immigration appears to be more stochastic.