The influence of beaver ponds on the temporal and spatial dynamics of southeastern stream fish assemblages

I examined the influence of beavers on a southeastern landscape and the fish assemblages occupying streams of this landscape. I used historical aerial photography to determine the extent and rate of beaver impoundment of streams over a 77,000 ha Upper Coastal Plain landscape (the Savannah River Site, SRS) during a 40-year period of beaver population recovery. A spatial survey of fishes in 38 impounded or previously impounded stream reaches and a temporal survey of fishes in and adjacent to two active ponds was used to investigate the influence of beavers on stream fish assemblage structure and dynamics. Beaver impoundment of low-order streams on the Upper Coastal Plain had relatively minor effects on the landscape when compared to their effects on northern boreal forest landscapes. Management of beaver populations by lethal trapping at SRS slowed the growth rate of individual patches associated with individual beaver colonies but did not influence the ultimate size of patches. Beaver impoundment greatly altered stream fish assemblage structure within impounded reaches: assemblages in impounded reaches were dominated by species other than yellowfin shiners (Notropis lutipinnis), the dominant species in free-flowing stream reaches, and assemblage structure was temporally less stable in impounded reaches when compared to free-flowing reaches. There were also large differences in assemblage structure among ponds. These differences were related to pond age and watershed position, and changes in the internal environment of ponds with pond age. The influence of beaver ponds on fish assemblage structure and dynamics in stream reaches adjacent to ponds was limited to blockage of fish movement and shifts in seasonal patterns of abundance, and was relatively small when compared to effects observed in a warmwater stream in Minnesota. These results indicate: (1) that the effects of beavers on North American streams can vary greatly between regions, (2) that patterns of assemblage structure and richness are altered by beaver impoundment of low-order streams, and (3) that management goals directed at protecting economic interest and biodiversity are compatible as long as the effects of management are monitored.