Spatial variation in fish assemblages across a beaver-influenced successional landscape

Beavers are increasingly viewed as "ecological engineers," having broad eff ects on physical, chemical, and biological attributes of north-temperate la ndscapes. We examine the influence of both local successional processes ass ociated with beaver activity and regional geomorphic boundaries on spatial Variation in fish assemblages along the Kabetogama Peninsula in Voyageurs N ational Park, northern Minnesota, USA. Fish abundance and species richness exhibited considerable variation among drainages along the peninsula. Geolo gical barriers to fish dispersal at outlets of some drainages has reduced f ish abundance and species richness. Fish abundance and species richness als o varied within drainages among local environments associated with beaver p ond succession Fish abundance was higher in upland ponds than in lowland po nds, collapsed ponds, or streams, whereas species richness was highest in c ollapsed ponds and streams. Cluster analyses based on fish abundance at sit es classified according to successional environment indicated that four spe cies (northern redbelly dace, Phoxinus eos; brook stickleback, Culaea incon stans; finescale dace, P. neogaeus; and fathead minnow, Pimephales promelas ), were predominant in all successional environments. Several less abundant species were added in collapsed ponds and streams, with smaller size class es bf large lake species (e.g., black crappie, Pomoxis nigromaculatus; smal lmouth bass, Micropertus dolomieui; yellow perch, Perca flavescens; and bur bot, Lota lota) being a component of these less abundant species. The addit ion of smaller size classes of large lake species indicates that dispersal of early life-history stages from Kabetogama Lake played a role in determin ing the species richness and composition of less abundant species in succes sional environments on the peninsula. Furthermore, collapsed-pond and strea m environments closer to Kabetogama Lake had higher species richness than s imilar successional sites located farther from the lake. Cluster analyses b ased on fish abundance at sites classified according to drainage indicated that species composition among drainages was influenced both by the presenc e or absence of geological barriers to fish dispersal and the nonrandom dis tribution of collapsed ponds and streams. Based on these results, we presen t a hierarchical conceptual model suggesting how geomorphic boundaries and beaver pond succession interact to influence fish assemblage attributes. Th e presence of a productive and diverse fish assemblage in headwater streams of north-temperate areas requires the entire spatial and temporal mosaic o f successional habitats associated with beaver activity, including those du e to the creation and abandonment of beaver ponds. The ultimate impact of t he local successional mosaic on fishes, however, will be strongly influence d by the regional geomorphic context in which the mosaic occurs.