THE RELATIONSHIP BETWEEN LARGE-SCALE PATTERNS IN GEOMORPHIC DISTURBANCE AND THE ECOLOGY OF STREAM FISHES

Current paradigms in management and regulation of forestlands relative to geomorphic processes and aquatic ecosystems focus on the mass of material generated per unit time, often at the scale of small watersheds. Increasing understanding of the dynamics of stream habitats and of the ecology of fishes in mountain basins suggests that a paradigm recognizing patch dynamics and the spatial and temporal distribution of conditions across watersheds may have greater utility in conservation of sensitive species. Metapopulation theory considers the interactions of local populations associated with habitat patches, whose extent can be defined by environmental controls such as passage barriers or thermal conditions in streams. Within patches, occupancy may be highly variable in time, but the probability of a global extinction across a metapopulation is much lower, except in the case of all local populations fluctuating in perfect synchrony. The size of habitat patches in relation to the size and continuity of disturbances will define the relative vulnerability of individual populations. Asynchrony and dispersal among patches can facilitate gene flow, demographic support, and even recolonization following catastrophic disturbance, important to the stability and persistence of the regional or “meta” - population. The application of metapopulation concepts in concert with a temporally and spatially explicit evaluation of severe geomorphic disturbance to channels may help explain the large-scale patterns of occurrence or persistence in some stream dwelling species. Comparison of bull trout population and genetic structure with sequential mapping of severe channel disturbance over the last 40 years across the entire Boise River basin reveals a concordance among the scales of disturbance, patch size effects, and patterns of gene flow. Recent observations of the recovery of populations following severe disturbances related to fire provide information about the relevant time scales for population responses. Our results support a metapopulation paradigm for sensitive species conservation with important implications for habitat fragmentation, the relative risk of wildfire versus the management intended to reduce its severity and spread, and the role geomorphic processes across entire river basins.