Spatial heterogeneity of Rana boylii habitat: Physical processes, quantification and ecological meaningfulness

Analysis of the heterogeneity of stream habitat and how biological communities respond to that complexity are fundamental components of ecosystem analysis that are often overlooked. The Foothill Yellow-legged Frog (Rana boylii ) is known to associate with various stream habitats throughout its lifecycle and thus may require some degree of habitat complexity at a larger reach scale for a population to persist. The physical processes driving stream hydraulic and geomorphic conditions, such as the relationship between the sediment supply and transport capacity, likely influence the degree of habitat heterogeneity that results. Through a series of three studies, this project addressed the relationships between stream habitat heterogeneity, the supply/capacity ratio, and the physical habitat requirements of R. boylii.; R. boylii habitat associations were quantified throughout a single season to obtain insight into local hydraulic and geomorphic conditions preferred by each lifestage. The best predictors of habitat associations by lifestage were velocity and substrate size, two key characteristics of geomorphic units such as riffles and pools. Results indicated R. boylii occurred in stream reaches with a variety of geomorphic habitats suitable to multiple lifestages. The spatial heterogeneity of geomorphic units was then quantified using several indices from landscape ecology. Indices of spatial composition, such as Shannon's Diversity Index, were found to correlate well with frog abundance, while indices of spatial configuration, such as Contagion, were not significant. Lastly, Shannon's Diversity Index, as an ecologically meaningful spatial metric, was compared with a supply/capacity ratio calculated for each study reach in order to assess how relative sediment supply correlated with varying degrees of habitat heterogeneity. Results indicated that in simple channels where only flow and alluvial sediment interacted to create bed topography, maximum heterogeneity occurred with a moderate relative sediment supply. In complex channels where structural elements, such as large woody debris and boulders, created local scour and deposition, habitat heterogeneity increased as the percent of structural elements increased. Project results imply restoration practices and land use changes that affect the relative sediment supply and local geomorphic processes in a stream may directly impact the suitability of habitat complexity required by R. boylii.