How do stream grazers partition their benthic habitat?

Abstract

Emphasizing species’ trait similarities over differences, functional feeding guilds underrepresent functional diversity within stream communities. Species within guilds commonly overlap in their distributions and must partition their shared resources to coexist. Guild partitioning strategies may be revealed by the dissimilar responses the species show to environmental conditions, and these responses may be predicted from species-specific traits. To test this, I examined a grazer guild in a mountain stream dominated by three mayflies, Baetis, Epeorus, and Cinygmula. My objective was to determine how these species responded to gradients of current velocity, stream depth, and rock size and the abundance of algal food. I sampled 35 streambed locations and measured near-bed current velocity and depth at nine points at each location before sampling macroinvertebrates with a Surber sampler. Nine of the sampled rocks were measured for size, and benthic algae were collected from four of them. Multiple regression and non-metric multi-dimensional scaling (NMDS) revealed that each mayfly had unique responses to current velocity, depth, rock size, and algal abundance. Baetis correlated with depth, current velocity, and the interaction of these variables. Epeorus correlated exclusively with current velocity, and Cinygmula correlated with depth in interaction with other variables. Algal abundance by itself was a poor predictor of mayfly abundance, and NMDS indicated that individual species responses were not good predictors of mayfly assemblage patterns. Nevertheless, the mayflies’ idiosyncratic responses to current velocity, depth, and rock size supported the notion that these variables are components of unique partitioning strategies.