Current velocity and spatial scale as determinants of the distribution andabundance of two rheophilic herbivorous insects

Organisms frequently show marked preferences for specific environmental con ditions, but these preferences may change with landscape scale. Patterns of distribution or abundance measured at different scales may reveal somethin g about an organism's perception of the environment. To test this hypothesi s, we measured densities of two herbivorous aquatic insects that differed i n body morphology and mobility in relation to current velocity measured at different scales in the upper Colorado River (Colorado, USA). Streambed den sities of the caddisfly larva Agapetus boulderensis (high hydrodynamic prof ile, low mobility) and mayfly nymph Epeorus sp. (low hydrodynamic profile, high mobility) were assessed at 3 spatial scales: whole riffles, individual cobbles within riffles, and point locations on cobbles. Riffles were sever al meters in extent, cobbles measured 10-30 cm in size, and the local scale was within a few centimeters of individual larvae (themselves ca. 0.5-1.0 cm in size). We also quantified the abundance of periphytic food for these herbivores at the cobble and riffle scales. Agapetus favored slow current ( < 30 cm s(-1)) across all scales. Epeorus, by contrast, favored fast curren t (60-80 cm s(-1)) at the local and riffle scale, but not at the cobble sca le. Only Agapetus showed a significant relationship to current at the cobbl e scale, with greatest larval densities occurring at velocities near 30 cm s(-1). We had predicted an inverse correlation between grazer density and p eriphytic abundance; however, this occurred only for Agapetus, and then onl y at the cobble scale. These data suggest that organisms respond to environ mental gradients at different spatial scales and that the processes driving these responses may change with scale, e.g., shifting from individual habi tat selection at local and cobble scales to population responses at the rif fle scale. This study also highlights the importance of using the appropria te scale of measurement to accurately assess the relationship between organ isms and environmental gradients across scale.