PARENT SESSION
Poster Session # 21: Biogeography, Biodiversity, Populations, and Genetics.

Thursday, August 7 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

Does heterogeneity in near-bed current mediate competition between stream herbivores?

Wellnitz, Todd¹, Poff, LeRoy¹, ¹ Biology Department, Colorado State University, Fort Collins, Colorado

ABSTRACT- Herbivore guilds in streams can be speciose despite the fact grazers are often food-limited. This suggests that variation in environmental conditions may influence the outcome of species interactions and thus potentially mediate herbivore co-existence. To examine the role of heterogeneity in near-bed current in this regard, we conducted a 3 week competition experiment in streamside channels along the upper Colorado River using 3 current velocities (5, 15 and 30 cm/s), and 5 densities (0, 0.5x, 1x, 2x and 3x ambient densities) for each of two grazers, the mayfly Drunella, and the caddisfly Glossosoma. Grazer weight gain and periphytic responses to herbivores were both influenced by current velocity. For example, Glossosoma grew faster and were more effective at removing periphyton as current velocity increased, suggesting higher feeding efficiency under high flows. At the slowest velocity, periphytic abundance actually increased with Glossosoma density. Caddisfly weight gain was inversely related to larval density, implying intraspecific competition. Interestingly, Glossosoma weight actually increased with increasing Drunella density, suggesting facilitation by this mayfly on Glossosoma feeding. Evidence from this study and others indicate that heterogeneity in near-bed current can influence species interactions among stream herbivores, and thereby potentially mediate patterns of species co-occurrence.

Key words: stream, competition, herbivory, facilitation