COS 108-5 - Top predators versus the abiotic environment: What determines community structure in arid-land streams?

Thursday, August 11, 2011: 2:50 PM
6A, Austin Convention Center
Kate S. Boersma, Biology, University of San Diego, San Diego, CA, Michael T. Bogan, Environmental Science, Policy & Management, University of California, Berkeley, Berkeley, CA and David A. Lytle, Zoology, Oregon State University, Corvallis, OR

Extreme abiotic conditions fundamentally structure arid-land aquatic communities. However, top-down biotic processes may be equally important determinates of community assembly in seasonally intermittent arid-land streams. In the southwestern United States, these habitats shrink to fragmented pools during the summer, restricting faunal movement and increasing the density of invertebrate top predator, Abedus herberti (Hemiptera: Belostomatidae). To explore relationships between abiotic factors, A. herberti abundance, and community structure, we conducted complementary observational and experimental studies. In our observational study, we documented invertebrate communities and abiotic factors in pools within a single fragmented arid-land stream in southeastern Arizona. In our experimental study, we established mesocosm communities, applied treatments with and without A. herberti, and recorded subsequent community changes.


Ordination of observational data reveals that pools with little canopy cover and pools with extensive canopy cover contain distinct invertebrate communities, and the presence of A. herberti is associated with unshaded pools. Experimental results suggest that A. herberti affect algal growth via a trophic cascade and the invertebrate community by altering the abundance and diversity of secondary predators. These top-down effects are apparent even under the extreme abiotic conditions found in arid-land streams. A warming and drying climate will cause many seasonally intermittent streams in the southwestern United States to become ephemeral, drying completely for portions of the year. These hydrologic changes are predicted to intensify abiotic extremes and trigger local extinctions of year-round stream inhabitants – mostly comprised of predatory taxa. Therefore, studying the relative influences of abiotic factors and top predators on aquatic communities is important to understand the future of arid-land streams.

Copyright © . All rights reserved.
Banner photo by Flickr user greg westfall.