PARENT SESSION
Oral Session # 71: Trophic Structure I: Theory; Aquatic Systems.
Presiding: D Chalcraft
Thursday, August 7. 8:00 AM to 11:30 AM, SITCC Meeting Room 202.
Within- and among-system differences in the strength of trophic cascades: Methodology or biology?
Borer, Elizabeth*,1, Seabloom, Eric2, Shurin, Jonathan2, Anderson, Kurt1, Blanchette, Carol1, Broitman, Bernardo1, Cooper, Scott1, Halpern, Benjamin1, 1 Department of Ecology, Evolution, and Marine Biology, Santa Barbara, CA2 National Center for Ecological Analysis and Synthesis, Santa Barbara, CA
ABSTRACT- A recent meta-analysis examining data across ecosystem types shows that trophic cascades, indirect positive effects of predators on plant biomass, are stronger in benthic marine and stream systems than they are in terrestrial or pelagic systems. This finding begs further investigation into the causes of this pattern. Using the same studies and meta-analysis techniques, we examined over 20 biological and methodological factors that have been proposed to explain variation in the strength of trophic cascades among studies within an ecosystem and among ecosystem types. After accounting for inherent biological differences among systems, we found that cascades are stronger when plant generation times are longer, and that predation on invertebrate herbivores is associated with stronger cascades than is predation on vertebrate herbivores. A few extremely efficient herbivores (e.g. sea urchins) are consistently associated with very strong trophic cascades. In addition, cascading effects tended to decline as the spatial and temporal scales of the studies increased. One strictly methodological factor was important: predators that are mobile with respect to an experimental unit produced weaker indirect effects on plants than did enclosed predators. Among ecosystem types, percent nitrogen in plants had the strongest effect size, suggesting that cascades are stronger in systems with a smaller stoichiometric imbalance between plants and herbivores.
Key words: meta-analysis, trophic cascades, indirect effects