Shurin etal 2002 (Eco Letters)

Shurin etal 2002 (Eco Letters) - Ecology Letters, (2002) 5:...

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REPORT A cross-ecosystem comparison of the strength of trophic cascades Jonathan B. Shurin 1 *, Elizabeth T. Borer 2 , Eric W. Seabloom 1 , Kurt Anderson 2 , Carol A. Blanchette 2 , Bernardo Broitman 2 , Scott D. Cooper 2 and Benjamin S. Halpern 2 1 National Center for Ecological Analysis and Synthesis, University of California- Santa Barbara, 735 State St., Suite 300, Santa Barbara, CA 93101, USA 2 Department of Ecology, Evolution, and Marine Biology, University of California- Santa Barbara, Santa Barbara, CA 93106, USA *Correspondence: E-mail: shurin@nceas.ucsb.edu Abstract Although trophic cascades (indirect effects of predators on plants via herbivores) occur in a wide variety of food webs, the magnitudes of their effects are often quite variable. We compared the responses of herbivore and plant communities to predator manipulations in 102 Feld experiments in six different ecosystems: lentic (lake and pond), marine, and stream benthos, lentic and marine plankton, and terrestrial (grasslands and agricultural Felds). Predator effects varied considerably among systems and were strongest in lentic and marine benthos and weakest in marine plankton and terrestrial food webs. Predator effects on herbivores were generally larger and more variable than on plants, suggesting that cascades often become attenuated at the plant– herbivore interface. Top-down control of plant biomass was stronger in water than on land; however, the differences among the Fve aquatic food webs were as great as those between wet and dry systems. Keywords Cross-system comparison, indirect effects, meta-analysis, predation, top-down control, trophic structure. Ecology Letters (2002) 5: 785–791 INTRODUCTION Hairston et al . (1960) touched off 40 years of debate when they proposed that predators maintain global plant biomass at high levels by limiting the densities of herbivores (the Ô green world Õ hypothesis). Trophic cascades have since been described in a wide variety of systems including lakes, streams, forests, grasslands, kelp beds, and marine plankton (Power 1990; Carpenter & Kitchell 1993; McClaren & Peterson 1994; Brett & Goldman 1996; Estes et al . 1998; Micheli 1999; Pace et al . 1999; Post et al . 1999; Schmitz et al . 2000; Halaj & Wise 2001). Because the most compelling examples of cascades come from aquatic systems, several authors have proposed that cascades are more prevalent in water than on land (Strong 1992; Polis 1999; Halaj & Wise 2001). Two recent meta-analyses of terrestrial trophic cascade experiments came to contrasting conclusions about whether trophic cascades are stronger in aquatic than in terrestrial systems (Schmitz et al . 2000; Halaj & Wise 2001). Differences among food webs in the strength of top-down control may have profound effects on the distribution of producer and consumer biomass among the earth’s ecosys- tems. The effects of human food web alterations, for instance predator extirpation through Fshing or hunting, on lower trophic levels may also vary among ecosystem types.
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Shurin etal 2002 (Eco Letters) - Ecology Letters, (2002) 5:...

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