Consumer versus resource control of producer diversity depends on ecosystem type and producer community structure

doi:10.1073/pnas.0701918104

Consumer versus resource control of producer diversity depends on ecosystem type and producer community structure

^†Institute for Botany, University of Cologne, Gyrhofstrasse 15, D-50931 Cologne, Germany;
^§Bodega Marine Laboratory, University of California at Davis, P.O. Box 247, 2099 Westside Road, Bodega Bay, CA 94923-0247;
^¶Department of Zoology, Cordley Hall 3029, Oregon State University, Corvallis, OR 97331-2914;
^‖Marine Science Center, Northeastern University, 430 Nahant Road, Nahant, MA 01908;
^††National Center for Ecological Analysis and Synthesis, University of California, 735 State Street, Suite 300, Santa Barbara, CA 93103;
^‡‡School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501;
^§§Department of Ecology and Evolutionary Biology, University of California, 321 Steinhaus Hall, Irvine, CA 92697; and
^¶¶Department of Zoology, University of British Columbia, 6270 University Boulevard, Vancouver, BC, Canada V6T 1Z4

Edited by James H. Brown, University of New Mexico, Albuquerque, NM, and approved May 17, 2007 (received for review March 2, 2007)

Abstract

Consumer and resource control of diversity in plant communities have long been treated as alternative hypotheses. However, experimental and theoretical evidence suggests that herbivores and nutrient resources interactively regulate the number and relative abundance of coexisting plant species. Experiments have yielded divergent and often contradictory responses within and among ecosystems, and no effort has to date reconciled this empirical variation within a general framework. Using data from 274 experiments from marine, freshwater, and terrestrial ecosystems, we present a cross-system analysis of producer diversity responses to local manipulations of resource supply and/or herbivory. Effects of herbivory and fertilization on producer richness differed substantially between systems: (i) herbivores reduced species richness in freshwater but tended to increase richness in terrestrial systems; (ii) fertilization increased richness in freshwater systems but reduced richness on land. Fertilization consistently reduced evenness, whereas herbivores increased evenness only in marine and terrestrial ecosystems. Producer community evenness and ecosystem productivity mediated fertilization and herbivore effects on diversity across ecosystems. Herbivores increased producer richness in more productive habitats and in producer assemblages with low evenness. These same assemblages also showed the strongest reduction in richness with fertilization, whereas fertilization increased (and herbivory decreased) richness in producer assemblages with high evenness. Our study indicates that system productivity and producer evenness determine the direction and magnitude of top-down and bottom-up control of diversity and may reconcile divergent empirical results within and among ecosystems.

Footnotes

^‡To whom correspondence should be addressed. E-mail: helmut.hillebrand{at}uni-koeln.de
Author contributions: H.H., D.S.G., and J.B.S. designed research; H.H., D.S.G., E.T.B., M.E.S.B., E.E.C., J.J.E., W.S.H., J.T.N., E.W.S., J.B.S., and J.E.S. performed research; H.H. and E.W.S. analyzed data; and H.H. and D.S.G. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0701918104/DC1.
Abbreviations:

GLM,

general linear model;

MA,

metaanalysis.