Ecology, 90(4), 2009, pp. 1073–1083 Ó 2009 by the Ecological Society of America Diversity has stronger top-down than bottom-up effects on decomposition DIANE S. SRIVASTAVA, 1,8 BRADLEY J. CARDINALE, 2 AMY L. DOWNING, 3 J. EMMETT DUFFY, 4 CLAIRE JOUSEAU, 5 MAHESH SANKARAN, 6 AND JUSTIN P. WRIGHT 7 1 Department of Zoology and Biodiversity Research Centre, University of British Columbia, 6270 University Boulevard, Vancouver, British Columbia V6T 1Z4 Canada 2 Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, California 93106 USA 3 Department of Zoology, Ohio Wesleyan University, Delaware, Ohio 43015 USA 4 Virginia Institute of Marine Science, The College of William and Mary, Gloucester Point, Virginia 23061 USA 5 Department of Ecology, Evolution and Environmental Biology, Columbia University, New York, New York 10027 USA 6 Institute of Integrative and Comparative Biology, Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT United Kingdom 7 Department of Biology, Duke University, Durham, North Carolina 27708 USA Abstract. The flow of energy and nutrients between trophic levels is affected by both the trophic structure of food webs and the diversity of species within trophic levels. However, the combined effects of trophic structure and diversity on trophic transfer remain largely unknown. Here we ask whether changes in consumer diversity have the same effect as changes in resource diversity on rates of resource consumption. We address this question by focusing on consumer–resource dynamics for the ecologically important process of decomposition. This study compares the top-down effect of consumer (detritivore) diversity on the consumption of dead organic matter (decomposition) with the bottom-up effect of resource (detrital) diversity, based on a compilation of 90 observations reported in 28 studies. We did not detect effects of either detrital or consumer diversity on measures of detrital standing stock, and effects on consumer standing stock were equivocal. However, our meta-analysis indicates that reductions in detritivore diversity result in significant reductions in the rate of decomposition. Detrital diversity has both positive and negative effects on decomposition, with no overall trend. This difference between top-down and bottom-up effects of diversity is robust to different effect size metrics and could not be explained by differences in experimental systems or designs between detritivore and detrital manipulations. Our finding that resource diversity has no net effect on consumption in ‘‘brown’’ (detritus–consumer) food webs contrasts with previous findings from ‘‘green’’ (plant–herbivore) food webs and suggests that effects of plant diversity on consumption may fundamentally change after plant death. Key words: biodiversity and ecosystem function; detrital processing; resource consumption; trophic structure; trophic transfer. INTRODUCTION Human activity has altered many natural food webs (Chapin et al. 2000, Jackson et al. 2001). Changes in both the structure and diversity of food webs are predicted to affect the flow of energy and nutrients through food webs (de Ruiter et al. 2005). For example, decades of research on trophic structure have demon- strated that changes in the biomass of a focal trophic level can affect the production of biomass in lower trophic levels (top-down effects) as well as the flow of energy up to higher trophic levels (bottom-up effects) (Pace et al. 1999, Shurin et al. 2002). This historical focus on how trophic structure influences the function- ing of food webs has recently been complemented by research that has focused on the functional role played by diversity within trophic levels (Hooper et al. 2005). New syntheses have shown that experimental reductions in species richness of a trophic level tend to reduce the consumption of resources and production of biomass by that trophic group (Balvanera et al. 2006, Cardinale et al. 2006). Regrettably, this latter body of research (often called ‘‘biodiversity and ecosystem functioning’’) has largely focused on species within a single trophic level and in isolation from their natural food webs. As a consequence, this area of research has often ignored the well-known role that trophic structure plays in control- ling the distribution of biomass among food web components. There is a growing sentiment that the study of biodiversity and ecosystem functioning must now be integrated with classical perspectives on trophic structure if we are to explain the transfer of energy and nutrients and the production of biomass within food webs (The´ bault and Loreau 2003, Srivastava and Vellend 2005, Duffy et al. 2007). Manuscript received 2 March 2008; revised 18 June 2008; accepted 27 June 2008. Corresponding Editor: D. S. Gruner. 8 E-mail: srivast@zoology.ubc.ca 1073