The role of species diversity in bottom-up and top-down interactions

Introduction The flow of energy, carbon, and nutrients through food webs is constrained by the abilities of constituent organisms to consume, assimilate, and excrete resources. There is a growing recognition that the range of these abilities is positively correlated with community diversity and thus, variation in diversity can directly impact the functioning of food webs and ecosystems (Loreau et al., 2001; Naeem, 2002; Hooper et al., 2005). For instance, the strong influence of trophic diversity, or the number of trophic levels in a food web, is recognized as an important predictor of the standing stock and productivity of primary producers and herbivores (Hairston et al., 1960; Carpenter et al., 1985). In addition to trophic diversity, the diversity of competitors, from genotypic diversity within a species (Whitham et al., 2006; Hughes et al., 2008), to species diversity (Loreau et al., 2001; Naeem, 2002; Hooper et al., 2005), to broad phylogenetic diversity (Cadotte et al., 2008), can strongly influence ecosystem functions, including the uptake and assimilation of limiting nutrients and the stability of food webs through time. In this chapter, I review the current understanding of the direct influences of competitor species richness on the transfer of energy and nutrients through food webs; a topic that has received a huge amount of attention over the past two decades (Naeem, 2008; Hooper et al., 2012). In part, the motivation to understand the influence of species richness on ecosystem functions has been prompted by a need to understand the ecosystem consequences of global species losses (Loreau et al., 2001; Naeem, 2002, Hooper et al., 2005; 2012; also, see Chapter 14). However, diversity varies across landscapes for a number of deterministic and stochastic reasons (MacArthur, 1967; Rosenzweig, 1995), and understanding the causes and consequences of this variation has resulted in a more broadly informative integration of community and ecosystem ecology with relevance beyond the implications of global species loss (Loreau et al., 2001).