Multitrophic diversity effects depend on consumer specialization and species‐specific growth and grazing rates

Abstract

Ecosystem functioning is affected by horizontal (within trophic groups) and vertical (across trophic levels) biodiversity. Theory predicts that the effects of vertical biodiversity depend on consumer specialization. In a microcosm experiment, we investigated ciliate consumer diversity and specialization effects on algal prey biovolume, evenness and composition, and on ciliate biovolume production. The experimental data was complemented by a process-based model further analyzing the ecological mechanisms behind the observed diversity effects. Overall, increasing consumer diversity had no significant effect on prey biovolume or evenness. However, consumer specialization affected the prey community. Specialist consumers showed a stronger negative impact on prey biovolume and evenness than generalists. The model confirmed that this pattern was mainly driven by a single specialist with a high per capita grazing rate, consuming the two most productive prey species. When these were suppressed, the prey assemblage became dominated by a less productive species, consequently decreasing prey biovolume and evenness. Consumer diversity increased consumer biovolume, which was stronger for generalists than for specialists and highest in mixed combinations, indicating that consumer functional diversity, i.e. more diverse feeding strategies, increased resource use efficiency. Overall, our results indicate that consumer diversity effects on prey and consumers strongly depend on species-specific growth and grazing rates, which may be at least equally important as consumer specialization in driving consumer diversity effects across trophic levels.Natural food webs represent a complex array of diversely cross-linked multiple trophic levels, in which ecosystem functioning is affected by horizontal (within trophic groups) and vertical (across trophic levels) biodiversity (Duffy et al. 2007). Numerous ecological studies have addressed the consequences of species diversity on ecosystem functioning. After an initial phase of studies within trophic levels, more recently, diversity effects across multiple trophic levels have been addressed (see studies by Downing and Leibold 2002, Naeem and Li 1997, see reviews by Duffy et al. 2007, Srivastava et al. 2009, Griffin et al. 2013. These multitrophic studies, however, yielded inconsistent conclusions regarding the strength and the direction of consumer diversity (i.e. richness) effects on biomass and composition within and across trophic levels (Gamfeldt et al. 2005, Steiner et al. 2005, Dzialowski and Smith 2008, indicating that these effects are highly context-dependent. Further studies demonstrated that diversity effects may be strongly determined by food web configuration and/or consumer identity, involving relevant species-specific traits, such as consumer specialization and grazing rates (Steiner 2001, Straub and Snyder 2006, Finke and Snyder 2008. For instance, Filip et al. (2012) demonstrated that altering species composition (and thus the degree of trait variation) in a microbial food ...

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