Effects of algal and herbivore diversity on the partitioning of biomass within and among trophic levels

Factorial manipulations of biodiversity on different trophic levels can reveal the mechanisms that govern how biomass is partitioned within and among trophic levels. Here I report results of a laboratory microcosm experiment crossing eight algal compositions (seven monocultures and a polyculture of all seven species) with five ciliate herbivore compositions (no herbivores, three monocultures, and a polyculture of all three species). Microcosms allowed comparison of short-, medium-, and long-term (dozens of generations) results. Niche differentiation allowed competing algae to coexist in the absence of herbivores. Surprisingly, algal polycultures produced more biomass than the most productive monocultures only in the medium term. A temporally variable relationship between the productivity of diverse and depauparate algal assemblages might reflect changes in competitive mechanisms over time, and/or transient dynamics, and contrasts with the predictions of simple competition models. Herbivores had little effect on total algal biomass or algal composition, in part because of the lack of a strong trade-off between algal competitive ability and edibility. Increasing algal diversity significantly reduced the biomass of one herbivore, apparently because competition from inedible algae reduced the productivity of edible algae. A negative effect of inedible algae on herbivore biomass accords with simple food web theory and indicates that increasing diversity on one trophic level can have negative consequences for other trophic levels. Herbivores varied little in their abilities to graze different algae. Total algal biomass, algal composition, and total herbivore biomass were independent of herbivore diversity and composition, and algal composition had only minor effects the outcome of competition among herbivores. However, the outcome of herbivore competition was predictably asymmetrical. indicating that herbivores varied in other ways, such as ability to consume alternative food sources (bacteria). Further factorial manipulations of diversity and trophic structure are needed to improve understanding of the likely consequences of species loss in food webs. These experiments should last as long as possible, since the relationship between biodiversity and ecosystem properties may depend as much on when the relationship is measured as on species identity and the degree of niche differentiation among species.