Prey diversity and prey identity affect herbivore performance on different time scales in a long term aquatic food‐web experiment

Abstract

The question whether and how diversity‐mediated productivity at the base of food‐webs influences adjacent trophic levels is still unclear. Experiments revealed negative effects on consumers due to the increasing dominance of inedible species under grazing pressure, and positive effects due to a greater variety of prey resources. We experimentally investigate two more hypotheses, which have not been addressed in detail so far: first, more diverse primary producer communities potentially use limiting resources more efficiently, and are, therefore, more productive. This effect can be considered functionally similar to a direct enrichment with limiting resources, potentially resulting in a higher stochastic risk of herbivore extinction (‘paradox of enrichment’). Second, in a stable environment, enclosed primary producer communities should evolve towards a ‘climax state’, eventually dominated by one or few prey species. Therefore, long‐term diversity effects in producer communities should more likely result from the specific traits of the dominating species, than from complementarity. To address these hypotheses, we conducted long‐term laboratory experiments, exposing the freshwater grazer Daphnia magna to a gradient of algal species richness (1, 2, 4 or 8 edible chlorophyte species). The experiments were run in batch cultures, without exchange of growth medium after the start of the experiment. Six parameters related to Daphnia population demography, biomass accrual, and stability were followed and determined over a period of up to 263 days. Producer diversity exhibited strong positive effects on the short‐term performance of grazers (first reproduction, first population peak), and on grazer mean standing stocks. However, herbivore long‐term dynamics (day of extinction and temporal stability) depended on prey species identity, namely the presence of Chlamydomonas reinhardtii. Our experiments suggest that both prey diversity and identity can have positive effects on consumer performance, but act on different time scales.