The effects of producer diversity on community biomass and stability depend on herbivore identity in aquatic microcosms
Over the past several decades, an increasing amount of research has been dedicated to predicting how changes in biodiversity will alter the productivity and stability of ecosystems. However, the majority of this work has focused on a single trophic level, and the smaller number of studies investigating impacts of biodiversity loss on the functioning of more realistic food webs has produced mixed results. Studies suggest that herbivory, a key link between trophic levels, can be either decreased, increased, or unaffected by producer diversity, depending on the herbivore. However, whether such diversity-herbivory effects are important for community biomass and stability is unresolved. We performed an experiment in freshwater laboratory microcosms to test the relative importance of diversity-herbivory effects versus more traditionally studied diversity effects within producers. Our experiment crossed an algal diversity treatment (1 vs. 4 species) with an herbivore treatment (herbivore-free control, Daphnia pulex, or Ceriodaphnia dubia) to allow parsing of the effects and to find whether the results were consistent with different herbivores. We measured the biomass and temporal stability (mean/SD) of biomass of the algae and herbivores over four weeks.
The effects of algal species richness on the biomass and stability of both the algae and the herbivores were highly dependent on herbivore identity. In the absence of herbivores, algal diversity increased algal biomass and stability by 21% and 27%, respectively (the “within-producer” diversity effect). Since herbivory (grazing intensity) by C. dubia was unrelated to algal diversity, the C. dubia population was unaffected by diversity, and the within-producer diversity effect dominated in the presence of that herbivore: polycultures had a higher biomass and stability than monocultures by 12% and 52%. In contrast, algal diversity more than doubled the grazing intensity by D. pulex, leading to a larger and more stable population of D. pulex but lower algal biomass and stability by 43% and 47%, respectively in polycultures versus monocultures. Thus a differential effect of diversity on herbivory led to completely opposite effects of algal diversity on community biomass and stability. These findings indicate that diversity effects via trophic interactions can overwhelm effects within trophic levels, leading to qualitatively different relationships between diversity and community biomass and stability depending on the herbivores present.