Biodiversity is often valued due its potential of insuring stable functioning of natural ecosystems. Theory suggests that increases in functional diversity should confer greater system-level resilience - ability to recover following a perturbation - as compared to taxonomic diversity. We test this idea experimentally in a closed lab system consisting of algae (a mix of several species) consumed by zooplankton (Daphnia magna), snails (Physa sp.), or both consumers representing, together, three functional, or trait-defined, groups. We perturbed this simple system by spiking cultures with nutrients - a eutrophication event – to stimulate rapid increase in algal concentrations. We expected dynamic responses to be moderated by consumers; more so where two consumer species were present. We examined sediment load, algal concentrations, and ammonium concentration as gauges of resilience. To measure resilience, we evaluated the percent difference between an undisturbed control and disturbed treatment, both in maximum values and in concentrations three weeks after the perturbation event.
Results/Conclusions
We found that only Daphnia magna had positive and significant impact on our measure of resilience. However, this effect was not consistent across ecosystem measures. Additionally, we found changes in the algal community that appear to represent significant differences in response to functional group treatment (Daphnia, Physa, both). These differences were present regardless of perturbation treatment and their magnitude did not dependent on the perturbation. Further, we have some evidence of shifting reproductive strategy in Daphnia magna in response to disturbance and in response in Physa sp. to the presence of the filter feeder, Daphnia. These shifts correspond to developing differences in density in Daphnia magna in the course of the experiment, and suggest a feedback loops between productivity, filter feeders and system level recovery. While these results fail linking diversity of trophic species to ecosystem resilience, they point to complexities and difficulties in establishing such a link: Indirect effects of one species on reproduction of another and different scales of response among components of the system, are just two examples of other dimensions that may be general, confounding, and thus equally important.