Keystone predators, by reducing the abundance of competitively superior prey, may have indirect positive effects on weak competitors, possibly increasing their abundance or preventing local competitive exclusion. By analogy to the Intermediate Disturbance Hypothesis, we would expect species diversity to peak at intermediate predator densities. In a replicated artificial pool experiment, we examined the relationships between density of the backswimmer Notonecta maculata (0, 1, 2, and 4 per 30 l pool) and invertebrate taxon diversity over an 11-week period of predator occupancy. Diversity reached high levels at high predator density sooner than at intermediate density. At the end of the experiment, taxon diversity was greatest at densities of 2 and 4 Notonecta per pool. While the overall predator density-diversity curve was in line with the intermediate disturbance hypothesis, the reduction in diversity from intermediate to high predator density was not statistically significant. Density of the preferred prey Daphnia magna decreased with Notonecta density, while densities of the smaller cladocerans Moina brachiata and Ceriodaphnia spp. increased. Suppression of Daphnia at high Notonecta densities may partially explain the increase in Moina and Ceriodaphnia densities. However, most of the relationship between Notonecta and the smaller cladocerans appears to be independent of Daphnia, suggesting complex interactions within the community. Our results suggest that keystone predation plays a strong role in structuring this community. Although diversity did not decrease significantly at the highest predator density as predicted, such a decrease may be more likely for pools with longer durations of predator occupancy or with higher predator densities.
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Acknowledgments We thank Jamie Kneitel, two anonymous reviewers and the subject editor for critical comments on the manuscript, Jonathan Chase, Moshe Kiflawi, and Marc Mangel for fruitful discussion, Julia Vider for statistical consultation, and Kay Van Damme and Koen Martens for taxonomic assistance. This study was supported by a Vataat Postdoctoral Fellowship awarded to Avi Eitam, and US-Israel Binational Science Foundation grant 98-390 awarded to Leon Blaustein and Marc Mangel.
- Intermediate disturbance hypothesis
- Keystone predation
- Predator-mediated coexistence
- Species diversity
ASJC Scopus subject areas
- Aquatic Science