Variation in the susceptibility of species to local extinction alters the effect of predator biodiversity on prey suppression

Background/Question/Methods

Anthropogenic activity is causing our planet’s biodiversity to decline at an alarming rate and it is essential to understand how this loss affects the functioning and stability of natural systems if society is interested in conserving or restoring ecosystems and species.  There is much evidence to suggest that important ecosystem processes, like primary productivity, vary with the number of plant species present, and an increasing amount of work suggests that a change in the number of predator species present can also have an important effect on the ability of predators to suppress their prey.  Nearly all of the existing studies, however, have examined the effects of random patterns of species loss.  Species loss in nature, however, is not random and more effort needs to be directed toward understanding how different non-random patterns of species loss affect ecological processes.  We conducted an experiment in artificial ponds to examine how non-random species loss from a guild of predatory insects found in temporary ponds affects the ability of the predators to suppress population size of their prey, tadpoles of the squirrel treefrog.

Results/Conclusions

We found that fewer prey survive when more predator species are present than when fewer species are present.  The effects of predator species loss on prey suppression depended on whether predator species remaining in the foodweb numerically increased in abundance to compensate for the lost species.  On average, the effect of predator species loss on prey suppression was greater when numerical compensation of remaining predators did not occur.  The effect of species loss on prey suppression also depended on the pattern of predator species loss .  Reducing the number of predator species by 50% had no effect on the ability of the predator assemblage to suppress prey when phylogenetic diversity (number of Orders) of predators was not altered.  Reductions in the number of predator species that were associated with the preferential loss of a taxonomic lineage (i.e., reducing phylogenetic diversity) or with the preferential loss of less abundant taxa adversely affected the ability of the predator assemblage to suppress prey.   The results reported here demonstrate the importance of predator species richness for suppressing prey population size, but the extent to which prey suppression is reduced depends on the particular pattern (i.e., which species are more predisposed to local extinction) of species loss.