Experimental reduction of plant abundance changes interaction frequency of a tri‐trophic micro‐food web: contrasting responses of generalists and specialists

Abstract

Abstract Species abundance is vulnerable to climate change and anthropogenic impact. Although numerous studies have examined the food web response to species loss, their response (e.g. in network topology and interaction frequency) to changes in species abundance has received little attention. We experimentally reduced the abundance (by c. 60%) of one host plant species (Saussurea nigrescens) in a tri‐trophic micro‐food web consisting of two common Asteraceae species (S. nigrescens and Anaphalis flavescens), their pre‐dispersal seed predators (tephritid flies) and the parasitoid wasps that feed on these seed predators. The neutral process hypothesis posits that network topology and interaction frequency are determined by the relative abundances of the participating species. Accordingly, we hypothesized that interaction frequency (indicated by the rate of flies infesting plants and of parasitoid wasps parasitizing flies) would decrease with decreasing abundance of host plant abundance. Consistent with the neutral process hypothesis, abundance reduction significantly decreased the rate of generalist flies infesting the manipulated plant host species, but increased the rate on unmanipulated plant species. In contrast, abundance reduction significantly increased the rate of specialist flies infesting the manipulated species, but it had no demonstrable effect on the specialists on the unmanipulated plant host. Moreover, abundance reduction significantly increased the rate of parasitoid wasps (as a species group) parasitizing flies in the manipulated species, but decreased the rate of the unmanipulated species. These results were not attributable to neutral processes, but can be explained by adaptive foraging theory. In addition, experimental manipulation did not shift the qualitative presence–absence of the linkages in the micro‐food web, and the change in the abundance of both fly and wasp species was smaller than the abundance changes of the manipulated plant species. Synthesis. Our results indicate that both neutral processes and/or adaptive foraging likely contributed to food web stability in responses to changes in species abundance. Because species abundance is vulnerable to abiotic and biotic environmental changes, we suggest more research should be conducted to understand how food webs respond to climate change and anthropogenic disturbance.