Fitness effects of symbiotic relationships among arthropod predators

Symbiotic relationships shape ecological communities and often involve more than two species. Yet few experimental studies examine the impact of symbioses involving three species, particularly any mediating role of third parties, and none involving symbioses of predators.

We investigated experimentally the synergistic and antagonistic fitness effects of three symbiotic spider predators across a broad latitudinal range and involving different species combinations.

The three-dimensional web-complex of Cyrtophora spiders is a habitat patch to different associates – species of Argyrodes and web-building Leucauge spider guests. Our field experiments, which manipulated the presence of each guest species and determined the subsequent host weight change, revealed a remarkable consistency in fitness outcomes across the three populations, with the consequences of the interactions between two species depending upon the services provided by a third.

Cyrtophora hosts intercepted more prey when web-building Leucauge guests were present and thus gained more weight. In contrast, Argyrodes guests exerted a fitness cost on their Cyrtophora host, but only when Leucauge guests were absent. A comparison of the prey consumed by Cyrtophora hosts and Argyrodes guests revealed that their diets (reflected in the size of prey) overlapped less in the presence of Leucauge web-building guests.

Our novel experimental study highlights the importance of exploring synergistic effects in multi-species symbioses.

Methods
The data were collected from field experiments that manipulated the presence/absence of Argyrodes and Leucage guests on the webs of their Cyrtophora host.  The data were analysed using the R platform.  Details can be found in the published paper describing these experiments.