Fitness effects of symbiotic relationships among arthropod predators

<p>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.</p> <p>We investigated experimentally the synergistic and antagonistic fitness effects of three symbiotic spider predators across a broad latitudinal range and involving different species combinations.</p> <p>The three-dimensional web-complex of <em>Cyrtophora</em> spiders is a habitat patch to different associates – species of <em>Argyrodes</em> and web-building <em>Leucauge</em> 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.</p> <p><em>Cyrtophora</em> hosts intercepted more prey when web-building <em>Leucauge</em> guests were present and thus gained more weight. In contrast, <em>Argyrodes</em> guests exerted a fitness cost on their <em>Cyrtophora</em> host, but only when <em>Leucauge</em> guests were absent. A comparison of the prey consumed by <em>Cyrtophora</em> hosts and <em>Argyrodes</em> guests revealed that their diets (reflected in the size of prey) overlapped less in the presence of <em>Leucauge</em> web-building guests.</p> <p>Our novel experimental study highlights the importance of exploring synergistic effects in multi-species symbioses.</p>