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.