Abstract
Group living animals invariably risk resource competition. Cooperation in foraging, however, may benefit individuals in groups by facilitating an increase in dietary niche. To test this, we performed a comparative study of social and solitary spider species. Three independently derived social species of Stegodyphus (Eresidae) occupy semi-arid savannas and overlap with three solitary congeners. We estimated potential prey availability in the environment and prey acquisition by spiders in their capture webs. We calculated dietary niche width (prey size) and breadth (taxonomic range) to compare resource use for these six species, and investigated the relationships between group size and average individual capture web production, prey biomass intake rate and variance in biomass intake. Cooperative foraging increased dietary niche width and breadth by foraging opportunistically, including both larger prey and a wider taxonomic range of prey in the diet. Individual capture web production decreased with increasing group size, indicating energetic benefits of cooperation, and variance in individual intake rate was reduced. However, individual biomass intake also decreased with increasing group size. While cooperative foraging did not completely offset resource competition among group members, it may contribute to sustaining larger groups by reducing costs of web production, increasing the dietary niche and reducing the variance in prey capture.
Highlights
Inter-specific competition for resources plays an important role in the evolution of foraging adaptations and the ecological niche of individuals and species, and may drive adaptive speciation[1,2,3,4]
Withinindividual niche variation was higher in social compared to solitary species (WIC in Table 3) and the proportional variance as well was higher for social species (Table 3)
The available prey collected from traps in the vicinity of each observed nest showed similar prey size spectrum and did not differ significantly between micro-habitats occupied by nests of social and solitary spiders (TNW environment, Table 3; and additional results in the Supplementary analyses A)
Summary
Inter-specific competition for resources plays an important role in the evolution of foraging adaptations and the ecological niche of individuals and species, and may drive adaptive speciation[1,2,3,4]. There is good support for exploitation of larger prey, i.e. a wider dietary niche, in cooperatively foraging Anelosimus, which is associated with their occurrence in lowland tropical areas with higher frequency of large prey (18, 29, and references above) Whether this is a common pattern in other social spider species, remains unknown. We present results from a comprehensive, comparative study of six species of the Old-World genus Stegodyphus (Eresidae), in which we investigated effects of cooperative foraging on resource acquisition, as measured by the dietary niche width and breadth, and potential energetic benefits of silk saving in producing a group capture web. Using traps to estimate prey availability in the habitat, and determining the prey captured and exploited by spiders, we obtained quantitative estimates of dietary niche width (prey size) and breadth (taxonomic range) in relation to potential prey in the nearby habitat[1,19]
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