A help or a hindrance? Consequences of the exploitation of ant-acacia mutualisms by orb-weaver spiders
Mutualism is a reciprocal interaction in which each of two species consumes a resource (material rewards or services) that the other provides. Mutualisms are often exploited by other species that consume one of the traded resources without providing any reward or service in return. The consequences of such third-party exploitation to the stability and viability of mutualisms have been considered theoretically, but empirical data are quite limited. We investigated the consequences of the exploitation of two closely-related ant-acacia mutualisms in central Panama by two sister species of orb-weaver spiders (Eustala oblonga and E. illicita). These spiders inhabit Acacia melanocerus and A. collinsii, respectively. The spiders do not consume the food rewards produced by the acacias nor do they capture the plant-guarding, mutualist ant workers; however, the spiders exploit the plant-guarding services of the ants for protection from their own natural enemies. We tested two non-mutually exclusive hypotheses to assess what effects the spiders may have on the ant-acacia mutualisms: 1) the spiders benefit the ant-acacia mutualisms by capturing flying herbivorous insects in their webs, and 2) the spiders harm the ant-acacia mutualisms by capturing winged virgin females and males as they disperse on nuptial flights.
1,726 flying insects were passively caught on sticky card traps placed in A. melanocerus and A. collinsii for 48-h periods in July and August 2014. 38.8% of insects trapped on A. melanocerus were strictly herbivorous (mostly leaf beetles [Chrysomelidae, 12.8%] and leafhoppers [Cicadellidae, 17.9%]. A greater percentage (70.6%) of insects trapped on A. collinsii was strictly herbivorous (again, mostly leaf beetles [28.4%] and leafhoppers [34.4%]). No alate ants of either species of plant-guarding ant mutualist were caught on sticky card traps. Analysis of 88 prey items collected from the webs of E. oblonga and E. illicita showed that 26.3% and 20.0%, respectively, of the insects the spiders actually captured were herbivores. However, alates of the plant-guarding ant mutualists also made up large percentages of prey (15.8% in E. oblonga webs and 42.0% in E. illicita webs). These results support both hypotheses: the two spider species potentially intercept insect herbivores, but they also capture dispersing virgin male and female ants. Because the ant defense is so effective, however, the cost of potentially hindering the establishment of new ant colonies may outweigh any plant protection by the spiders.