Protection Mutualisms and the Community: Geographic Variation in an Ant-Plant Symbiosis and the Consequences for Herbivores

Mutualistic interactions between species can strongly affect the structure of ecological communities (Stachowicz, 2001; Bruno et al. 2003; Hay et al., 2004). For example, animal seed dispersal can determine the composition of tropical-tree communities (Terborgh et al., 2008), and pollinator preferences can favor the spread of certain plant species (Chittka and Schürkens, 2001). Protection mutualisms, in which a mutualist defends its partner against natural enemies, are frequently shown to affect the growth and fitness of individual organisms, but very little is known about how these interactions structure the communities in which they are embedded. The paucity of such studies may give the false impression that the community effects of protection mutualisms are rare Abstract Protection mutualisms mediate trophic interactions in many systems, but their effects on the surrounding community are rarely studied. Ant-plant symbioses are classic examples of protection mutualisms: myrmecophytic plants provide nesting space and food for symbiotic ants in exchange for ant defense. Ant defense should thus reduce the abundance of herbivores, but studies of ant-plant symbioses usually measure damage to the plant without quantifying the herbivores themselves. In this study, we investigated whether geographic variation in the quality of ant defense in a symbiotic mutualism between Cordia alliodora trees and Azteca ants was associated with the abundance and species richness of plant herbivore communities. In three tropical-dry-forest sites in Middle America, we found that the density of Azteca ants within trees was negatively associated with the levels of leaf herbivory. At sites where ants were effective tree defenders, tree herbivores were less abundant and herbivore assemblages on trees exhibited lower species richness than at a site where ants were poor defenders. In addition, in a site where ants reduced herbivory, herbivore communities were less abundant and diverse in the presence of ants than in their absence, where as in a site where ants did not reduce herbivory, there were no differences in herbivore abundance or richness between trees with or without ants. We conclude that geographic variation in the quality of ant defense drives variation in myrmecophytic-plant herbivore communities. Moreover, ant-plant protection mutualisms should have important but rarely considered effects on herbivore population dynamics and food-plant specialization. Sociobiology An international journal on social insects