Keystone mutualism: A case study of ant-plant mutuailsm

Background/Question/Methods

Keystone interactions are species interactions that exert a disproportionate effect on the community in relation to their abundance.  We know much about predatory and competitive keystone interactions. However, less is known about whether a mutualism can function in this manner. We tested the interaction between tree cholla (Opuntia imbricata) and its mutualistic ant partner (Liometopum apiculatum) for effects on the ant community structure. We sampled the ant community across a tree cholla density gradient with the hypothesis that ant partner abundance and occurrence will increase with an increase with plant partner abundance and this keystone mutualism will result in a change in ant community composition. 

Results/Conclusions

Analysis revealed that tree cholla density influences the occurrence of the dominant mutualistic ant partner (L. apiculatum). An increase in plant partner density correlated with a greater presence of the ant partner. This suggests that the mutualism facilitates the occurrence of the ant partner. There was no clear effect of cactus density on ant partner abundance nor on community composition. Thus, in this case study we do not find a clear indication for a keystone mutualism. However, it is possible that considering this mutualism through time may yield interesting results on how mutualisms may influence community structure.