Are ant-plant mutualisms stronger at lower latitudes? A case study using the castor bean plant

The Biotic Interactions Hypothesis (BIH) predicts that species interactions will intensify from the poles to the tropics. We studied the ant-plant defensive interaction mediated by extrafloral nectaries (EFNs) in the castor bean plant, Ricinus communis, across a ~ 21° latitudinal gradient in its naturalized range in India. Among our study sites, we found that the ant-EFN mutualism in castor bean gets stronger from sub-tropical to tropical latitudes. Investment in mutualism by plants and visitation by ants increased from higher to lower latitudes. Further, latitude significantly explained ant community composition. However, contrary to the predictions of the BIH, standing herbivory increased with latitude, perhaps because plants invest less in biotic defence at higher latitudes. To our knowledge, our study is the first to test for the patterns predicted by the BIH for an ant-plant interaction in the Paleotropics. Results from our study can also help inform sustainable pest control practices for R. communis since India is the world's largest producer of castor oil.