Ant Mutualism Increases Long-Term Growth and Survival of a Common Amazonian Tree

Abstract

How ecological context shapes mutualistic relationships remains poorly understood. We combined long-term tree census data with ant censuses in a permanent 25-ha Amazonian forest dynamics plot to evaluate the effect of the mutualistic ant Myrmelachista schumanni (Formicinae) on the growth and survival of the common Amazonian tree Duroia hirsuta (Rubiaceae), considering its interactions with tree growth, population structure, and habitat. We found that the mutualist ant more than doubled tree relative growth rates and increased odds of survival. However, host tree size and density of conspecific neighbors modified the effect of the ant. Smaller trees hosting the mutualist ant consistently grew faster when surrounded by higher densities of conspecifics, suggesting that the benefit to the tree outweighs any negative effects of high conspecific densities. Moreover, our findings suggest that the benefit afforded by the ant diminishes with plant age and also depends on the density of conspecific neighbors. We provide the first long-term large-scale evidence of how mutualism affects the population biology of an Amazonian tree species.

The Impact of Mutualisms on Species Richness

“…A good example is the mutualism between the common Amazonian tree Duroia hirsuta and Myrmelachista schumanni ants, which kill all vegetation other than their host plants in so-called devil's gardens. An 18 year study revealed that ants significantly increase the growth and survival of this common tree [68], thus negatively affecting species coexistence. This acts as a 'destabilizing' mechanism by making the mutualist competitively superior.…”

Does ant–plant mutualism have spillover effects on the non‐partner ant community?

“…Despite the potential importance of EFN for ants, the majority of ant-plant systems are studied unilaterally from the plant's perspective (Bronstein, 1994;Lanan & Bronstein, 2013). There is strong evidence for ant partners reducing herbivory (Rudgers et al, 2010) and herbivore diversity (Pringle & Gordon, 2013), and enhancing nutrient availability (Mayer et al, 2014;Wagner & Fleur Nicklen, 2010), growth, and survival (Báez et al, 2016), all of which benefit the plant partners. Much less is known about the consequences of ant-plant mutualism from the ant's perspective (but see Byk &Del-Claro, 2011 andBronstein, 2013).…”