The effects of mutualist partner identity on plant demography are context-dependen

Background/Question/Methods

Mutualisms play a central role in the origin and maintenance of biodiversity. Because many mutualisms have strong demographic effects, interspecific variation in partner quality could have important consequences for population dynamics. Nevertheless, few studies have quantified how a mutualist partner influences population growth rates, the demographic impacts of multiple partner species, and how context-dependent variation in the dynamics of mutualisms alters demographic trends. Mutualisms between ants and specialized plants known as myrmecophytes are a defining feature of tropical forests, with plants from over 100 genera having leaf pouches, swollen petioles, hollow stems, or other 'domatia' in which a suite of obligately associated ant species establish colonies. Resident ants defend their host plants from herbivores or competitors, and the loss of ant colonies can results in severe defoliation or plant death. We used integral projection models parameterized with three years of census data to compare the demographic effects of two ant species – Crematogaster laevis and Pheidole minutula – on populations of the Amazonian ant-plant Maieta guianensis.

Results/Conclusions

Ant identity has a strong and positive effect on plant population growth rate (i.e., lambda was always >1), but which species had the largest positive effect on lambda varied by habitat. In plots near streams, lambda was 5-15% greater for when Maieta populations were colonized by Pheidole minutula, irrespective of the levels of canopy disturbance . In contrast, Maieta populations in uplands colonized by Crematogaster had population growth rates 30% greater than those colonized by Pheidole, with the effects of canopy disturbance again being negligible. Our results suggest partner identity in obligate mutualisms has strong but context-dependent effects on plant demography and population dynamics. These results underscore the importance of expanding the study of mutualisms beyond the study of pair-wise interactions to consider the demographic costs and benefits of interacting with different partners and ecological contexts.