Why do models of plant demography in fragmented landscapes fail to include missing mutualists and their modified movements?

Background/Question/Methods

Mutualisms play a central role in the origin and maintenance of plant diversity. Because many mutualisms have strong effects on plant demography, interspecific variation in benefits provided by different partner species could have important consequences for demography and population dynamics. Without information on the movement of these different partners, however, generalizations about their relative demographic impacts will continue to prove elusive. For instance, differences in how pollinators or seed dispersers move through space, their maximum dispersal distance, or tradeoffs between dispersal ability and the quality of benefits they provide could all have major consequences for plant demography. The importance of partner movement could be especially pronounced in fragmented landscapes, where any effects of differential movement will be compounded by inter-patch distances, the behavioral responses of partners to the matrix structure, and if the biotic and abiotic changes in fragments might alter the outcome of the interactions. I quantified how often studies of plant demography incorporated empirical data on mutualist movements. I then used data on the movement of ants that establish colonies in and defend plants against herbivores, and the demography of their host plants, to disentangle how variation in partner movement influenced plant demography in fragmented tropical forests.

Results/Conclusions

I found that few demographic studies incorporated data on partner movements. Similarly, few studies assessing the outcome of mutualistic interactions in fragmented landscapes quantified partner movement. Failing to consider inter-partner movement differences could lead to overly-optmistic conclusions regarding population persistence. Partner identity in these obligate mutualisms has strong but context-dependent effects on plant demography and population dynamics. In particular, the effects of inter-patch distance were highly dependent on the relative abundance of the different ant-partners because of the tradeoff between dispersal ability and the capacity to defend host plants against herbivores. These results underscore the importance of expanding the study of mutualisms beyond the study of pairwise interactions to consider (1) the demographic costs and benefits of interacting with different partners, (2) the different ecological contexts in which these interactions take place, and (3) the movement ability of different partners, and (4) how context-dependence of movement’s consequences.