On the need to evaluate costs and benefits of synzoochory for plant populations

Abstract

Many species interactions occur along a continuum from mutualistic to antagonistic. Among these interactions, the effects of seed-dispersing granivores (synzoochory) are especially interesting because the benefits and costs are so striking. Here, we respond to a commentary by C. M. Moore and J. W. Dittel (‘On mutualism, models, and masting: the effects of seed-dispersing animals on the plants they disperse’). We focus on the points that were most thought-provoking to us, specifically the notions that (a) synzoochory is widespread, and therefore ought to be beneficial for plants, (b) dispersal should reduce negative density dependence and (c) plant life spans are at least an order of magnitude longer than typical study lengths. We argue that (a) widespread occurrence of an interaction cannot serve as an argument that this interaction is mutualistic, (b) dispersal effects on density dependence are variable and need to be tested for every particular species pair and (c) Short-term studies can be used to gain mechanistic understanding of interactions even without risky long-term extrapolations. Synthesis. Evaluating the costs and benefits of synzoochory for plant populations under different ecological contexts will help to understand the ecological principles of the interaction but also allows to move the interaction towards each end of the antagonism–mutualism continuum in nature management and conservation.