Measuring the net outcome of conditional mutualism: a case study with oaks and scatterhoarding rodents

Numerous interactions between plants and animals vary in their outcome between antagonism and mutualism, but it has proven to be difficult to quantify their final outcome. Interactions between plants and scatterhoarding animals provide a prime example of this phenomenon. Scatterhoarders consume large quantities of seeds (potentially reducing plant establishment), yet also disperse seeds and bury them in shallow caches (potentially improving recruitment). However, it has been rarely determined which role prevails for particular plant species. We demonstrate how plant-scatterhoarder interactions can be placed at the antagonism-mutualism continuum, with interactions between rodents and two oaks species (sessile oak Quercus petraea, and red oak Q. rubra) as an empirical example. Our approach consists of quantifying the net outcome of the interaction through assembling different vital rates (e.g. probability of seedling recruitment with and without rodents; near and far from conspecific trees; with and without seed pilferage) piecewise with a simple mathematical model. Our results indicate that during the period of the study, interactions between scatterhoarding rodents and both focal oaks were antagonistic. Even though acorn burial increased the likelihood of seedling establishment, this effect was not strong enough to compensate for the costs of seed predation. Furthermore, we found no evidence that the short-distance transportation that is usually provided by small mammals benefited early oak recruitment. Our study demonstrates how readily accessible field data can be used to gauge the outcomes in conditional mutualisms.