Small-scale indirect eVects determine the outcome of a tripartite plant–disperser–granivore interaction

Abstract

The microhabitat in which plants grow aVects the outcome of their interactions with animals, particularly non-specialist consumers. Nevertheless, as most research on this topic has dealt with either mutualists or antagonists, little is known about the indirect eVects of plant microhabi- tats on the outcome of tripartite interactions involving plants and both mutualists (e.g. seed dispersers) and antag- onists (e.g. granivores). During three consecutive years, we analysed small-scale variations in the interaction of a perennial myrmecochore, Helleborus foetidus, with its seed dispersers and consumers as a function of the intensity of plant cover. Most seeds were released during the day and were rapidly removed by ants. Nevertheless, the proportion of ant-removed seeds was higher for plants located in open microhabitats than for plants surrounded by dense vegeta- tion and rocky cover. Ant sampling revealed that seed removers were equally abundant, irrespective of the level of cover. By contrast, a few tiny ant species that feed on the reward without transporting the seeds were more abundant in highly covered microhabitats, irrespective of hellebore diaspore availability. These “cheaters” decrease the chance of removal by removers and increase the probability of seeds remaining on the ground until night, when granivore mice Apodemus sylvaticus become active. Mice also pre- ferred foraging in covered microhabitats, where they con- sumed a larger proportion of seeds. Therefore, the density of cover indirectly increased seed predation risk by attract- ing more seed predators and cheater ants that contribute to increase seed availability for seed predators. Our results emphasize the importance of considering the indirect eVects of plant microhabitat on their dispersal success. They highlight the indirect eVect of cheaters that are likely to interfere in mutualisms and may lead to their collapse unless external factors such as spatio-temporal heterogene- ity in seed availability constrain their eVect.