Monkey and dung beetle activities influence soil seed bank structure

In Neotropical forests, dung beetles act as efficient secondary dispersers of seeds that are dispersed primarily by red howler monkeys. Here, we investigated the origins of soil seed bank variability in relation to monkey and dung beetle activity, to assess the impact of dung beetles on seed fate, and their adaptability to resource availability. This question is important to better understand the process of tree regeneration, and is especially timely in the current context of threat to primates. We characterized soil seed bank structures in sites differing in monkey frequentation, and conducted field experiments with artificial beads to monitor bead fate. We also conducted experiments on specific roller and tunneller beetle species to examine bead processing behavior and its variability among and within species. We found that seed number and diversity increased with monkey frequentation, but seed viability was optimal under moderate monkey frequentation. We showed for the first time that dung provisioning yielded higher beetle activity in sites more often visited by monkeys, which calls for further investigation to understand the mechanisms of attraction to resource and potential spatial structuration of beetle populations. Although all beetle species involved in the experiments actively excluded beads from dung reserves, selectivity was higher for small than large beetle species, and for large compared to small bead sizes. It also increased when per-capita dung resource decreased, suggesting that intraspecific competition could alter seed fate. Altogether, our results support a major role of dung beetles in soil seed bank structure and dynamics. They reveal interesting interspecific variability within the dung beetle community and a complex interplay with primary dispersal.