Seed fate in ant-mediated dispersal: Seed dispersal effectiveness in the Ectatomma ruidum (Formicidae)—Zanthoxylum ekmanii (Rutaceae) system

Plants are often dispersal limited relying on passive or active agents to find suitable microhabitats for germination. Seeds of pioneer tree species, for example, require light gaps for growth but have short median dispersal distances and often do not provide a food reward to encourage animal dispersal. Zanthoxylum ekmanii seeds are frequently moved by ants but evaluating the effectiveness of ant-mediated seed removal requires knowledge of the species moving the seeds, how far they are moved, and the deposition site. To assess the effectiveness of ants as seed dispersers of Z. ekmanii, we utilized the seed dispersal effectiveness framework. We tracked the movement of seeds from caches on the forest floor, revealing that foragers of Ectatomma ruidum moved 32.8% of seeds an average first distance of 99.8 cm with 68.3% of those seeds taken into a colony. The quality of deposition location was assessed using a seedling emergence study where freshly germinated seeds were buried at different depths. Seedlings were primarily able to emerge from the shallowest depths. Wax castings of E. ruidum colonies demonstrated that seeds brought into the colony were deposited in chambers that had larvae present and experienced more damage than seeds unhandled by ants. Foragers, however, did not have a strong enough bite force to rupture Z. ekmanii seeds likely because their muscle morphology is not structured to maximize force generation. Overall, E. ruidum may help fine tune deposition location, incorporating seeds into the topsoil, though few seeds will likely emerge if soil bioturbation is low. Abstract in Spanish is available with online material.