Effectiveness of seed dispersal by ants in a Neotropical tree

Seed dispersal and subsequent recruitment is the template on which forest regeneration takes place. Hence, considering the scale over which ecological processes occur is key for understanding the overall impact of various dispersal agents. To explore leafcutter ant (Atta colombica) dispersal effectiveness in space and time, seed movement and subsequent recruitment of a large-seeded predominately vertebrate-dispersed tree, Simarouba amara (Aubl. Simaroubaceae), was investigated on Barro Colorado Island, Panama. At each of 218 reproductive-sized adults (≥20 cm diameter at breast height), presence or absence of a leafcutter ant colony was noted, with extensive checks for Atta activity taking place at or in close proximity to seed and seedling transects, which extended 4 cardinal directions for 30 m from each reproductive female tree (n= 74). Only at 2 S. amara trees were nests observed, and in these areas a dense S. amara seedling carpet was observed. Although nearby nest and dump sites might increase local S. amara recruitment in the short term, mortality at these sites is complete or nearly so. Hence, the seed dispersal effectiveness by leafcutter ants appears to be ephemeral and likely contributes inconsequentially to the long-term recruitment and distribution patterns of the species. This finding highlights the importance of evaluating disperser effectiveness at ecologically relevant spatiotemporal scales.