Effects of habitat disturbance on ant community composition and seed dispersal by ants in a tropical dry forest in Costa Rica

Habitat alteration can have far-reaching consequences for natural communities and can alter existing species interactions in profound ways. Working in a tropical dry forest ecosystem in the Guanacaste Province of Costa Rica, we measured seed removals and seed dispersal distances for Acacia collinsii and papaya seeds to determine if habitat disturbance affects ant community composition and the associated interactions between ants and seeds. Two hundred and forty experimental seed depots were observed in four sites that differed in land-use history and disturbance severity: secondary forest, forest edge, Acacia collinsii edge, and open-pasture. Both seed removals and average dispersal distances achieved by ants differed among habitats. Habitat disturbance did not negatively affect seed removals by ants in our study; seed removals were highest in the most disturbed habitat, the open pasture (38.3% of seeds removed within 2 h). Ant community composition and the relative abundance of key seed dispersers also differed among habitats. In all sites combined, ten species of ant were observed dispersing seeds, with two species, Pheidole fallax and Ectatomma ruidum, in combination being responsible for 92%) of all observed seed removals. The abundance of P. fallax and E. ruidum differed among habitats, with E. ruidum being the most abundant ant species collected in the open-pasture habitat and P. fallax being the most abundant in the edge habitats. Pheidole fallax ants dispersed seeds significantly further (mean +/- SD = 2.11 +/- 1.35 m, maximum = 5.2 m) than did E. ruidum (mean +/- SD = 0.7 +/- 0.81 m, maximum = 3.85 m), a difference we attribute to recruitment behaviour and not body size. Habitat disturbance thus alters the ant community and the relative abundance of key seed-dispersing ant species, with cascading effects on seed removals and seed dispersal distances.