Plant Killing by Mutualistic Ants Increases the Density of Host Species Seedlings in the Dry Forest of Costa Rica

Some species of plant-mutualistic ants kill the vegetation growing in the vicinities of their host plant, creating an area of bare ground (clearing). The reduced competition in the clearing may facilitate the establishment of host species sprouts (clones and seedlings), which in turn benefits the ants with additional food and shelter (“sprout-establishment hypothesis”). To test this hypothesis, the density and origin ofAcacia collinsiisprouts growing inside clearings and in the vicinities of acacia plants without clearings were compared. Also, to assess the pruning selectivity of acacia ants (Pseudomyrmex spinicola), seedlings were transplanted into clearings. The reaction of ants towards unrewarding acacia seedlings (without food and shelter) was also tested. The density of acacia sprouts growing inside clearings was almost twice that in the vicinities of host plants without clearings, and sprouts were inhabited by nestmates of the colony that made the clearing. Clones and seedlings were found in similar proportions in the clearings, and ants did not kill unrewarding acacia seedlings or seedlings unrelated to their host. The benefit reported here for the ants could be in conflict with the host plant, especially when the plant has rhizomal reproduction.

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 forAcacia collinsiiand 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 collinsiiedge, 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 fallaxandEctatomma ruidum, in combination being responsible for 92% of all observed seed removals. The abundance ofP. fallaxandE. ruidumdiffered among habitats, withE. ruidumbeing the most abundant ant species collected in the open-pasture habitat andP. fallaxbeing the most abundant in the edge habitats.Pheidole fallaxants dispersed seeds significantly further (mean ± SD = 2.11 ± 1.35 m, maximum = 5.2 m) than didE. 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.