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Recent evidence suggests that the traditional view of myrmecochory as a highly diffuse interaction between diaspores and a wide range of ant species attracted to their elaiosomes may not be correct. The effectiveness of dispersal varies markedly among ant species, and combined with differential attractiveness of diaspores due to elaiosome size and composition, this raises the potential for myrmecochorous plants to target ant species that offer the highest quality dispersal services. We ask the question: Do particular physical and chemical traits of elaiosomes result in disproportionate removal of Euphorbiaceae diaspores by high-quality disperser ants in Caatinga vegetation of north-eastern Brazil? We offered seeds of five euphorb species that varied in morphological and chemical traits of elaiosomes to seed-dispersing ants. High-quality seed-disperser ants (species of Dinoponera, Ectatomma and Camponotus) were identified as those that rapidly collected and transported diaspores to their nests, often over substantial distances, whereas low-quality disperser ants (primarily species of Pheidole and Solenopsis) typically fed on elaiosomes in situ, and only ever transported diaspores very short distances. Low-quality disperser ants were equally attracted to the elaiosomes of all study species. However, high-quality dispersers showed a strong preference for diaspores with the highest elaiosome mass (and especially proportional mass). As far as we are aware, this is the first study to identify a mechanism of diaspore selection by high-quality ant dispersers based on elaiosome traits under field conditions. Our findings suggest that myrmecochorous plants can preferentially target high-quality seed-disperser ants through the evolution of particular elaiosome traits.

Anthropogenic disturbance can have important indirect effects on ecosystems by disrupting species interactions. Here we examine the effects of anthropogenic disturbance on distance dispersal by ants for the diaspores of myrmecochorous Euphorbiaceae in Brazilian Caatinga. Rates of diaspore removal and distances removed of Croton sonderianus and Jatropha mollissima were observed at 24 sites ranging from low to very high disturbance (primarily grazing by livestock, hunting and firewood collection). Despite a large number of seed-disperser ant species, there were only two species providing high-quality distance-dispersal services, Dinoponera quadriceps (40% of all observed seed removals) and Ectatomma muticum (33%). D. quadriceps was responsible for 97% of all removals >2 m, and 100% of all removals >5 m. Removal rates did not vary with disturbance for C. sonderianus (small elaiosome), but declined with increasing disturbance for J. mollissima (large elaiosome). The number of removals by Ectatomma was highest at intermediate levels of disturbance, whereas those by Dinoponera decreased systematically with increasing levels of disturbance. Mean dispersal distance was four times higher at sites experiencing low disturbance, where removals >5 m represented a third of all removal events, compared with very highly disturbed sites, where no removals >5 m were observed. Despite high overall diversity there is very limited functional redundancy in disperser ant species, resulting in low disperser resilience in relation to disturbance. This is likely to have important implications for recruitment by myrmecochorous plants, and therefore on vegetation composition and structure, at sites subject to high anthropogenic disturbance.

Although seed dispersal by ants might reduce seed predation near the parent plants, predation on discarded seeds clustered on nest refuse piles may reduce any initial benefit provided by seed removal. Here we examine the fate of Croton sonderianus seeds that were discarded by Pheidole fallax Mayr ants on their nest refuses in caatinga vegetation of northeast Brazil. We collected all seeds discarded in refuse piles of 20 nests and within a radius of 50 cm from their borders, and examined them for evidence of predation. A total of 3,017 seeds were recorded either located in the P. fallax refuse piles (89.1%) or nest vicinity (10.9%). Predation was three fold higher in nest vicinity as compared to refuse piles. By removing seeds from beneath parent plants and relocating then to refuse piles, P. fallax is possibly providing double protection services for C. sonderianus seeds. Our findings represent the first evidence for predator-avoidance as benefit for plants resulting from ant seed-dispersal in the neotropics.