COMPLEX ANT PLANT INTERACTIONS - RAIN-FOREST ANTS AS SECONDARY DISPERSERS AND POSTDISPERSAL SEED PREDATORS

We describe the role of leaf-litter ants (Pheidole spp.) in the seed bank dynamics of several small-seeded shrubs in a Costa Rican lowland rain forest. These ants harvest seeds from frugivore feces and cache them in their nests in partially decomposed twigs. Most harvested seeds are eaten but some are placed in viable condition on refuse piles. From 24 to 38% of the colonies (depending on ant species) contain cached seeds and 25-32% have seeds on refuse piles. Experiments with captive colonies of Pheidole nebulosa and P. nigricula demonstrated that almost-equal-to 6% of harvested Miconia nervosa and M. centrodesma seeds are deposited on refuse piles. Because seeds generally retain viability longer than the nest twig remains intact, harvested seeds are not trapped inside twigs. Experimental plantings of 4-d-old Miconia nervosa seedlings on two types of substrate (ant refuse pile vs. topsoil) under two light levels (equivalent to small and large clearings) demonstrated that seedlings grew faster and survived better on refuse piles under light levels typical of small clearings. Light levels typical of large gaps are not necessarily advantageous for establishment of understory rain forest plants. These results illustrate the dynamic nature of a tropical soil seed bank and the complexity of plant-animal interactions that occur there. Ants are simultaneously antagonistic and mutualistic towards seeds, killing most but significantly benefiting some. This interaction is extremely common (Pheidole density > 300 individuals/m2) and likely influences plant recruitment patterns. Our results challenge the generalizations that small seeds are largely protected from predation because of their size, that post-dispersal seed harvesting is equivalent to seed predation, that competition among seedlings from a frugivore defecation is common, and that small seeds accumulate over long periods of time in the seed bank. We hypothesize that differential harvesting and treatment of seeds by ant species may be a mechanism underlying community-level patterns of regeneration in small-seeded plants. Also, the incidental benefits to uneaten seeds deposited on refuse piles provide additional evidence that myrmecochory evolved in plants that strengthened rare but consistent benefits of being harvested by ''granivorous'' ants.