Seed preferences by ants and seed abundance determine associational effects between plant species in a semiarid grassland

Abstract

Plant populations in semi-arid ecosystems rely heavily on seeds to endure environmental uncertainty. Granivores, like ants, cause significant seed loss but can also have a positive impact. Most studies on this topic examine seed consumption in isolation, failing to consider how the presence of other seeds can influence granivory. However, in nature, seeds of different species share space, which can lead to "associational susceptibility" or "resistance" effects. These effects depend on ant preferences for both the focal and neighboring species. Preferred seeds can act as a "magnet," attracting ants and increasing the predation of nearby seeds. Conversely, less preferred seeds can have a "deterrent" effect. The magnitude of these associative effects may also depend on the preference of the focal seed itself: more desirable seeds will be less affected by neighboring seeds than less preferred ones. This study examined how the presence of heterospecific seeds affects ant seed removal in 28 plant species. We found that preferred seeds experienced a greater reduction in removal probability in the presence of other species. Additionally, plant population densities were positively correlated with the overall associative effect of neighboring seeds. This is the first study to report these indirect interactions between seeds affecting ant removal. Our results suggest that the impact of ants is a complex interaction of predation and dispersal, and their net effect on plant populations can be positive, even though seed removal is often considered a negative interaction.