Seedling establishment is site limited in perennial grasslands except when disturbances such as drought create gaps in grass cover. This is when seed harvest by granivores may play an important role in seedling recruitment. Seed limitation occurs by post-dispersal seed predators resulting in lowered plant populations and altered community structure as foragers differentially harvest seeds. We hypothesized that seed removal by the granivore Pogonomyrmex barbatus (red harvester ant) would reduce densities of herbaceous species’ seedlings directly after a severe drought in a semiarid savanna. To assess seedling recruitment affected by P. barbatus foragers, an ant-protected exclosure study was established in two colony densities, low and high. A pair of ant-protected and open arenas was established at 20 ant nests during the end of the drought and was sampled for seedling density immediately after average rainfall returned and one year later during spring and fall plant recruitment.
Results/Conclusions
Perennial dicot seedlings were more abundant in exclosures compared to open arenas at all three sampling periods. During spring when annual dicot recruitment was highest, seedling density was significantly higher in exclosures compared to open arenas. In contrast, during summer when most perennial grass seedlings are recruited, they were significantly higher in open arenas than exclosures. Few differences were observed between low and high colony density likely because P. barbatus colonies protect their foraging areas. Our results show that seed harvest by ants modify seedling establishment in temporally different ways. While seed limitation by P. barbatus was important in dicot seedling recruitment following drought, it was especially important in reducing annual dicot recruitment in the spring. Consequently, competition was reduced between annual dicots and perennial grass seedlings resulting in indirect facilitation by ants next summer. Harvester ants can play an important role in plant community dynamics when disturbance alleviates site limitation for seedling establishment.
