Agricultural legacies constrain germinant diversity on mounds of a seed-harvesting ant (Pogonomyrmex badius)
The foraging behavior of seed-harvesting ants can facilitate plant dispersal and recruitment, thereby impacting local patterns of plant diversity. However, the manner in which agricultural legacies alter the strength and direction of consumer effects on plant diversity is largely unknown. Here, we evaluate differences in the abundance and diversity of viable seeds deposited on mounds of seed-harvesting Pogonomyrmex badius among sites with differing land-use histories. We surveyed the diversity and abundance of vegetation surrounding Pogonomyrmex badius mounds at 26 remnant and post-agricultural sites located in longleaf pine savannas at Fort Bragg, North Carolina. At each site, we collected soil samples from a focal Pogonomyrmex badius mound and an adjacent undisturbed area. Soil samples were maintained under controlled greenhouse conditions for 12 months and surveyed regularly for germinants. All germinants were counted and identified by species. We compared the composition of mound germinants with that of the surrounding vegetation to evaluate the manner in which agricultural legacies can alter the effects of seed-harvesting ant foraging behavior on local patterns of plant diversity.
Vegetation surrounding Pogonomyrmex badius mounds differed substantially between the two land-use categories. Specifically, local plant species diversity and abundance were greater among remnant than post-agricultural sites. For both land-use categories, soil collected from Pogonomyrmex badius mounds yielded a greater density and diversity of germinants than nearby control samples. Across all sites, mound samples yielded more than 10,000 germinants divided among 75 plant species. Both the density and richness of mound germinants were similar between remnant and post-agricultural sites despite differences in vegetation surrounding the focal mounds. However, mound germinant composition differed substantially between the two land-use categories. At remnant sites, mounds largely yielded species indicative of undisturbed longleaf pine understory plant communities, whereas ruderal plant species indicative of disturbed habitats were more abundant among mounds at post-agricultural sites. Further, for each land-use category, mound germinant composition differed substantially from that of the surrounding vegetation, suggesting that Pogonomyrmex badius foraging behavior can affect local diversity patterns and that the nature of these effects can be contingent on historical land-use activities. Evaluating the role of historical land use in mediating consumer effects on diversity can provide novel insight into the factors shaping plant communities in human-modified landscapes.