Abstract
Vegetation composition is often dictated by grazing intensity in semiarid savannas; recovery following drought may depend on pre drought species composition. Nests of the red harvester ant, Pogonomyrmex barbatus, affect the dynamics, composition and recovery of post drought communities due to their larger size, greater seed production and higher perennial grass richness. We hypothesized that vegetation at ant nests would survive drought and recover faster than vegetation in the surrounding grasslands, but that individual and population recovery would depend on plant species composition, which, in turn, would be influenced by grazing intensity. We assessed nest influence on density, cover, number of inflorescences and dynamics of grass and forb species. Disk margins (area encircling the ant nest) were compared with grassland locations in unreplicated heavy, light and ungrazed treatments before, immediately after and one year after a severe drought. Significantly greater aerial and basal cover of grasses was found on disk margins compared to grasslands in each treatment. Grass cover and number of inflorescences increased faster on disk margins compared to grasslands. Fastest grass growth was seen on margins in the ungrazed treatment. There was greater diversity in ungrazed treatments of grazing intolerant mid-grasses compared to the grazed treatments, suggesting that mid-grasses may persist belowground, leading to faster productivity in the ungrazed treatment. Grass densities were generally higher and increased faster in grasslands resulting in smaller grasses compared to the large, more robust grasses on disk margins. Forbs showed significantly lower abundance and cover on margins compared to grasslands. These observations suggest that red harvester ant nests may serve as drought refugia for grass survival and a seed source for recovering grass populations after drought in semiarid savannas.
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