Academic journal article The American Midland Naturalist

Harvester Ant Nests Improve Recovery Performance of Drought Impacted Vegetation in Grazing Regimes of Semiarid Savanna, Texas

Academic journal article The American Midland Naturalist

Harvester Ant Nests Improve Recovery Performance of Drought Impacted Vegetation in Grazing Regimes of Semiarid Savanna, Texas

Article excerpt

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.

INTRODUCTION

Ants are well documented as influential agents of soil change. As ecosystem engineers, they create nest patches altering resource availability for plant species. Harvester ant nest patches often increase plant diversity and performance (MacMahon et al, 2000). The genus Pogonomyrmex contains species of harvester ants that create bare soil disks approximately 1 m diameter by clearing all vegetation (Rissing, 1988; Whitford and DiMarco, 1995; Wagner, 1997; Gordon, 1999; MacMahon et al, 2000). Plants encircling the disk, however, tend to be larger and produce more seeds than plants in surrounding habitats (Golley and Gentry, 1964; Wight and Nichols, 1966; Rissing, 1986; Whitford, 1988; Soule and Knapp, 1996). Vegetation composition around the disks often differs from adjacent areas: shrub and perennial forb richness is lower, whereas grass richness is higher (Rissing, 1986; Soule and Knapp, 1996; Lei, 1999). Although most studies of vegetation at the edge of disks focus on structure and composition, the disk and the plants at its margin are a unique patch type that may have a distinctive ecosystem function: to provide refugia and accelerate recovery during and following environmental stresses.

At landscape scales, vegetation structure is driven by large scale factors. Before European settlement, natural herbivory, periodic fires and weather variability were major factors in maintaining the liveoak savanna parkland of the Edwards Plateau of Texas (Smeins and Merrill, 1988; Fuhlendorf et al, 2001). With settlement the suppression of fire, demise of large, free roaming herbivores and the introduction of confined grazing by livestock transformed the structure of these parklands to woodlands (Smeins and Merrill, 1988; Fuhlendorf and Smeins, 1997). Confined, intense grazing altered the structure and composition of the grassland community to early-successional, short grasses with a low diversity of forbs and low total cover (Smeins and Merrill, 1988; Briske and Hendrickson, 1998; Fuhlendorf et al, 2001). …

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