Ant diversity and body size patterns across an altitudinal gradient in the Drakensberg Mountains, South Africa

Abstract

Patterns of invertebrate species richness and body size across elevational gradients have been well-documented in a variety of studies. However, very little is known about the factors that govern these patterns along elevational gradients. A number of studies have have reported a monotonic decrease in species richness with increasing elevation; while others have observed a hump-shaped pattern, where the peak of species richness occurs at an intermediate elevation. For body size, studies have recorded larger size at higher elevations; some smaller and some have recorded no change across elevational gradients. Even though these patterns have been well-documented, the majority of them did not study invertebrates. This is surprising given that invertebrates such as ants are widely regarded as powerful monitoring tools in environmental management because they are abundant, diverse, easy to sample, sensitive to perturbation and they can indicate long-term general ecosystem change. Elevational gradients are isolated fragmented, spatially complex, comprise harsh environmental conditions and often retain comparatively intact habitats. The steep elevational gradients enable species to track climatic changes over short distances. Temperature varies across elevational gradients and variation in rates of growth and development at different temperatures may lead in differences in the mean body size of an insect species along elevational gradients. Body size can also be associated with species range size and geographic patterns of distribution and diversity. Ant diversity and body size patterns were investigated using data collected during March and November 2011 across an elevational gradient at Mariepskop. Pitfall traps were used to sample ants at five elevational sites and environmental variables were also collected at each sampling site. Body size was measured for some of the individuals selected from all ant species collected and simple regressions were used to examine altitudinal body size patterns within and across species. A total of 92 ant species were collected from 30 genera. Ant species density decreased monotonically with increasing elevation and abundance also declined as elevation increased. Altitude, vegetation complexity, height of grass, proportion of bare ground and clay in the soil were the five environmental variables that contributed significantly to the variance explained in the ant assemblages. Body size increased with altitude within species for Plagiolepis sp. 45, Pheidole sp. 1, Monomorium sp. 12 and Tetramorium sp. 3. No body size relationship was established across species when using the across species method whereas the Stevens method indicates a decrease in body size with elevation. A number of factors are responsible for structuring ant assemblages and body size along elevational gradients and it is not easy to establish to what extent each of the factors is responsible for the variation. Factors such as availability of resources, starvation resistance, accelerated maturation and adaptation to local environment are associated with positive relationship between body size and altitude. Related mechanisms such as desiccation resistance, metabolic rate, competition and predation contributing to the observed responses to elevation are discussed.