Invertebrate species turnover and body size patterns across an altitudinal gradient

Abstract

Understanding patterns of biodiversity over geographical gradients is an important aspect of conservation science. Latitudinal and altitudinal gradients are among the preferred gradients selected to study due to their ecological significance. Studying diversity along these gradients may assist in understanding why some areas are more species rich than others. A similarity in climatic conditions can be observed between latitudinal and altitudinal gradients in terms of both gradients representing a climatic cline. However, within an altitudinal gradient a faster rate of change within environmental variables e.g. temperature can be observed compared to latitudinal gradients. As a result, altitudinal studies may be a preferred choice to investigate patterns in biodiversity. Climate change is a topic that has received considerable research attention due to its uncertain impacts on taxa. One species may respond positively to the changes while another may respond negatively. Altitudinal gradients can be used to investigate these possible responses of species under changes in temperature. Many studies have investigated species richness patterns across altitudinal gradients, while species turnover and body size patterns have received less attention, especially in South Africa. Abstract iii Furthermore, few studies have focussed on comparing species richness and turnover patterns between taxa. The aim of this study was to investigate and compare species richness and species turnover patterns of spiders and ants across an altitudinal gradient in the Drakensberg Mountains. The altitudinal gradient studied ranged from 900 to 3 000 m.a.s.l. Overall, a clear difference in spider and ant species richness patterns was observed, ant species richness decreased significantly with increasing altitude, while no spider species richness pattern was detected. Spider species turnover was higher than the ant species turnover across the altitudinal gradient, between seasons, and between sampling occasions. I also investigated spider body size patterns in relation to Bergmann’s rule at an inter-and intraspecific level. Bergmann’s rule states that the body size of populations and species within a broadly distributed taxonomic clade is of a larger size in cooler climates than their small-bodied relatives found in warmer regions. No significant relationship between spider body size and altitude was observed when using a midpoint or mean body size method. At the intraspecific level, a weak but significant relationship was observed, as spider body size increased with altitude. This study has provided some valuable insights into spider and ant diversity across an altitudinal gradient and has provided further insights into spider body size patterns in relation to altitude. Keywords: Altitude, Spiders, Ants, Species Richness, Species Turnover, Body Size