Ecophysiology, Range Contraction and Survival of a Geographically Restricted African Dung Beetle (Coleoptera: Scarabaeidae)

Abstract

1. Habitat degradation and the overexploitation of many taxa currently represent serious threats to biological diversity. In species-rich foodwebs, trophically specialized species are particularly prone to risk if their prey species are removed. This appears to be true of Circemllium bacchus (Scarabaeinae: Canthonini), a large, flightless dung beetle, which feeds on the dung of large mammals such as elephant, buffalo and rhinoceros. It was once widespread in the southern African savanna but within the last century has become restricted to a small, xeric area in the south-eastern Cape Province of South Africa. In this study we examine the original distribution of this species and reasons for its subsequent range contraction. 2. Because temperature and water availability play such important roles in determining the abundance and distribution of animals, and because the thermal biology of dung beetles is so important in determining their competitive ability, we compare the desiccation resistance and thermal biology of C. bacchus with a large, winged, savanna dung beetle, Pachylomerus femoralis, as well as that reported in the literature for other dung beetles. We also compare respiratory metabolism of the two species because an elevated factorial aerobic scope is indicative of competitive superiority. In addition, we compare the distribution and habitat preference of C. bacchus with that of closely related scarabaeine beetles to gain insight into its original distribution. 3. Circellium bacchus appears to have originated in mesic, east-coast wooded savanna and forests. It is unlike any other large dung beetle because its flightless condition makes it a strict ectotherm. This means that it stands little chance in competing for dung with heterothermic, winged dung beetles which have a competitive advantage at dung pads. This advantage is conferred on winged species because of high post-flight activity levels associated with high thoracic temperatures. 4. The penetration of other, wooded savanna areas by C. bacchus is ascribed to the presence of a constant dung source which would ameliorate the effects of competition from other dung beetles. The only known resource of this type is Black Rhinoceros middens. Range contraction of the Black Rhinoceros seems to have precipitated range contraction of C. bacchus. Collection records of C. bacchus correspond with areas known to have supported Black Rhinoceros populations throughout this century. 5. The survival of C. bacchus in the xeric southern and eastern Cape seems to have been a result both of its large body size, which has endowed it both with a lower conductance and greater desiccation resistance than other savanna-dwelling dung beetles, and the presence of a small population of elephants in the thick Valley Bushveld of the eastern Cape throughout this century