Remarkable elevational tolerance in an African odonata larvalassemblage

摘要

The spatial patterns in species richness and abundance were investigated here at a series of reservoirs at different elevations, to establish which factors determine species distributions along this topographic gradient. Larvae of 18 spp. were sampled in small reservoirs across a 1250 m elevational gradient at one latitude. Most spp. occurred throughout all elevations indicating that this subtropical odon, assemblage as a whole is remarkably tolerant of elevational changes. Although Anisoptera larval species richness and abundance increased significantly with increasing elevation, there was no change in Zygoptera species richness, while Zygoptera abundance decreased significantly. Species-site-variable triplots for Anisoptera and Zygoptera larvae indicated that no measured site variable on an individual basis clearly accounted for larval species assemblage distribution patterns. Nevertheless, canonical axes and their respective intra-set correlation coefficients showed that some measured site variables e.g. floating/submerged vegetation, turbidity, pH, water temperature (resulting from sunny or shade habitat conditions), marginal grasses, water depth as well as elevation to some extent, explained the main variation in species assemblage composition/distribution in a broadly similar manner for both suborders. Generally, the reservoirs recruited spp. from the regional pool, irrespective of the elevation of the pool. These spp. were all geographically widespread spp. that took advantage of the presence of these man-made reservoirs, and included only one national endemic. Although these artificial water bodies are not increasing the 'extent of species occurrence', they play a major role in increasing 'area of occupancy'. Furthermore, these spp. are remarkably vagile, habitat-tolerant, as well as elevationally-widespread. A reasonable explanation is that this assemblage is the historical survivor over many millennia of oscillating wet/dry periods and natural selection. The present-day spp. are those that readily recolonise pools after drought has been broken, and are pioneering residents of new water bodies over a wide elevational range.