VERTEBRATE POPULATIONS AS INDICATORS OF ENVIRONMENTAL CHANGE IN SOUTHERN AFRICA

SUMMARY

Two categories of historical change in vertebrate populations are analysed: decreases leading to species being included in local Red Data Books and changes in distribution and abundance of what are often more common species, as revealed by regional surveys. The former category of change tends to reflect radical anthropogenic influences (e.g. habitat transformation, persecution and overuse). The latter generally reflects the more widespread forms of habitat modification (e.g. ‘bush encroachment’, the spread of alien trees into formerly ‘treeless’ biomes, and ‘desertification’ through overgrazing of semi-arid rangelands). Destruction of vleis and marshes has been important throughout the region. Any long-term climatic change that might have occurred over the last two centuries is not reflected by changes in vertebrate populations. Several vertebrate changes do, however, indicate a general tendency for the woody plant component of savanna communities to increase in density. This trend is consistent with the predicted response of these communities to the observed global increase in the atmospheric concentration of carbon dioxide. Short-term variations in annual rainfall have given rise to significant fluctuations in vertebrate populations. Vertebrates could be particularly useful in monitoring future climatic change where the effects of such change can be predicted in terms of alterations in the relative abundance of woody and herbaceous vegetation. Continuous monitoring of bird populations would probably be the most effective approach to detecting such changes. The effects of a wide range of other factors, as detailed in this analysis, must be filtered out before variations in vertebrate populations can be attributed to climatic change.