Birds of the two largest national parks in South Africa : a method to generate estimates of population size for all species and assess their conservation ecology : research article

Most assessments of biodiversity are based on lists that record only the presence or absence of selected species within a given area. Information on the actual numbers of individuals that live or breed within such an area is rarely available, despite being essential data to understand the ecology and conservation of species. Estimates of population size are usually only available for a few large vertebrate and plant species, even in the two most extensive conservation areas in South Africa, the Kruger National Park (KNP) and the South African section of the Kgalagadi Transfrontier Park (KTP, previously the Kalahari Gemsbok National Park). We present a method that provides preliminary estimates of population size, based on a series of population thresholds that increase by the power of 2, or by doubling. The main characteristic of such estimates is that the proportional margin of error or interval width remains the same, at half or double the estimated population threshold, even though the absolute estimated population sizes and their errors increase exponentially by the power of 2. We present such population estimates for each bird species that has been recorded within the KNP (517 species) and KTP (313 species). We were able to assess the accuracy of our estimates using various techniques, including comparison with the independently obtained reporting rates that were available for most species from the Southern African Bird Atlas Project. Our annotated species lists consolidate bird information for the KNP and KTP, two important areas for ecotourism. Our population estimates allow the first ecological comparisons of the residency, numbers, biomass, diet and nesting habits of the component avifaunas. They show, in particular, the dominance by numbers and biomass of ostrich (Struthio camelus) in the KTP and red-billed quelea (Quelea quelea) in the KNP. Our estimates should be most accurate for those smaller populations that are of most conservation concern, rather than for larger, less-vulnerable populations, and so will indicate the role of these parks for avian conservation in South Africa. Our rationale for providing these preliminary estimates is that they generate hypotheses and should encourage more accurate population surveys.