Effects of land-use on avian demography in the Kalahari area of the North-West Province, South Africa
Desertification, whether due to anthropogenic pressures, climate change or other factors, has become a global concern. The far-reaching effects of desertification have prompted the formation of the United Nations Convention for the Control of Desertification (UNCCD) and the initiation of the Desert Margins Programme (DMP) in order to attempt to control desertification. This study forms part of the first phase of the DMP and will thus aim to keep to the objectives of the DMP. The principal aims of this study was to determine what effects, if any, land use types in the desert margins areas of the North-West Province, South Africa, will have on avian demography of the area, and to ascertain whether these changes in avian demography can be used in order to indicate land degradation in these areas. Vegetation structure is widely known to influence avian demography, along with factors such as food availability, nesting sites, water availability and climate. Vegetation structure was also found to be dramatically altered by the effects of land-use in the study area. The hypotheses formulated for this study were that: 1) Bird populations are noticeably influenced by the vegetation structure of the area they inhabit; 2) bird species diversity as well as bird numbers decline with an increase in land degradation in the study area; and 3) Bird species diversity will act as a good surrogate for land degradation in the study area. In order to test these hypotheses, the study area was selected in the Molopo district of the North-West Province. This district falls within the desert margin area and is earmarked as one of the target areas for the Desert Margins Programme in South Africa. Within the study area four sites were chosen to represent different degrees of degradation. Vegetation structure analyses were carried out at each of the sites in order to determine the degree of change in vegetation structure brought about by land use in the area. The birds at each of these sites were surveyed using three transects. Surveys were repeated over four seasons to give some indication of the effects of seasonality on bird populations of the different sites. The results showed a definite decline in bird species diversity with an increase in land degradation, especially due to the simplification of the vegetation structure because of anthropogenically induced alteration of the vegetation structure of the area. Both bird species diversity and the number of birds occurring at the sites declined with an increase in land degradation. The guild analysis done showed that, although the actual number of species occurring at the various sites changed, aggregations remained relatively similar with regard to feeding guilds. At all the sites, analysis of feeding guilds showed that insectivores were the guild most represented, with granivores second most and then a variation in other guilds at each site. Breeding guilds showed a much greater variation in percentage composition of the guilds. At sites with less shrub and tree strata, ground nesting species were most represented, whereas the sites with a more well developed tree and shrub strata had a greater occurrence of tree nesting birds than the other guilds. The deduction to be made from this is that bird species composition changes can be attributed to their nesting needs, to a much greater extent than their feeding needs. Bird species varied in their response to changes in the vegetation structure at different sites with specialist species, such as raptors and specialist insectivores, being more vulnerable to changes in vegetation structure than generalist species, such as granivores and generalist insectivores. From the results of this study it appears that vegetation structure played the most important role in determining species diversity, in the Molopo district of the North-West Province. Many of the other factors that have been shown to influence bird species diversity in other studies were shown to be negated due to the uniqueness of the study area. The result of this study showed that bird species diversity is definitely influenced by the effects of land use on vegetation structure due to land degradation in the desert margin areas of the North-West Province. This also appears to indicate that bird species diversity will be a good surrogate for the indication of land degradation in the study area. More studies are however needed in order to adequately understand how and why species diversity is affected by vegetation structure and how the changes in avian diversity will affect the ecosystem processes in the desert margins areas. Due to the decrease in species diversity on, what was supposed to be, a well managed commercial farm, this study has also shown that more studies need to be done on the long term effects of management in the desert margin areas. Bird species diversity has also been shown by this study to have potential as a cost effective, easy way of determining the degree of degradation occurring in an area as well as a possible tool for monitoring the effectiveness of restoration of degraded areas. Molopo Nature Reserve was found to be more important to the bird species of the area than first anticipated. The results seem to indicate that Molopo Nature Reserve acts as a refuge for resident bird species in the colder, drier winter months. This was most clearly shown by the increase in bird numbers at Molopo Nature Reserve during the winter survey, when bird numbers at all the other sites declined.