Avifaunal responses to environmental conditions and land-use changes in South Africa: diversity, composition and body size
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In macroecology, body sizes in assemblages have traditionally been investigated from two perspectives: body size frequency distributions (BSFDs) and geographic variation in body size. Neither of these has been investigated for the South African avifauna; one objective of this study was therefore to explore these. The regional BSFD of South African birds was found to be right-skewed. The Southern African Bird Atlas database was used to calculate median body sizes of avian assemblages in quarter degree grid cells. Median sizes were then used to investigate geographic variation in body size across the country. Of the mechanisms previously proposed to explain geographic variation in body size, only the starvation resistance hypothesis, which states that large size confers starvation resistance during seasonally resource shortages, was supported, though weakly, as median body size decreased with increasing productivity. However, much of the variation in median size of assemblages could be predicted by randomly drawing species from the regional BSFD, particularly at high species richness values. This provides empirical support for a continuum between the dominance of niche-based processes at low richness and neutral processes at higher richness. In addition it emphasizes the need to consider null expectations in investigations of the geographic variation in size. The importance of the regional BSFD and species richness for body sizes of local assemblages is highlighted. Previously, it has also been suggested that body size may affect the sensitivity of organisms to human activities. World-wide, landscapes are increasingly being altered by people. Few studies have investigated the effect of such disturbances on the avifauna of South Africa. The consequence of land-use changes on avian assemblages was therefore assessed in three South African regions which experience different environmental conditions and are threatened by different land-use changes. Birds were recorded in transects in undisturbed protected areas and the disturbed landscape outside the protected areas in the three regions. The effect of land-use change on avian assemblages varied between regions, and avian assemblages were most affected where disturbance was most intense. While species richness was not affected in a consistent manner across regions, species composition always changed in response to disturbance. This lead to higher regional species richness as natural and disturbed areas supported different avian assemblages, and heterogeneity of assemblages between vegetation types usually became less pronounced in disturbed areas. Functional diversity was also compromised by land-use changes: the relative proportion of feeding guilds was altered, although the mean body size of birds did not change in disturbed landscapes. This study therefore highlights the importance of natural and protected areas for conserving species, assemblages and ecosystem processes.