Avifaunal responses to environmental conditions and land-use changes in South Africa : diversity, composition and body size

[摘要] ENGLISH ABSTRACT: In macroecology, body sizes in animal assemblages have traditionally beeninvestigated from two perspectives: body size frequency distributions (BSFDs) andgeographic variation in body size. Neither of these has been investigated for theSouth 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, as is usuallyfound for assemblages at large scales. This suggests that mechanisms driving theshape of BSFDs elsewhere are also acting for the South African avifauna. TheSouthern African Bird Atlas database was used to calculate median body sizes ofavian assemblages in quarter degree grid cells. Median sizes were then used toinvestigate geographic variation in body size across the country. Of the mechanismspreviously proposed to explain geographic variation in body size, only the starvationresistance hypothesis, which states that large size confers starvation resistanceduring seasonally resource shortages, was supported, though weakly, as medianbody size decreased with increasing productivity. The ability of null models to predictthe variation in body size was subsequently explored, and it was found that much ofthe variation in median size of assemblages could be predicted by randomly drawingspecies from the regional BSFD, particularly at high species richness values. Thisprovides empirical support for a continuum between the dominance of niche-basedprocesses (where assemblages are a product of organisms' response to theirenvironment) at low richness and neutral processes (where organisms assemble at random) at higher richness. In addition, it emphasizes the need to consider nullexpectations in investigations of the geographic variation in size. The importance ofthe regional BSFD and species richness for body sizes of local assemblages ishighlighted.Body size is one of several life history and community characteristics of animalsthat may be affected by anthropogenic disturbance to the environment. World-wide,landscapes are increasingly being altered by people, though few studies haveinvestigated the effect of such disturbances on the avifauna of South Africa. Theconsequence of land-use changes on avian assemblages was therefore assessed inthree South African regions which experience different environmental conditions andare threatened by different land-use changes. Birds were recorded in transects inundisturbed protected areas and the disturbed landscape outside the protected areasin the three regions. The effect of land-use change on avian assemblages variedbetween regions, and avian assemblages were most affected where disturbance wasmost intense. While species richness was not affected in a consistent manner acrossregions, species composition always changed in response to disturbance. This led tohigher regional species richness as natural and disturbed areas supported differentavian assemblages, and heterogeneity of assemblages between vegetation typesusually became less pronounced in disturbed areas. Functional diversity was alsocompromised by land-use changes: the relative proportion of feeding guilds wasaltered, indicating that changes in food availability affect composition ofassemblages. In contrast, mean body size of birds did not change in disturbedlandscapes, which suggests that habitat architecture has little effect on body size.This study therefore highlights the importance of natural and protected areas forconserving species, assemblages and ecosystem processes.

[发布机构] Stellenbosch University