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

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.

在宏观生态学领域，群落的体型大小通常从两个角度进行研究：体型大小频率分布（BSFDs）以及体型大小的地理变化。然而，这两种研究方法在南非鸟类的生态研究中尚未得到应用；因此，本研究的一个目标便是探索这两个方面。研究发现，南非鸟类的区域BSFD呈现右偏态分布。本研究利用南方非洲鸟类图谱数据库，计算了四分之一度网格单元中鸟类群落的平均体型大小。随后，通过这些平均体型大小数据，探讨了全国范围内体型大小的地理变化。在先前提出的解释体型大小地理变化的机制中，只有“抗饥饿性假设”得到了弱支持，该假设认为大型体型在季节性资源短缺期间赋予生物抗饥饿的能力，随着生产力的增加，平均体型大小呈下降趋势。然而，群落平均体型大小的很大一部分可以通过从区域BSFD中随机抽取物种来预测，尤其是在物种丰富度较高的情况下。这为低丰富度下基于生态位的过程在较高丰富度下中性过程之间的连续性提供了经验支持。此外，这也强调了在研究体型大小的地理变化时，考虑无差异期望的必要性。研究突出了区域BSFD和物种丰富度对本地群落体型大小的重要性。先前的研究也提出，体型大小可能影响生物对人类活动的敏感性。全球范围内，景观正日益受到人类活动的影响。目前，很少有研究调查了这种干扰对南非鸟类生态的影响。因此，本研究评估了三个不同环境条件、受到不同土地利用变化威胁的南非地区的土地利用变化对鸟类群落的影响。在三个地区，研究人员分别在未受干扰的保护区和保护区外的受干扰景观中对鸟类进行了记录。土地利用变化对鸟类群落的影响在不同地区之间存在差异，在干扰最严重的地区，鸟类群落受到的影响最大。尽管物种丰富度在不同地区没有发生一致的变化，但物种组成总是会对干扰作出反应。这导致了区域物种丰富度的增加，因为自然区域和受干扰区域支持不同的鸟类群落，并且在受干扰区域内，不同植被类型之间的群落异质性通常变得不那么明显。土地利用变化也损害了功能多样性：摄食类群的相对比例发生了变化，尽管在受干扰景观中鸟类的平均体型大小没有发生变化。因此，本研究强调了自然和保护区对保护物种、群落和生态系统过程的重要性。