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.
To determine what effects body sizes of bird assemblages in South Africa, and the effect of land-use changes on the South African avifauna

在宏观生态学领域，群落的生物体型研究传统上从两个角度进行：生物体型频率分布（BSFDs）和生物体型在地理上的变化。对于南非的鸟类群落，这两种角度尚未得到充分探究；因此，本研究的一个目标便是对此进行探索。研究发现，南非鸟类的BSFD呈现出右偏态。本研究利用南非鸟类图鉴数据库，计算了鸟类群落在中度网格单元中的中值体型。随后，通过这些中值体型数据，对全国范围内的生物体型地理变化进行了研究。在先前提出的解释生物体型地理变化的机制中，仅有对饥饿抵抗假说得到了支持，尽管支持力度较弱，即随着生产力的提高，中值体型呈下降趋势。然而，群落中值体型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值。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的连续性提供了经验支持。此外，它还强调了在研究体型地理变化时考虑零期望值的重要性。强调了区域BSFD和物种丰富度对本地群落生物体型的重要性。此前，也有人提出生物体型可能会影响生物对人类活动的敏感性。全球范围内，人类活动正日益改变景观。关于此类干扰对南非鸟类群落的效应的研究却寥寥无几。因此，本研究评估了三个不同环境条件且受不同土地利用变化威胁的南非地区土地利用变化对鸟类群落的影响。在三个地区，记录了未受干扰的保护区内以及保护区外的受干扰景观中的鸟类。土地利用变化对鸟类群落的影响在不同地区间存在差异，受干扰最严重的地区鸟类群落受影响最大。尽管物种丰富度在各个地区并未保持一致，但物种组成在受到干扰后总是会发生改变。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，且受干扰区域中植被类型之间的异质性通常变得更加不显著。土地利用变化也损害了功能多样性：摄食 guild 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并未改变。因此，本研究突出了自然和保护区域在保护物种、群落和生态系统过程中的重要性。