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.在宏生态学（Macroecology）研究中，生物群落的体型特征传统上主要从两个视角开展探究：一是体型频率分布（Body Size Frequency Distributions, BSFDs），二是体型的地理变异。目前尚未有针对南非鸟类区系（South African avifauna）的相关研究，因此本研究的目标之一便是对这两个维度进行探索。研究发现南非鸟类的区域体型频率分布呈右偏态。本研究借助南非鸟类图谱数据库（Southern African Bird Atlas database），计算了四分之一度网格单元（quarter degree grid cells）内鸟类群落的体型中位数。随后以体型中位数为指标，探究了南非全国范围内鸟类体型的地理变异规律。在以往提出的用以解释体型地理变异的各类机制中，仅抗饥饿假说（Starvation Resistance Hypothesis）获得了微弱支持——该假说指出，体型较大的个体在季节性资源短缺时期具备更强的抗饥饿能力，但本研究观测到体型中位数随生产力水平提升而降低，与该假说仅存在微弱的契合度。然而，若从区域体型频率分布中随机抽取物种模拟群落，便可在很大程度上复现群落体型中位数的变异规律，在物种丰富度较高的情况下这一模拟效果尤为显著。这一结果为低丰富度下以生态位过程（Niche-based Processes）为主导、高丰富度下以中性过程（Neutral Processes）为主导的连续体假说提供了实证支持。此外，本研究还强调，在开展体型地理变异相关研究时，需纳入零假设预期（Null Expectations）作为参照。本研究同时凸显了区域体型频率分布与物种丰富度对本地群落体型特征的重要意义。此前已有研究提出，生物体型可能会影响其对人类活动的敏感性。全球范围内，人类活动正日益改变自然景观。目前针对此类干扰对南非鸟类区系影响的研究相对匮乏。因此，本研究选取了三个环境条件各异、面临不同土地利用变化威胁的南非区域，评估了土地利用变化对鸟类群落的影响。研究人员在这三个区域的未受干扰保护区及保护区外受干扰景观中，通过样线法记录了鸟类群落信息。土地利用变化对鸟类群落的影响存在区域差异，且干扰强度最高的区域中，鸟类群落受到的影响也最为显著。尽管不同区域的物种丰富度未呈现一致的变化规律，但鸟类群落的物种组成始终会随干扰发生改变。由于自然区域与受干扰区域拥有不同的鸟类群落，这使得区域整体的物种丰富度有所提升；同时，不同植被类型间的群落异质性在受干扰区域通常会有所降低。土地利用变化同时也损害了群落的功能多样性（Functional Diversity）：鸟类取食功能群（Feeding Guilds）的相对占比发生了改变，尽管受干扰景观中鸟类的平均体型并未出现明显变化。因此，本研究凸显了自然区域与保护区在保护物种、群落及生态系统过程中的重要价值。