Data from: Large mammal declines and the incipient loss of mammal-bird mutualisms in an African savanna ecosystem

Over the past half-century, large mammal populations have declined substantially throughout East Africa, mainly due to habitat loss and unsustainable direct exploitation. While it has been acknowledged that the loss of large mammals can have direct and cascading effects on community composition and ecosystem characteristics, limited quantitative work has been done on how declines of large herbivore populations impacts the abundance of mutualistic symbionts. Using a space-for-time observational approach, we quantified the large mammal community alongside the densities, host preferences and behaviors of mutualistic red-billed oxpeckers (Buphagus erythrorhynchus), and yellow-billed oxpeckers (Buphagus africanus) in northern Tanzania. At the landscape scale, mammal community composition was substantially less diverse in highly human-dominated areas when compared with more protected areas, with an observed complete loss of large wild mammal species in two study areas. Mirroring this trend, oxpecker densities were lowest in the least protected areas, and highest in fully protected areas. Using resource selection functions implemented via generalized linear models at different scales, we found that oxpeckers (1) were predominantly (67% of red-billed oxpeckers; 70% of yellow-billed oxpeckers) feeding on larger (between 500kg and 1500kg) ungulate host species within the mammal community, (2) usually preferred feeding on larger individuals (adults and males) within a specific host species population, and (3) preferred hosts that were more tolerant of their presence. In particular, cattle were especially intolerant of oxpecker presence and were relatively effective in displacing oxpeckers. We found little evidence that oxpecker feeding was parasitic across all host species; wound feeding was only observed on giraffe, comprising 6% and 4% of feeding behavior in red-billed and yellow-billed oxpeckers respectively. Thus, a loss of large-bodied and oxpecker tolerant host species is a likely explanation for declines of oxpecker populations in human dominated landscapes, which may have further cascading effects.

过去半个世纪以来，东非全境的大型哺乳动物种群均出现大幅衰减，核心诱因为栖息地丧失与不可持续的直接猎捕活动。尽管学界已达成共识：大型哺乳动物的消亡会对群落组成与生态系统特征产生直接及级联效应，但针对大型草食动物种群衰减如何影响互利共生共生生物丰度的定量研究仍较为匮乏。本研究采用空间替代时间的观测方法（space-for-time observational approach），针对坦桑尼亚北部区域，对大型哺乳动物群落，以及互利共生的红嘴牛椋鸟（Buphagus erythrorhynchus）、黄嘴牛椋鸟（Buphagus africanus）的种群密度、宿主偏好与取食行为进行了量化分析。在景观尺度下，相较于保护程度更高的区域，人类活动主导程度较强的区域内哺乳动物群落多样性显著更低，且两个研究区域内的大型野生哺乳动物物种已完全消失。这一趋势在牛椋鸟种群密度上得到了呼应：牛椋鸟密度在保护程度最低的区域处于最低水平，而在完全受保护区域则达到最高。通过在不同尺度下基于广义线性模型（generalized linear models）构建的资源选择函数（resource selection functions），本研究发现牛椋鸟存在以下取食偏好：（1）主要以哺乳动物群落中体重介于500kg至1500kg的大型有蹄类（ungulate）宿主为食（红嘴牛椋鸟的取食对象中该类宿主占比67%，黄嘴牛椋鸟占比70%）；（2）在特定宿主物种种群内，偏好取食体型更大的个体（成年个体与雄性个体）；（3）更倾向选择对其存在耐受度更高的宿主。尤为值得注意的是，家牛对牛椋鸟的耐受度极低，且能有效驱赶牛椋鸟。我们未发现足够证据表明牛椋鸟的取食行为对所有宿主物种均具有寄生性：仅在长颈鹿身上观察到其取食宿主伤口的行为，该行为分别占红嘴牛椋鸟与黄嘴牛椋鸟总取食行为的6%与4%。因此，大型体型且对牛椋鸟耐受的宿主物种的流失，或是人类活动主导景观中牛椋鸟种群衰减的合理解释，这一变化或进一步引发级联生态效应。