Centre for African Conservation Ecology, Department of Zoology, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa.
PLoS One. 2013 Jul 17;8(7):e69771. doi: 10.1371/journal.pone.0069771. Print 2013.
In African large herbivore assemblages, megaherbivores dominate the biomass and utilise the greatest share of available resources. Consequently, they are considered a separate trophic guild that structures the food niches of coexisting large herbivores. However, there exists little empirical evidence on how food resources are shared within this guild, and none for direct competition for food between megaherbivores. Using the histological analysis of faeces, we explore this phenomenon for African elephant Loxodonta africana and black rhinoceros Diceros bicornis in the Addo Elephant National Park, South Africa, where the accumulated impacts of elephant have reduced browse availability. Despite being unable to generalise beyond our study sites, our observations support the predictions of competition theory (as opposed to optimality theory) by showing (1) a clear seasonal separation in resource use between these megaherbivores that increased as resource availability declined, and (2) rhinoceros changed their selectivity in the absence of elephant (using an adjacent site) by expanding and shifting their diet along the grass-browse continuum, and in relation to availability. Although black rhinoceros are generally considered strict browsers, the most significant shift in diet occurred as rhinoceros increased their preferences for grasses in the presence of elephant. We speculate that the lack of specialised grazing adaptations may increase foraging costs in rhinoceros, through reduced harvest- and handling-efficiencies of grasses. In the short-term, this may be off-set by an enhanced tolerance for low quality food and by seasonally mobilising fat reserves; however, the long-term fitness consequences require further study. Our data suggest that managing elephant at high densities may compromise the foraging opportunities of coexisting browsers. This may be particularly important in small, fenced areas and overlapping preferred habitats where impacts intensify.
在非洲大型草食动物组合中，巨型草食动物主导生物量，并利用最大份额的可用资源。因此，它们被认为是一个独立的营养阶层，构建了共存大型草食动物的食物生态位。然而，关于这个阶层内食物资源是如何共享的，以及巨型草食动物之间是否存在直接的食物竞争，几乎没有实证证据。本研究使用粪便的组织学分析，探索了南非阿多大象国家公园的非洲象（Loxodonta africana）和黑犀牛（Dicerobicornis）的这种现象，在那里，大象的累积影响降低了可食用植物的可用性。尽管我们的观察结果无法推广到我们的研究地点之外，但我们的观察结果支持竞争理论（而非最优理论）的预测，表明（1）这些巨型草食动物在资源可用性下降时，在资源利用上有明显的季节性分离；（2）在没有大象（使用相邻的地点）的情况下，犀牛通过扩大和沿着草-可食用植物连续体以及与可用性相关的方向改变其选择性，从而改变其选择性。尽管黑犀牛通常被认为是严格的食草动物，但饮食的最大变化发生在犀牛增加了对大象存在时的草的偏好。我们推测，缺乏专门的放牧适应可能会增加犀牛的觅食成本，降低对草的收获和处理效率。在短期内，这可能会通过提高对低质量食物的耐受性和季节性动员脂肪储备来得到补偿；然而，长期的健康后果需要进一步研究。我们的数据表明，在高密度下管理大象可能会损害共存食草动物的觅食机会。在小的、有围栏的区域和重叠的首选栖息地中，这种情况可能更为严重，因为这些区域的影响会加剧。
