Centre for African Conservation Ecology, Department of Zoology, Nelson Mandela University, P.O. Box 77000, Port Elizabeth 6031, South Africa.
Centre for African Conservation Ecology, Department of Zoology, Nelson Mandela University, P.O. Box 77000, Port Elizabeth 6031, South Africa.
Curr Biol. 2018 Aug 6;28(15):2493-2499.e3. doi: 10.1016/j.cub.2018.05.088. Epub 2018 Jul 19.
The loss of apex consumers (large mammals at the top of their food chain) is a major driver of global change [1]. Yet, research on the two main apex consumer guilds, large carnivores [2] and megaherbivores [3], has developed independently, overlooking any potential interactions. Large carnivores provoke behavioral responses in prey [1, 4], driving prey to distribute themselves within a "landscape of fear" [5] and intensify their impacts on lower trophic levels in low-risk areas [6], where they may concentrate nutrients through localized dung deposition [7, 8]. We suggest, however, that megaherbivores modify carnivore-induced trophic cascades. Megaherbivores (>1,000 kg [9]) are largely invulnerable to predation and should respond less to the landscape of fear, thereby counteracting the effects of fear-triggered trophic cascades. By experimentally clearing plots to increase visibility and reduce predation risk, we tested the collective role of both apex consumer guilds in influencing nutrient dynamics in African savanna. We evaluated whether megaherbivores could counteract a behaviorally mediated trophic cascade by redistributing nutrients that accumulate through fear-driven prey aggregations. Our experiment showed that mesoherbivores concentrated fecal nutrients in more open habitat, but that megaherbivores moved nutrients against this fear-driven nutrient accumulation by feeding within the open habitat, yet defecating more evenly across the risk gradient. This work adds to the growing recognition of functional losses that are likely to have accompanied megafaunal extinctions by contributing empirical evidence from one of the last systems with a functionally complete megaherbivore assemblage. Our results suggest that carnivore-induced trophic cascades work differently in a world of giants.
顶级消费者（食物链顶端的大型哺乳动物）的消失是全球变化的主要驱动因素之一[1]。然而，对大型食肉动物[2]和巨型食草动物[3]这两个主要顶级消费者群体的研究是独立进行的，忽略了任何潜在的相互作用。大型食肉动物会引起猎物的行为反应[1,4]，促使猎物在“恐惧景观”中分布[5]，并在低风险地区加强对低营养级别的影响[6]，在这些地区，它们可能通过局部粪便沉积来集中养分[7,8]。然而，我们认为巨型食草动物会改变由食肉动物引发的营养级联。巨型食草动物（>1000 公斤[9]）基本上不受捕食的影响，对恐惧景观的反应应该较小，从而抵消由恐惧引发的营养级联的影响。通过实验清除斑块以增加可见度和降低捕食风险，我们测试了两个顶级消费者群体在影响非洲稀树草原养分动态方面的集体作用。我们评估了巨型食草动物是否可以通过重新分配因恐惧驱动的猎物聚集而积累的养分来抵消由行为介导的营养级联。我们的实验表明，中型食草动物将粪便养分集中在更开放的栖息地，但巨型食草动物通过在开放栖息地进食，同时在风险梯度上更均匀地排便，从而对抗了这种由恐惧驱动的养分积累。这项工作增加了对功能丧失的认识，这些功能丧失很可能伴随着巨型动物的灭绝而发生，并从最后一个具有功能完整巨型动物组合的系统之一提供了经验证据。我们的研究结果表明，在一个充满巨兽的世界中，由食肉动物引发的营养级联的作用方式不同。
