Department of Wildlife, Fish and Environmental Studies, Swedish University of Agricultural Sciences, Umeå, Sweden.
PLoS One. 2013 Aug 21;8(8):e72599. doi: 10.1371/journal.pone.0072599. eCollection 2013.
Cascading trophic interactions are often defined as the indirect effects of a predator on primary producers through the effect of the predator on herbivores. These effects can be both direct through removal of herbivores [density-mediated indirect interactions (DMIIs)] or indirect through changes in the behavior of the herbivores [trait-mediated indirect interactions (TMIIs)]. How the relative importance of these two indirect interactions varies with predator diversity remains poorly understood. We tested the effect of predator diversity on both TMIIs and DMIIs on phytoplankton using two competitive invasive dreissenid mussel species (zebra mussel and quagga mussel) as the herbivores and combinations of one, two or all three species of the predators pumpkinseed sunfish, round goby, and rusty crayfish. Predators had either direct access to mussels and induced both TMII and DMII, or no direct access and induced only TMII through the presence of risk cues. In both sets of treatments, the predators induced a trophic cascade which resulted in more phytoplankton remaining with predators present than with only mussels present. The trophic cascade was weaker in three-predator and two-predator treatments than in one-predator treatments when predators had direct access to dreissenids (DMIIs and TMIIs). Crayfish had higher cascading effects on phytoplankton than both pumpkinseed and round goby. Increased predator diversity decreased the strength of DMIIs but had no effect on the strength of TMIIs. The strength of TMIIs was higher with zebra than quagga mussels. Our study suggests that inter-specific interference among predators in multi-species treatments weakens the consumptive cascading effects of predation on lower trophic levels whereas the importance of predator diversity on trait mediated effects depends on predator identity.
级联营养相互作用通常被定义为捕食者通过捕食者对食草动物的影响对初级生产者的间接影响。这些影响既可以是通过去除食草动物直接产生的[密度介导的间接相互作用（DMII）]，也可以是通过食草动物行为的变化间接产生的[性状介导的间接相互作用（TMII）]。捕食者多样性对这两种间接相互作用的相对重要性如何变化仍知之甚少。我们使用两种竞争入侵的贻贝类（斑马贻贝和斑马贻贝）作为食草动物，以及三种捕食者（南瓜籽太阳鱼、圆鳍鱼和铁锈蟹）的一个、两个或全部三个物种的组合，测试了捕食者多样性对 TMII 和 DMII 的影响。捕食者要么直接接触贻贝类并同时诱导 TMII 和 DMII，要么由于存在风险线索而没有直接接触，仅通过 TMII 诱导。在这两组处理中，捕食者都引发了营养级联，导致有捕食者存在时比只有贻贝类存在时留下了更多的浮游植物。当捕食者可以直接接触贻贝类时（DMII 和 TMII），三食者和二食者处理中的营养级联较弱于一食者处理。与南瓜籽和圆鳍鱼相比，蟹对浮游植物的级联效应更高。增加捕食者多样性会降低 DMII 的强度，但对 TMII 的强度没有影响。与斑马贻贝相比，TMII 的强度在斑马贻贝中更高。我们的研究表明，在多物种处理中，捕食者之间的种间干扰会削弱捕食对较低营养级别的消耗性级联效应，而捕食者多样性对性状介导效应的重要性取决于捕食者的身份。
