Selection past impacts the strength of an aquatic trophic cascade

1. In complex food webs, interactions among species in different trophic levels can generate cascading indirect effects that couple top predators with primary producers, thereby affecting ecosystem functioning. Natural selection imposed by top predators on intermediate predators may play a role in shaping the strength of these trophic cascades, but this conjecture remains largely untested. 2. To determine the effects of natural selection on the strength of trophic cascades we conducted a two-part experiment in a four-level aquatic trophic system involving a top predator (fish), an intermediate predator (damselflies), herbivores (zooplankton), and primary producers (algae). We first quantified how predation by fish generated selection on damselfly activity levels after controlling for phenotypic plasticity. We then measured the indirect effects of this selection on primary production (phytoplankton biomass). In both experiments we varied the density of predators, allowing us to elucidate...

1. 在复杂的食物网中，不同营养级（trophic levels）的物种种间相互作用可产生级联间接效应，将顶级捕食者与初级生产者联结起来，进而影响生态系统功能。顶级捕食者对中级捕食者施加的自然选择，可能在塑造此类营养级联（trophic cascades）强度的过程中发挥作用，但这一推测迄今尚未得到充分验证。
2. 为明确自然选择对营养级联强度的影响，我们在包含四级营养结构的水生营养系统（aquatic trophic system）中开展了两阶段实验，该系统涵盖顶级捕食者（鱼类）、中级捕食者（豆娘）、植食者（浮游动物）以及初级生产者（藻类）。我们首先在控制表型可塑性（phenotypic plasticity）的前提下，量化了鱼类捕食对豆娘活动水平所产生的自然选择作用。随后，我们测定了该自然选择对初级生产（浮游植物生物量（phytoplankton biomass））产生的间接效应。在两项实验中，我们均设置了不同的捕食者密度梯度，以此得以阐明……