Data from: 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 both trait-mediated and density-mediated indirect effects. 3. We found that as fish density increased, damselfly survivorship declined, which generated natural selection favoring less active damselflies. These results are robust after taking into account latent effects of plasticity in response to fish predator cues. The surviving damselflies likely foraged less, freeing herbivores from predation, which in turn reduced primary production. This selection driven trait-mediated indirect effect was only apparent at low damselfly densities, because the consumptive effect of damselflies at high densities overwhelmed the effects of past selection. 4. These results demonstrate that the past action of natural selection can affect the strength of a trophic cascade. Natural selection can therefore act as a mechanism coupling ecological dynamics across trophic levels, which ultimately influences ecosystem functioning.1. 在复杂食物网中，不同营养级（trophic level）的物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，可能是调控此类营养级联（trophic cascade）强度的潜在机制，但该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，我们在四级水生营养系统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，damselflies）、植食者（浮游动物，zooplankton）以及初级生产者（藻类，algae）。首先，我们在控制表型可塑性的前提下，量化了鱼类捕食对豆娘活动水平施加的选择压力；随后，我们测定了该选择对初级生产（浮游植物，phytoplankton生物量）的间接效应。两项实验均通过调控捕食者密度，得以阐明性状介导与密度介导的两类间接效应。 3. 我们发现，随着鱼类密度升高，豆娘的存活率显著下降，由此产生的自然选择倾向于选择活动水平更低的豆娘个体。在考虑了豆娘对鱼类捕食信号产生可塑性响应的潜在效应后，上述结果依然稳健。存活下来的豆娘取食频率大概率降低，从而使植食者摆脱了捕食压力，进而降低了初级生产水平。该由选择驱动的性状介导间接效应仅在豆娘低密度条件下才显著，这是因为高密度下豆娘的捕食消耗效应掩盖了过往自然选择的作用。 4. 上述结果证实，过往发生的自然选择作用能够影响营养级联的强度。因此，自然选择可作为联结跨营养级生态动态的机制，最终对生态系统功能产生调控作用。