Predator effects on metamorphosis: The effects of scaring versus thinning at high prey densities.

Organisms with complex life cycles face the challenge of when to switch between habitats and foraging strategies over ontogeny in ways that improve their fitness. Metamorphosis is a well-studied life history event in animals and ecologists have spent decades trying to understand how the size at and time to metamorphosis are altered by natural stressors such as competition and predation. The challenges in interpreting the effects of predators on metamorphic decisions include the need to compare predator species that pose different levels of risk, compare the roles of predators inducing fear versus thinning of the density of prey, and examine prey life history traits and behavior over ontogeny. We addressed these challenges in a mesocosm experiment in which we introduced a high initial density of hatchling Northern Leopard Frogs (Rana pipiens) and exposed them to three different species of caged predators (to induce three different levels of fear), three rates of hand-thinning (to mimic the thinning effect of each predator), or three species of lethal predators (to cause induction and thinning). Under these initial high densities, we found that caged predators had no effects on tadpole activity, growth, and development. This outcome was likely due to the high density of tadpoles causing high competition, which can inhibit anti-predator responses. High rates of hand thinning caused decreased tadpole activity, greater mass, and faster development. Interestingly, lethal predators caused phenotypic changes that were largely in line with the hand thinning effects alone. These results suggest that at high initial prey densities, the thinning process of predation appears plays a much more important role in prey metamorphosis than induction from predatory chemical cues.

具有复杂生活史的生物，在个体发育（ontogeny）过程中面临着如何在不同生境与觅食策略间切换以提升自身适合度（fitness）的难题。变态发育（metamorphosis）是动物中被广泛研究的生活史事件（life history event），生态学家数十年来一直致力于探明自然胁迫因子（natural stressor）如竞争与捕食作用（predation）如何改变生物的变态发生时间与变态时的体型。解析捕食者对变态决策（metamorphic decision）的影响时面临诸多挑战：需要比较不同风险等级的捕食者物种、区分捕食者诱导的恐惧效应与猎物种群密度降低的作用，同时还需考察猎物在个体发育过程中的生活史性状与行为变化。本研究通过一项中型实验生态系统（mesocosm）实验应对上述挑战：我们以较高初始密度投放刚孵化的北方豹蛙（Rana pipiens），并设置三类处理：一是三种不同物种的笼养捕食者（caged predator），以诱导三种不同等级的恐惧效应；二是三种梯度的人工疏伐（hand-thinning）处理，以模拟各类捕食者的种群密度降低效应；三是三种不同物种的致命捕食者（lethal predator），可同时诱导恐惧效应与种群密度降低。在初始高密度条件下，我们发现笼养捕食者对蝌蚪的活动、生长与发育均无显著影响。该结果很可能源于蝌蚪高密度导致的激烈种内竞争，这种竞争会抑制猎物的反捕食响应（anti-predator response）。高梯度的人工疏伐处理则使蝌蚪活动强度降低、体型更大且发育更快。值得注意的是，致命捕食者诱导的表型变化（phenotypic change）整体上与单独的人工疏伐处理效应基本一致。本研究结果表明，在猎物种群初始密度较高的情况下，捕食作用导致的种群缩减过程，相较于捕食者化学信号（chemical cue）诱导的效应，对猎物变态发育的影响更为显著。