Temporal changes in predator density are linked to shifts in prey behavior, mortality, and abundance in the field

Predators suppress prey populations and elicit defensive phenotypes in
prey. The magnitude of predator effects depends upon several factors,
including the density of predators, and their cue concentrations, in the
environment. Predator density manipulations have often relied on
laboratory studies that use unrealistic densities of predators and prey
over unnatural temporal and spatial scales. Field studies can provide
insights into predator-prey interactions under more realistic scenarios.
However, field studies linking predator density and prey populations are
limited by the challenge of manipulating predator densities or predicting
predator densities in dynamic/stochastic environments. We exploited a
somewhat predictable rise in predatory crab communities associated with
ocean warming to evaluate the impacts of swimming crab density on
ecologically important horn snails. Our approach combined long-term
monitoring of crabs and snails with snail behavior surveys and snail
tethering experiments repeated during and after a marine heat wave. Near
the end of a marine heat wave in 2016, swimming crabs were found in 25% of
marsh tidal creeks. No swimming crabs were found in these tidal creeks
during cooler water conditions in 2012 and 2021. When swimming crabs were
more abundant (e.g. 2016), horn snails experienced 7,533% more mortality
and were 91% less abundant. The proportion of snails climbing vegetation
was 454% higher when swimming crabs were more abundant. Thus, higher
predator densities occurring during a marine heat wave were associated
with changes in snail abundance, mortality, and behavior. Such changes
could influence marsh food webs and nutrient cycling. Our findings
highlight the value of exploiting climatic anomalies to understand
ecological patterns linked to the top-down effects in ecosystems - a
critical need in predator-prey ecology.

捕食者会抑制猎物种群数量，并诱导猎物产生防御表型。捕食者效应的强度取决于多种因素，包括环境中捕食者的密度及其信号浓度。捕食者密度操控研究通常依赖实验室实验，但这类实验使用的捕食者与猎物密度往往不切实际，且时间和空间尺度也不符合自然场景。野外研究则能在更贴近现实的情境下，揭示捕食者-猎物间的相互作用机制。然而，在动态/随机环境中，操控或预测捕食者密度的难度限制了探讨捕食者密度与猎物种群关系的野外研究。我们利用海洋变暖导致捕食性蟹类群落数量可预测性上升这一现象，评估了梭子蟹密度对生态关键物种角螺的影响。我们的研究方法结合了对蟹类与角螺的长期监测、角螺行为调查，以及在海洋热浪期间和之后重复进行的角螺系留实验。2016年海洋热浪末期，25%的沼泽潮沟中发现了梭子蟹；而在2012年和2021年水温较低的时期，这些潮沟中未发现梭子蟹。当梭子蟹数量较多时（如2016年），角螺的死亡率增加了7533%，种群数量则减少了91%；同时，攀爬植被的角螺比例提高了454%。因此，海洋热浪期间捕食者密度的上升与角螺种群数量、死亡率及行为的变化相关。这些变化可能会影响沼泽生态系统的食物网及养分循环（nutrient cycling）。我们的研究结果凸显了利用气候异常现象，解析生态系统中与自上而下效应相关的生态模式的价值——这是捕食者-猎物生态学领域的关键需求。