Warming strengthens food web effects of predator phenotypic variation

Intraspecific variation modifies ecological processes and ecosystem functioning. Still, we know relatively little of how the nature and strength of ecosystem effects caused by intraspecific variation may interact with climate change.

We conducted a mesocosm experiment to test if, and to what extent, ocean warming modifies the ecological impacts of intraspecific variation in a predatory fish. The mesocosms consisted of a simplified coastal food web with threespine stickleback (Gasterosteus aculeatus) as the top predator, from a population where two stickleback phenotypes with either complete or incomplete lateral armour plating coexist and display differentiated predation behavior: The completely plated phenotype often feeds more on invertebrate herbivores compared to the incompletely plated phenotype.

Presence of stickleback reduced biomass of arthropod shredders (crustaceans, insect larvae). Warming (+4 °C) strengthened this predation, releasing benthic primary producers (diatoms) from top-down control, causing a trophic cascade. This trophic cascade was attributed to one of the plate phenotypes: the completely plated stickleback increased their predation on shredders under warming, while the incompletely plated stickleback instead decreased their predation. Diatom biomass responded accordingly: warming increased diatom biomass in the presence of completely plated stickleback but not when incompletely plated stickleback were present.

Our results suggest that different plate phenotypes of threespine stickleback differentially affect lower trophic levels, and that warming may exacerbate these cascading effects. These trait-dependent effects on trophic cascades highlight the consequences of intraspecific variation on ecosystem functioning.

种内变异可改变生态过程与生态系统功能，但目前我们对种内变异所引发的生态系统效应的性质与强度，如何与气候变化产生交互作用，仍知之甚少。

我们开展了一项中型实验生态系统（mesocosm）实验，以探究海洋变暖是否会，以及在多大程度上，改变捕食性鱼类的种内变异所带来的生态影响。该中型实验生态系统构建了简化的近岸食物网，以三棘刺鱼（Gasterosteus aculeatus）作为顶级捕食者；其所在种群共存有两种表型：侧甲完全发育与侧甲不完全发育的刺鱼，二者捕食行为存在分化——相较于侧甲不完全发育的表型，侧甲完全发育的刺鱼通常更多以无脊椎草食动物为食。

刺鱼的存在会降低碎屑食性节肢动物（甲壳类、昆虫幼虫）的生物量。海水升温（+4℃）强化了这一捕食作用，使底栖初级生产者（硅藻）摆脱自上而下的调控，进而引发营养级联效应。该营养级联效应可归因于其中一种侧甲表型：升温环境下，侧甲完全发育的刺鱼对碎屑食性者的捕食量增加，而侧甲不完全发育的刺鱼捕食量则有所下降。硅藻生物量也随之产生响应：当存在侧甲完全发育的刺鱼时，升温会提升硅藻生物量；但当存在侧甲不完全发育的刺鱼时，升温并未产生此类效应。

我们的研究结果表明，三棘刺鱼的不同侧甲表型会对低营养级生物产生差异化影响，而升温可能会加剧这类级联效应。这种依赖于性状的营养级联效应，凸显了种内变异对生态系统功能的重要意义。