Data_Sheet_1_Diurnal Changes in Hypoxia Shape Predator-Prey Interaction in a Bird-Fish System.DOCX

Animals often face changing environments, and behavioral flexibility allows them to rapidly and adaptively respond to abiotic factors that vary more or less regularly. However, abiotic factors that affect prey species do not necessarily affect their predators. Still, the prey’s response might affect the predator indirectly, yet evidence from the wild for such a classical bottom-up effect of abiotic factors shaping several trophic levels remains sparse. In many aquatic environments, daily changes in oxygen concentrations occur frequently. When oxygen levels drop to hypoxic levels, many fishes respond with aquatic surface respiration (ASR), during which they obtain oxygen by skimming the upper, oxygenated surface layer. By increasing time at the surface, fish become more vulnerable to fish-eating birds. We explored these cascading effects in a sulfidic spring system that harbors the endemic sulphur molly (Poecilia sulphuraria) as prey species and several fish-eating bird species. Sulfide-rich springs pose harsh conditions as hydrogen sulfide (H2S) is lethal to most metazoans and reduces dissolved oxygen (DO). Field sampling during three daytimes indicated that water temperatures rose from morning to (after)noon, resulting in the already low DO levels to decrease further, while H2S levels showed no diurnal changes. The drop in DO levels was associated with a decrease in time spent diving in sulphur mollies, which corresponded with an increase in ASR. Interestingly, the laboratory-estimated threshold at which the majority of sulphur mollies initiate ASR (ASR50: <1.7 mg/L DO) was independent of temperature and this value was exceeded daily when hypoxic stress became more severe toward noon. As fish performed ASR, large aggregations built up at the water surface over the course of the day. As a possible consequence of fish spending more time at the surface, we found high activity levels of fish-eating birds at noon and in the afternoon. Our study reveals that daily fluctuations in water’s oxygen levels have the potential to alter predator-prey interactions profoundly and thus highlights the joined actions of abiotic and biotic factors shaping the evolution of a prey species.

动物常面临环境变化，而行为可塑性使其能够快速且适应性地响应那些或多或少规律性波动的非生物因子。然而，作用于猎物物种的非生物因子未必会对其捕食者产生同等影响。尽管猎物的响应可能会间接影响捕食者，但野外环境中，关于非生物因子通过经典的上行效应（bottom-up effect）塑造多个营养级的相关证据依然稀少。

在诸多水生环境中，溶解氧浓度的日间波动十分常见。当氧含量降至低氧水平时，众多鱼类会启动水生表面呼吸（aquatic surface respiration, ASR）——此时它们通过掠过水面上层的富氧层来获取氧气。随着鱼类在水面停留时间增加，其被捕食性鸟类捕食的风险也会随之升高。

我们在一处栖息有特有硫鳉（Poecilia sulphuraria）作为猎物、且分布有多种食鱼鸟类的硫化物泉生态系统中，探究了这类级联效应。富含硫化氢（H₂S）的泉水环境极为严苛：硫化氢对绝大多数后生动物均具有致死性，且会降低水体溶解氧（DO）含量。我们在三个日间时段开展野外采样，结果显示水温从清晨至午间（及午后）逐渐升高，导致本就偏低的溶解氧水平进一步下降，而硫化氢浓度未出现昼夜变化。溶解氧水平的下降与硫鳉的潜水时长缩短相关，这也伴随着水生表面呼吸行为的增多。

有趣的是，实验室测算得到的多数硫鳉启动水生表面呼吸的临界阈值（ASR₅₀：溶解氧浓度<1.7 mg/L）不受温度影响，且随着午间低氧胁迫加剧，该阈值每日都会被突破。随着鱼类进行水生表面呼吸，日间水面会逐渐形成大规模的鱼群聚集。由于鱼类在水面停留的时间增加，我们观察到食鱼鸟类在午间及午后的活动频率显著升高。

本研究揭示，水体溶解氧的日间波动具备深刻改变捕食者-猎物相互作用的潜力，从而凸显了非生物因子与生物因子共同作用、共同塑造猎物物种演化的过程。