Abstract Understanding the role of species interactions within communities is a central focus of ecology. A key challenge is to understand variation in species interactions along environmental gradients. The stress gradient hypothesis posits that positive interactions increase and competitive interactions decrease with increasing consumer pressure or environmental stress. This hypothesis has received extensive attention in plant community ecology, but only a handful of tests in animals. Furthermore, few empirical studies have examined multiple co‐occurring stressors. Here we test predictions of the stress gradient hypothesis using the occurrence of mixed‐species groups in six common grazing ungulate species within the Serengeti‐Mara ecosystem. We use mixed‐species groups as a proxy for potential positive interactions because they may enhance protection from predators or increase access to high‐quality forage. Alternatively, competition for resources may limit the formation of mixed‐species groups. Using more than 115,000 camera trap observations collected over 5 yr, we found that mixed‐species groups were more likely to occur in risky areas (i.e., areas closer to lion vantage points and in woodland habitat where lions hunt preferentially) and during time periods when resource levels were high. These results are consistent with the interpretation that stress from high predation risk may contribute to the formation of mixed‐species groups, but that competition for resources may prevent their formation when food availability is low. Our results are consistent with support for the stress gradient hypothesis in animals along a consumer pressure gradient while identifying the potential influence of a co‐occurring stressor, thus providing a link between research in plant community ecology on the stress gradient hypothesis, and research in animal ecology on trade‐offs between foraging and risk in landscapes of fear.

中文翻译：

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塞伦盖蒂食草动物的混合物种群：对压力梯度假设的检验

摘要 了解群落内物种相互作用的作用是生态学的核心焦点。一个关键的挑战是了解物种相互作用沿环境梯度的变化。压力梯度假说认为，随着消费者压力或环境压力的增加，积极互动会增加，而竞争互动会减少。这一假说在植物群落生态学中受到广泛关注，但在动物身上的试验却屈指可数。此外，很少有实证研究检查了多种同时发生的压力源。在这里，我们使用塞伦盖蒂-马拉生态系统内六种常见放牧有蹄类动物物种中混合物种群的出现来测试压力梯度假设的预测。我们使用混合物种群体作为潜在积极相互作用的代表，因为它们可以增强对捕食者的保护或增加获得优质草料的机会。或者，对资源的竞争可能会限制混合物种群体的形成。使用超过 5 年收集的 115,000 多个相机陷阱观察，我们发现混合物种群体更有可能出现在危险地区（即靠近狮子有利位置的地区和狮子优先捕猎的林地栖息地）和资源水平很高。这些结果与来自高捕食风险的压力可能有助于形成混合物种群体的解释一致，但当食物供应不足时，对资源的竞争可能会阻止它们的形成。