Two predator – one prey systems are widespread in nature, and have been important to both ecological theory and practice. A primary question for these systems is whether the two predators produce “emergent effects”: greater or less prey mortality than expected based on the two predators acting alone. We conducted an in-lab experiment on a two predator (the crab Cancer productus and the sea-star Evasterias troschelii), one prey (the bivalve Mytilus trossulus) system common to the marine rocky intertidal of the northeastern Pacific. In 156 mesocosm experimental trials, mortality rates of M. trossulus were compared between Crabs only, Sea-stars only, and a Mixed treatment with both predators present. The experiment used a substitutive design, i.e., predator density was held constant. Based on a Linear Mixed Effects Model, the interactive effects of the number of predator species and water temperature had the greatest effect on mussel mortality. For the observed temperature range, the two predators in the Mixed treatment killed fewer mussels than expected based on the average mussel mortality of the one-predator treatments. As the two predators were never observed to interfere with each other directly, the reduced prey mortality is possibly a trait-mediated indirect interaction, involving behavioral modification (mutual avoidance) on the part of the predators. Temperature may mediate these results; crabs consumed more mussels at higher temperatures while sea-stars did so at lower temperatures. As these experiments were conducted over a large part of the known temperature range for this region, these results suggest that mussels may experience seasonal shifts in both predation rates and the identity of the predator, which should be investigated in future field studies. Increasingly warmer temperatures anticipated in the coming century could increase mussel mortality due to predation by cancrid crabs, which appear to have temperature responsive feeding rates. Understanding this two-predator one-prey system better is particularly important given that mussels are important foundational species, and recent climate change has affected abundances of the taxa involved.

两种捕食者-一种捕食系统在自然界很普遍，对生态学理论和实践都非常重要。这些系统的主要问题是两个捕食者是否产生“紧急效应”：比两个捕食者单独行动时的预期死亡率高或低。我们在两个捕食者（蟹癌产品和海星Evasterias troschelii），东北太平洋海洋岩石潮间带共同的一个猎物（双壳类Mytilus trossulus ）系统上进行了室内实验。在156次中观宇宙试验中，trossulus的死亡率仅在螃蟹，仅海星和同时存在两种捕食者的混合处理之间进行了比较。实验采用替代设计，即捕食者密度保持恒定。基于线性混合效应模型，捕食物种数量和水温的相互作用对贻贝死亡率影响最大。在观察到的温度范围内，基于一种捕食者处理的平均贻贝死亡率，混合处理中的两种捕食者杀死的贻贝比预期少。由于从未观察到两个捕食者直接相互干扰，因此降低的猎物死亡率可能是性状介导的间接相互作用，涉及捕食者方面的行为改变（相互避免）。温度可能会介导这些结果。螃蟹在较高温度下消耗更多的贻贝，而海星在较低温度下消耗更多。由于这些实验是在该地区大部分已知温度范围内进行的，因此这些结果表明，贻贝在捕食率和捕食者身份方面都可能经历季节性变化，应在以后的野外研究中进行调查。预计下个世纪温度会越来越高，这可能是由于螃蟹捕食而导致贻贝死亡的原因，螃蟹似乎具有对温度敏感的摄食速率。鉴于贻贝是重要的基础物种，并且更好地了解这种具有两个捕食者的一猎物系统特别重要，而且最近的气候变化已经影响了所涉分类单元的数量。由于这些实验是在该地区大部分已知温度范围内进行的，因此这些结果表明，贻贝在捕食率和捕食者身份方面都可能经历季节性变化，应在以后的野外研究中进行调查。预计下个世纪温度会越来越高，这可能是由于螃蟹捕食而导致贻贝死亡的原因，螃蟹似乎具有对温度敏感的摄食速率。鉴于贻贝是重要的基础物种，并且更好地了解这种具有两个捕食者的一猎物系统特别重要，而且最近的气候变化已经影响了所涉分类单元的数量。由于这些实验是在该地区大部分已知温度范围内进行的，因此这些结果表明，贻贝在捕食率和捕食者身份方面都可能经历季节性变化，应在以后的野外研究中进行调查。预计下个世纪温度会越来越高，这可能是由于螃蟹捕食而导致贻贝死亡的原因，螃蟹似乎具有对温度敏感的摄食速率。鉴于贻贝是重要的基础物种，并且更好地了解这种具有两个捕食者的一猎物系统特别重要，而且最近的气候变化已经影响了所涉分类单元的数量。这些结果表明，贻贝在捕食率和捕食者身份方面都可能经历季节性变化，应在以后的野外研究中进行调查。预计下个世纪温度会越来越高，这可能是由于螃蟹捕食而导致贻贝死亡的原因，螃蟹似乎具有对温度敏感的摄食速率。鉴于贻贝是重要的基础物种，并且更好地了解这种具有两个捕食者的一猎物系统特别重要，而且最近的气候变化已经影响了所涉分类单元的数量。这些结果表明，贻贝在捕食率和捕食者身份方面都可能经历季节性变化，应在以后的野外研究中进行调查。预计下个世纪温度会越来越高，这可能是由于螃蟹捕食而导致贻贝死亡的原因，螃蟹似乎具有对温度敏感的摄食速率。鉴于贻贝是重要的基础物种，并且更好地了解这种具有两个捕食者的一猎物系统特别重要，而且最近的气候变化已经影响了所涉分类单元的数量。