Data from: Density-dependent indirect effects: apparent mutualism and apparent competition coexist in a two-prey system

A predator consuming multiple prey species usually causes indirect effects. Apparent mutualism results when multiple prey species reduce predation risk for each other by altering a predator’s functional response. Short-term apparent competition occurs when multiple prey species increase predation risks for each other through the numerical response, i.e. increasing the predator’s birth rate, or aggregative response, i.e. attracting a higher density of predators. Our objectives in this study were to determine the aggregative response and 2-prey functional response of a predator and to examine indirect effects over a range of prey densities. We used the clam Macoma balthica and juvenile blue crabs Callinectes sapidus as prey for adult blue crabs. In laboratory experiments, we determined the single-prey functional responses of the crabs to each prey species and the 2-prey functional response. We combined the 2-prey functional response with the known blue crab aggregative response to clams to estimate field predation rates. Our model predicts that at low prey densities, clams and juvenile blue crabs exhibit apparent mutual- ism, whereas at high clam densities, this relation switches to short-term apparent competition. These unexpected results highlight the need to incorporate multiple aspects of predation at multiple scales when considering indirect effects.

捕食者取食多种猎物物种通常会引发间接效应。当多种猎物通过改变捕食者的功能反应（functional response）来相互降低被捕食风险时，便会产生表观互利共生（apparent mutualism）。当多种猎物通过数量反应（numerical response，即提升捕食者的出生率）或聚集反应（aggregative response，即吸引更高密度的捕食者）来相互增加被捕食风险时，则会出现短期表观竞争（short-term apparent competition）。本研究的目标在于确定捕食者的聚集反应以及双猎物功能反应，并探究不同猎物密度下的间接效应。我们以白樱蛤（Macoma balthica）与幼年蓝蟹（Callinectes sapidus）作为成年蓝蟹的猎物开展实验。在室内实验中，我们测定了蟹类对每种猎物的单猎物功能反应，以及双猎物功能反应。我们将双猎物功能反应与已知的蓝蟹对白樱蛤的聚集反应相结合，以估算野外捕食速率。我们的模型预测，在低猎物密度下，白樱蛤与幼年蓝蟹会表现出表观互利共生关系；而在白樱蛤密度较高时，这种关系会转变为短期表观竞争。这些出乎意料的研究结果表明，在探究间接效应时，需要从多个尺度纳入捕食作用的多方面特征。