1. Theory suggests that intraspecific trait variation will alter species interaction strengths through nonlinear averaging when interaction strengths are nonlinear functions of individuals' traits. This effect is expected to be widespread, yet what factors mediate its magnitude in nature and hence its potential effects on ecosystems and communities are unclear.
2. We sought to quantify how nonlinear predator functional responses, variation in prey densities and counteracting variation in attack rates and handling times among predator individuals of similar body size alter their population-level feeding rates through nonlinear averaging in a natural system, and to determine the processes influencing the net magnitude of this effect.
3. We used a field caging experiment in the rocky intertidal of Oregon, USA to quantify attack-rate variation and feeding rates of the whelk Nucella ostrina on its barnacle and mussel prey. We also used empirically parameterized simulations to examine the effects of handling-time variation among individuals on population-level feeding rates.
4. Within cages, individual attack-rate variation reduced population-level whelk feeding rates. However, the magnitude of this reduction differed among prey species and cages depending on cage-specific magnitudes of attack-rate variation and functional-response nonlinearity. The inferred effects of handling-time variation among individuals were of smaller magnitude than those of attack-rate variation, yet counteracted them to cause a net weakening of the effect of individual attack-rate variation on population-level feeding rates. Across cages, attack-rate and prey-density variation had non-additive effects that produced greater feeding-rate reductions at the experiment scale relative to the cage scale.
5. Our results indicate that the effects of trait variation via nonlinear averaging depend critically on the features of systems that determine the magnitudes of nonlinearities and trait variation. Because of counteracting trait variation, nonlinear-averaging effects may be quite complex, involving both the variances and covariances of all traits and environmental variables influencing the ecological process of interest.

中文翻译：

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通过非线性平均量化种内变异对捕食者摄食率的影响

1. 理论认为，当相互作用强度是个体特征的非线性函数时，种内性状变异将通过非线性平均改变物种相互作用强度。预计这种影响将是普遍的，但究竟是什么因素在自然界中调节了它的大小，因此它对生态系统和社区的潜在影响尚不清楚。
2. 我们试图量化非线性捕食者功能反应、猎物密度的变化以及抵消攻击率和处理时间的变化如何通过自然系统中的非线性平均来改变相似体型的捕食者个体之间攻击率和处理时间的变化，并确定这些过程影响这种效应的净幅度。
3. 我们在美国俄勒冈州的岩石潮间带进行了野外笼养实验，以量化海螺 Nucella ostrina对其藤壶和贻贝猎物的攻击率变化和摄食率。我们还使用经验参数化模拟来检查个体之间处理时间变化对种群水平喂养率的影响。
4. 在笼子内，个体攻击率的变化降低了种群水平的海螺摄食率。然而，这种减少的幅度在猎物物种和笼子之间有所不同，具体取决于笼子特定的攻击率变化幅度和功能响应非线性。个体之间处理时间变化的推断影响比攻击率变化的影响要小，但抵消了它们，导致个体攻击率变化对种群水平摄食率的影响净减弱。在整个笼子中，攻击率和猎物密度的变化具有非加性效应，相对于笼子规模，在实验规模上产生了更大的喂食率降低。
5. 我们的结果表明，通过非线性平均的特征变异的影响主要取决于决定非线性和特征变化幅度的系统特征。由于抵消性状变异，非线性平均效应可能非常复杂，涉及所有性状的方差和协方差以及影响感兴趣的生态过程的环境变量。