Traits have been used extensively to predict and understand performance in response to the abiotic environment, but their role for understanding competitive interactions is less understood, especially in nonplant systems. In this study, we evaluate how traits interact with intraspecific density to modulate performance (per capita birth rate) and whether the traits associated with intraspecific competitive ability are similar across multiple species. We used an experimental system of four cladoceran zooplankton species, experimentally manipulated the density of conspecifics, and measured a range of morphological and life history characteristics (body mass, body length, second antenna length, eye diameter, relative growth rate, age at first reproduction, and birth rate). With causal modeling, we identified significant trait–density relationships for three out of four species, although the specific traits that predicted birth rate varied from species to species. In general, individuals at higher densities displayed smaller morphological traits and shifts towards slower relative growth rates and delayed onset of reproduction. We also asked more generally if there are consistent trait‐mediated impacts of density across multiple species. The interspecific model identified significant trait–density relationships for body length, age at first reproduction, and relative growth rate. Unexpectedly, we found little evidence for trait‐based competition due to mechanisms such as limiting similarity or hierarchical competition, and rather noted the potential for trait plasticity and constraints on plasticity affecting performance in response to the competitive environment.

中文翻译：

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性状-密度关系解释了锁骨浮游动物的表现

特质已被广泛用于预测和理解对非生物环境的反应，但对于理解竞争性相互作用的作用却知之甚少，尤其是在非植物系统中。在这项研究中，我们评估性状如何与种内密度相互作用以调节性能（人均出生率），以及与种内竞争能力相关的性状在多个物种之间是否相似。我们使用了四个锁骨浮游动物物种的实验系统，通过实验控制了物种的密度，并测量了一系列形态和生活史特征（体重，体长，第二触角长度，眼睛直径，相对生长率，初次繁殖的年龄）以及出生率）。通过因果建模，尽管可以预测出生率的具体特征因种而异，但我们确定了四个物种中三个物种的显着性状-密度关系。通常，高密度个体表现出较小的形态特征，并朝着相对较低的相对生长速率和延迟的繁殖开始转变。我们还更普遍地问，跨多个物种的密度是否受到一致的性状介导影响。种间模型确定了显着的性状-密度关系，包括体长，初次繁殖的年龄和相对生长率。出乎意料的是，由于诸如限制相似性或等级竞争，