Data from: Density-dependent energy use contributes to the self-thinning relationship of cohorts

In resource-limited populations, an increase in average body size can occur only with a decline in abundance. This is known as self-thinning, and the decline in abundance in food-limited populations is considered proportional to the scaling of metabolism with body mass. This popular hypothesis may be inaccurate, because self-thinning populations can also experience density-dependent competition, which could alter their energy use beyond the predictions of metabolic scaling. This study tested whether density-dependent competition has an energetic role in self-thinning, by manipulating the abundance of the fish Macquaria novemaculeata and tank size to partition the effects of competition from metabolic scaling. We found that self-thinning can be density dependent and that changes in intraspecific competition may be more influential than metabolic scaling on self-thinning relationships. The energetic mechanism we propose is that density-dependent competition causes variation in the allocation of energy to growth, which alters the energetic efficiency of self-thinning cohorts. The implication is that food-limited cohorts and populations with competitive strategies that encourage fast-growing individuals will have less body mass at equilibrium and higher mortality rates. This finding sheds light on the processes structuring populations and can be used to explain inconsistencies in the mass-abundance scaling of assemblages and communities (the energetic-equivalence rule).

在资源受限种群中，平均个体体型的增大仅能伴随种群丰度的下降，这一现象被称为自疏效应（self-thinning）。在食物受限种群中，丰度的下降被认为与代谢速率随个体体重的缩放关系呈正比。这一广为接受的假说可能并不准确，因为发生自疏效应的种群同时也会经历密度依赖型种内竞争，这一过程可能会改变种群的能量利用方式，超出代谢缩放理论的预测范围。本研究通过调控麦夸里鲈（Macquaria novemaculeata）的种群密度与养殖箱尺寸，分离竞争效应与代谢缩放效应，以此检验密度依赖型种内竞争在自疏效应中是否发挥能量学作用。研究结果表明，自疏效应可具有密度依赖性，且种内竞争的变化对自疏关系的影响可能比代谢缩放更为显著。我们提出的能量学机制为：密度依赖型种内竞争会导致个体能量分配至生长的模式发生变化，进而改变自疏同龄群的能量利用效率。该研究的启示在于，食物受限的同龄群以及采取偏好快速生长个体竞争策略的种群，在达到平衡时的个体体重更低，死亡率更高。这一发现为解析种群结构的形成过程提供了新的视角，同时可用于解释集合群落与群落的质量-丰度缩放关系中存在的不一致性（能量等价法则（energetic-equivalence rule））。