Biotic interactions are central to both ecological and evolutionary dynamics. In the vast majority of empirical studies, the strength of intraspecific interactions is estimated by using simple measures of population size. Biologists have long known that these are crude metrics, with experiments and theory suggesting that interactions between individuals should depend on traits, such as body size. Despite this, it has been difficult to estimate the impact of traits on competitive ability from ecological field data, and this explains why the strength of biotic interactions has empirically been treated in a simplistic manner. Using long-term observational data from four different populations, we show that large Trinidadian guppies impose a significantly larger competitive pressure on conspecifics than individuals that are smaller; in other words, competition is asymmetric. When we incorporate this asymmetry into integral projection models, the predicted size structure is much closer to what we see in the field compared with models where competition is independent of body size. This difference in size structure translates into a twofold difference in reproductive output. This demonstrates how the nature of ecological interactions drives the size structure, which, in turn, will have important implications for both the ecological and evolutionary dynamics.

生物相互作用对于生态和进化动力学都是至关重要的。在绝大多数的实证研究中，种内相互作用的强度是通过使用简单的种群规模度量来估算的。生物学家早就知道这些是粗略的指标，实验和理论表明，个体之间的相互作用应取决于诸如体型之类的特征。尽管如此，很难根据生态田间数据估算出性状对竞争能力的影响，这解释了为什么以经验的方式简单地对待生物相互作用的强度。使用来自四个不同人群的长期观察数据，我们表明，特立尼达大孔雀鱼对同种动物的竞争压力比较小的个体更大。换一种说法，竞争是不对称的。当我们将这种不对称性纳入整体投影模型时，与竞争独立于车身尺寸的模型相比，预测的尺寸结构与我们在现场所看到的更加接近。规模结构的这种差异转化为生殖产出的两倍差异。这证明了生态相互作用的本质是如何驱动大小结构的，而大小结构又将对生态动力学和进化动力学产生重要影响。