Data from: Does intraspecific competition promote variation? A test via synthesis

Competitive diversification, that is, when increasing intraspecific competition promotes population niche expansion, is commonly invoked in evolutionary studies and currently plays a central role in how we conceptualize the process of adaptive diversification. Despite the frequency with which this idea is cited, the empirical evidence for the process is somewhat limited, and the findings of these studies have yet to be weighed objectively through synthesis. Here, we sought to fill this gap by reviewing the existing literature and collecting the data necessary to assess the evidence for competition as a diversifying force. Additionally, we sought to test a more recent hypothesis, which suggests that competition can act to both promote and inhibit dietary diversification depending on the degree to which a consumer depletes its resources. The surprising result of this synthesis was that increasing competition did not have a mean positive effect on population-level diet breadth or the degree of individual specialization. Instead, we found that increasing intraspecific competition had a restricting effect on population-level diet breadth in as many cases as it had a diversifying effect. This wide disparity in the effect of competition on consumer diet variation was negatively related to a metric for consumer resource depletion. Altogether, these findings call into question a long-standing assumption of basic evolutionary models and lend some support to recent theoretical predictions. Specifically, these findings support the idea that competition is primarily diversifying for species with a small effect (per unit biomass) on their resources and that resource depletion limits the diversifying effect of competition for consumers with larger ecological effects.

竞争分化（Competitive diversification），即种内竞争（intraspecific competition）加剧推动种群生态位扩张（population niche expansion），是进化研究中常被援引的核心概念，目前也是我们阐释适应性分化（adaptive diversification）过程的核心理论框架。
尽管这一观点被频繁引用，但该过程的实证证据仍较为有限，且现有研究结论尚未通过系统性综述得到客观评估。
为此，我们通过梳理现有文献、收集必要数据，旨在评估竞争作为分化驱动力的相关证据，以此填补这一研究空白。
此外，我们还检验了一项新近提出的假说：该假说认为，竞争对饮食分化（dietary diversification）的作用兼具促进与抑制效果，具体取决于消费者对其资源的消耗程度。
本综述得出的出人意料结果显示，竞争加剧并未对种群水平食谱宽度（population-level diet breadth）或个体特化（individual specialization）程度产生显著的正向平均效应。
相反，我们发现，种内竞争加剧在同等比例的案例中，既可能限制种群食谱宽度，也可能推动种群食谱分化。
竞争对消费者食谱变异的影响存在显著差异，且这种差异与消费者资源消耗（consumer resource depletion）的衡量指标呈负相关关系。
综上，本研究结论对经典进化模型的一项长期预设提出了质疑，同时为近期的理论预测提供了一定支持。
具体而言，本研究结果支持如下观点：当消费者（单位生物量）对资源的影响较小时，竞争主要发挥分化作用；而资源消耗会限制那些对资源具有更强生态效应（ecological effects）的消费者的竞争分化效果。