Frequency dependence and ecological drift shape coexistence of species with similar niches

The coexistence of ecologically similar species might be counteracted by ecological drift and demographic stochasticity, both of which erode local diversity. With niche differentiation, species can be maintained through performance trade-offs between environments, but trade-offs are difficult to invoke for species with similar ecological niches. Such similar species might then go locally extinct due to stochastic ecological drift but there is little empirical evidence for such processes. Previous studies have relied on biogeographical surveys and inferred process from pattern, while experimental field investigation of ecological drift are rare. Mechanisms preserving local species diversity, such as frequency-dependence (e. g. rare-species advantages), can oppose local ecological drift, but the combined effects of ecological drift and such counteracting forces have seldom been investigated. Here, we investigate mechanisms between coexistence of ecologically similar but strongly sexually differentiated damselfly species (Calopteryx virgo and C. splendens). Combining field surveys, behavioral observations, experimental manipulations of species frequencies and densities, and simulation modelling, we demonstrate that species coexistence is shaped by the opposing forces of ecological drift and negative frequency-dependence (rare species advantage), generated by interference competition. Stochastic and deterministic processes therefore jointly shape coexistence. The role of negative frequency-dependence in delaying the loss of ecologically similar species, such as those formed by sexual selection, should therefore be considered in community assembly, macroecology, macroevolution and biogeography.

生态位相似的物种共存，可能会受到生态漂变（ecological drift）与种群统计随机性（demographic stochasticity）的拮抗，二者均会削弱局域生物多样性。借助生态位分化（niche differentiation），物种可通过不同环境下的适合度权衡（performance trade-offs）实现存续，但对于生态位高度相似的物种而言，此类权衡难以发挥作用。此类生态位相似的物种或因随机生态漂变走向局域灭绝，但目前针对该过程的实证研究仍较为匮乏。以往研究多借助生物地理学调查（biogeographical surveys），通过群落格局反推生态过程，但针对生态漂变的野外实验探究仍较为稀缺。维持局域物种多样性的机制（如频率依赖（frequency-dependence）效应，例如稀有种优势）可对抗局域生态漂变，但生态漂变与这类拮抗力量的联合作用却鲜有研究。本研究聚焦生态位相似但性别分化显著的两种豆娘（色蟌属：Calopteryx virgo与C. splendens）的共存机制。本研究整合野外调查、行为观测、物种频率与密度的实验操控，以及模拟建模手段，证实物种共存由生态漂变与干扰竞争（interference competition）介导的负频率依赖（negative frequency-dependence）效应（即稀有种优势）这两种拮抗力量共同塑造。因此，随机过程与确定性过程共同调控物种共存格局。在群落构建（community assembly）、宏生态学（macroecology）、宏进化（macroevolution）与生物地理学研究中，应纳入负频率依赖效应在延缓生态位相似物种（如由性选择演化而来的类群）局域灭绝中的作用。