Data from: Interactive effects of productivity and predation on zooplankton diversity

Recent studies suggest the necessity of understanding the interactive effects of predation and productivity on species coexistence and prey diversity. Models predict that coexistence of prey species with different competitive abilities can be achieved if inferior resource competitors are less susceptible to predation and if productivity and/or predation pressure are at intermediate levels. Hence, predator effects on prey diversity are predicted to be highly context dependent: enhancing diversity from low to intermediate levels of productivity or predation and reducing diversity of prey at high levels of productivity or predation. While several studies have examined the interactive effects of herbivory and productivity on primary producer diversity, experimental studies of such effects in predator-prey systems are rare. We tested these predictions using an aquatic field mesocosm experiment in which initial density of the zooplankton predator Notonecta undulata and productivity were manipulated to test their interactive effects on diversity of seven zooplankton, cladoceran species that were common in surrounding ponds. Two productivity levels were imposed via phosphorus enrichment at levels comparable to low and intermediate levels found within neighboring natural ponds. We used open systems to allow for natural dispersal and behaviorally-mediated numerical responses by the flight-capable predator. Effects of predators on zooplankton diversity depended on productivity level. At low and high productivity, prey species richness declined while at high productivity it showed a unimodal relationship with increasing the predator density. Effects of treatments were weaker when using Pielou's evenness index or the inverse Simpson index as measures of prey diversity. Our findings are generally consistent with model predictions in which predators can facilitate prey coexistence and diversity at intermediate levels of productivity and predation intensity. Our work also shows that the functional form of the relationship between prey diversity and predation intensity can be complex and highly dependent on environmental context.

近期研究指出，亟需阐明捕食作用与生产力对物种共存及猎物多样性的交互影响效应。模型预测：若资源竞争劣势种对捕食的敏感性更低，且生产力和/或捕食压力处于中等水平，则具备不同竞争能力的猎物物种可实现共存。据此，捕食者对猎物多样性的影响具有高度情境依赖性：在生产力或捕食压力从低水平提升至中等水平的过程中，捕食者可提升猎物多样性；而当生产力或捕食压力处于高水平时，则会降低猎物多样性。尽管已有多项研究探讨了草食作用与生产力对初级生产者多样性的交互影响，但针对捕食者-猎物系统中此类效应的实验研究仍较为匮乏。本研究依托一项水生野外中型实验生态系统（mesocosm）实验验证上述预测：通过调控浮游动物捕食者仰泳蝽（Notonecta undulata）的初始密度与系统生产力，探究二者对周边池塘常见的7种枝角类（cladoceran）浮游动物物种多样性的交互影响。本研究通过磷富集设置两种生产力水平，其梯度与邻近天然池塘中观测到的低、中等生产力水平相当。本实验采用开放系统，以允许具备飞行能力的捕食者发生自然扩散与行为介导的种群数量响应。研究结果显示，捕食者对浮游动物多样性的影响取决于生产力水平：在低生产力与高生产力条件下，猎物物种丰富度均出现下降；而在中等生产力条件下，物种丰富度随捕食者密度升高呈现单峰关系。当采用皮卢均匀度指数（Pielou's evenness index）或逆辛普森指数（inverse Simpson index）作为猎物多样性的衡量指标时，各处理组的效应强度相对较弱。本研究结果与模型预测总体一致：即捕食者可在中等生产力与捕食强度水平下促进猎物共存与多样性提升。本研究同时表明，猎物多样性与捕食强度之间的关系函数形式可能较为复杂，且高度依赖于环境情境。