Data from: Herbivory enhances the diversity of primary producers in pond ecosystems

Diversity of primary producer is often surprisingly high, despite few limiting factors such as nutrients and light to facilitate species coexistence. In theory, the presence of herbivores could increase the diversity of primary producers, resolving this “paradox of the plankton”. Little experimental evidence supports this natural enemies hypothesis, but previous tests suffer from several deficiencies. Previous experiments often did not allow for multigeneration effects; utilized low diversity assemblages of herbivores; and limited opportunities for new primary producer and herbivore species to colonize and undergo species sorting that favors some species over others. Using pond plankton, we designed a mesocosm experiment that overcame these problems by allowing more time for interactions over multiple generations, openness to allow new colonists, and manipulated higher diversity of primary producers and grazers than have previous studies. With this design, the presence of zooplankton grazers doubled phytoplankton richness. The additional phytoplankton species in grazed mesocosms were larger, and therefore likely more grazer-resistant. Furthermore, phytoplankton richness in grazed mesocosms was similar to that observed in natural ponds whereas it was much lower in mesocosms without grazers. However, stoichiometric imbalance caused by variation in nitrogen:phosphorus ratios and light supply did not alter phytoplankton richness. Therefore, grazers enhanced primary producer richness more strongly than ratios of nutrient supply (even though both grazing and ratios of resource supply altered composition of primary producer assemblages). Taken together, these experimental and field data show that grazing from a diverse assemblage of herbivores greatly elevated richness of phytoplankton producers in pond ecosystems.

尽管营养盐、光照等限制性因子匮乏，难以维持物种共存，但初级生产者（primary producer）的多样性往往高得出人意料。理论上，植食动物（herbivores）的存在可提升初级生产者多样性，从而破解这一“浮游生物悖论（paradox of the plankton）”。鲜有实验证据支持这一天敌假说（natural enemies hypothesis），且既往相关实验存在诸多缺陷：既往实验往往未考虑多代效应，采用了低多样性的植食动物集合群，且限制了新的初级生产者与植食动物物种的定殖，以及对部分物种更有利的物种分选过程的发生机会。本研究以池塘浮游生物为研究对象，设计了中型实验生态系统（mesocosm）实验，通过延长多代种间互作的实验时长、保留环境开放性以允许新物种定殖、设置较既往研究更高的初级生产者与牧食者（grazers）多样性水平，克服了上述局限。采用该实验设计后，浮游动物（zooplankton）牧食者的存在使浮游植物（phytoplankton）的物种丰富度提升了一倍。在有牧食者的中型生态系统中，新增的浮游植物个体更大，因此对牧食者的抗性更强。此外，有牧食者的中型生态系统内浮游植物物种丰富度与自然池塘的观测结果相近，而无牧食者的中型生态系统的丰富度则显著更低。不过，由氮磷比与光照供给变化引发的化学计量失衡（stoichiometric imbalance）并未改变浮游植物的物种丰富度。由此可见，相较于养分供给比率，牧食者对初级生产者物种丰富度的提升作用更为显著——尽管牧食作用与资源供给比率均会改变初级生产者集合群的群落组成。综合上述实验与野外观测数据可知，多样的植食动物牧食作用可显著提升池塘生态系统中浮游植物生产者的物种丰富度。