Data from: Effect of phytoplankton richness on phytoplankton biomass is weak where the distribution of herbivores is patchy

Positive effects of competitor species richness on competitor productivity can be more pronounced at a scale that includes heterogeneity in ‘bottom-up’ environmental factors, such as the supply of limiting nutrients. The effect of species richness is not well understood in landscapes where variation in ‘top-down’ factors, such as the abundance of predators or herbivores, has a strong influence competitor communities. I asked how phytoplankton species richness directly influenced standing phytoplankton biomass in replicate microcosm regions where one patch had a population of herbivores (Daphnia pulicaria) and one patch did not have herbivores. The effect of phytoplankton richness on standing phytoplankton biomass was positive but weak and not statistically significant at this regional scale. Among no-Daphnia patches, there was a significant positive effect of phytoplankton richness that resulted from positive selection effects for two dominant and productive species in polycultures. Among with-Daphnia patches there was not a significant effect of phytoplankton richness. The same two species dominated species-rich polycultures in no- and with-Daphnia patches but both species were relatively vulnerable to consumption by Daphnia. Consistent with previous studies, this experiment shows a measurable positive influence of primary producer richness on biomass when herbivores were absent. It also shows that given the patchy distribution of herbivores at a regional scale, a regional positive effect was not detected.竞争物种丰富度对竞争物种生产力的正向效应，在包含上行环境因子（bottom-up environmental factors）异质性的尺度下更为显著，这类因子包括限制性养分的供给等。在下行因子（top-down factors，如捕食者或植食者丰度）对竞争群落具有强烈调控作用的景观中，物种丰富度的效应尚未得到充分阐明。本研究旨在探究：在设置了重复微宇宙（microcosm）区域的实验中，浮游植物物种丰富度对浮游植物现存量生物量的直接影响——其中一组斑块存在透明溞（Daphnia pulicaria）种群，另一组斑块则无植食者存在。在该区域尺度下，浮游植物丰富度对现存量生物量的正向效应虽存在，但强度较弱且未达到统计学显著性水平。在无溞类斑块组中，浮游植物丰富度表现出显著的正向效应，这源于多物种混养群落中两种优势且高生产力物种所产生的正向选择效应。在有溞类斑块组中，浮游植物丰富度未表现出显著效应。无论是无溞类还是有溞类斑块的物种丰富混养群落，均由相同的两个物种占据优势地位，但这两个物种对溞类的捕食均具有相对较高的易感性。与既往研究结果一致，本实验证实：在植食者缺失的情境下，初级生产者（primary producer）丰富度可对生物量产生可检测的正向影响。研究同时表明，当植食者在区域尺度呈斑块状分布时，无法检测到区域层面的正向效应。