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.