To test the hypothesis that the impacts of grazers on plant species richness reverse under contrasting nutrient richness, we analyzed unpublished and published data from lake, stream, marine, grassland, and forest ecosystems. We analyzed data from 30 studies providing 44 comparisons of plant species richness under low vs. high grazing pressure in enriched or nutrient‐rich and non‐enriched or nutrient‐poor ecosystems. All 19 comparisons from non‐enriched or nutrient‐poor ecosystems exhibited significantly lower species richness under high grazing than under low grazing. In contrast, 14 of 25 comparisons from enriched or nutrient‐rich ecosystems showed significantly higher species richness under high grazing than under low grazing. However, nine of these 25 comparisons showed no significant impact of grazers on species richness, while two comparisons showed declines in species richness under high grazing. Based on all the comparisons, plant species richness decreases with high grazing in nutrient‐poor ecosystems, while it increases with high grazing in nutrient‐rich ecosystems. Although nutrient‐rich ecosystems seemed to produce more variable responses to grazers than did nutrient‐poor ecosystems, in rare cases high grazing produced a decline in species richness in nutrient‐rich environments. We suggest that species richness declines with high grazing in nutrient‐poor ecosystems because a limitation of available resources prevents regrowth of species after grazing, which may not be the case in nutrient‐rich ecosystems. It is also possible that an increase in species richness under high grazing in nutrient‐rich ecosystems may be due to an increase in the dominance of inedible species. Our observation of a grazer reversal of plant species richness under contrasting nutrient richness may have important implications for management of species diversity.

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The response of phytoplankton to nutrient (N, P) and planktivorous fish additions (Phoxinus sp.) was compared between two sets of enclosures (8 m in diameter) installed at different depths in a dimictic temperate lake. One set of eight enclosures was located in the deep part of the lake (12 m) where water thermally stratifies during the summer. The other set of eight enclosures was located in a shallow area of the lake (3.5 m) where the water does not thermally stratify. During the summer months, phytoplankton cell number and biomass increased significantly with both nutrient additions (P = 0.004) and the presence of planktivorous fish (P = 0.011) in the deep, thermally stratified enclosures. On average, total algal biomass in the nutrient and fish treatments was 2.6 times higher than in the nutrient—only treatments. In contrast, in the shallow enclosures, total phytoplankton biomass increased with nutrient additions (P = 0.04) but was not significantly affected by the addition of fish (P = 0.34). However, fish additions increased the number of algal cells in both the shallow and deep enclosures, independently of nutrients, because of an increase in small cells: the proportion of nanoplankton cells was significantly greater in the presence of fish. Fertilization increased the large nanoplankton fraction (10—20 μm) in the deep enclosures, but had no significant effect on the size distribution of algal biomass in the shallow enclosures. In general, large (> 64 μ m) cells were a minor component of the phytoplankton biomass in all treatments. Fertilization stimulated the growth of Chlorophyta in the deep (P = 0.018) and in the shallow (P = 0.012) enclosures. Cyanobacteria were conspicuous in the fertilized shallow enclosures, but the effect on total biomass was not statistically significant. The presence of fish stimulated a significant increase in Cryptophyta at both depths and Pyrrophyta were more common. Nutrients and planktivorous fish had different effects on phytoplankton depending on the thermal regime. In terms of total algal biomass there was no significant difference between deep and shallow enclosures within a given treatment, but depth did affect the size distribution of algal biomass among treatments. The effects of fish were more evident with community structure variables (size or taxonomic distribution) than with coarser variables such as algal biomass.

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