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that the impacts of grazers on plant species richness reverse under contrasting nutrient richness, we analyzed unpublished and published data from lake, stream, marine, grassland, and forest ecosystems.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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We measured concentrations of essential fatty acids (EFAs) in four size categories of planktonic organismsseston (10-64 m), microzooplankton (100-200 m), mesozooplankton (200-500 m), and macrozooplankton (Ͼ500 m)-and in rainbow trout (Oncorhynchus mykiss) in coastal lakes. Size-dependent patterns in concentrations of specific fatty acids (FAs) are important for ecosystem function, because planktivorous fish and some invertebrates are size-selective predators. We demonstrate that the retention of individual FAs differs among the four size categories of planktonic organisms in our study systems. Changes in individual EFA concentrations within the planktonic food web were similar in all coastal lakes sampled, which indicates the generality of our findings. Although concentrations of arachidonic acid, eicosapentaenoic acid (EPA), and linoleic acid increased steadily with plankton size, the concentration of ␣-linolenic acid decreased slightly in larger size fractions of zooplankton. Concentrations of another EFA, docosahexaenoic acid (DHA), declined sharply from mesozooplankton to the cladoceran-dominated macrozooplankton size class. Our results indicate that the retention of EFAs, as a function of plankton size, is related, in part, to the taxonomic composition of planktonic food webs. We suggest that, in general, zooplankton exhibit an EPA-retentive metabolism with increasing body size, whereas different taxonomic groups within the planktonic food web retain DHA differently. Finally, we conclude that EPA is highly retained in zooplankton, whereas in rainbow trout DHA is highly retained.

Divergence in habitat use among closely related species is a common characteristic of adaptive radiations. Large differences in the size structure of prey between habitats could strengthen disruptive selection on generalist predators and lead to a divergence in trophic position among species in an adaptive radiation. Using threespine stickleback (Gasterosteus aculeatus) in freshwater lakes as a model system, we examined whether divergence in habitat use coincides with shifts in trophic position. We examined the habitat use and trophic position of individual sticklebacks from divergent lake environments that have only one stickleback species (allopatric lakes) and from lakes that have a pair of benthic and limnetic stickleback species (sympatric lakes). In two sympatric lakes, the limnetic species had a higher trophic position than the benthic species, and in both allopatric and sympatric lakes, sticklebacks specializing on pelagic prey had a higher trophic position for a given size than sticklebacks specializing on benthic prey. Furthermore, the trophic position of pelagic specialists was correlated with individual variation in their gill raker length. Our results indicate that gill raker length is an important trait that underlies differentiation in both habitat use and trophic position among stickleback species, populations, and individuals.

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