Summary1 Coexistence theory predicts that greater heterogeneity of resources or other fitnessconstraining environmental factors will promote species diversity, yet this classic mechanism of coexistence has rarely been tested in manipulative field experiments. 2 Here we present results from the fourth year of a long-term experiment designed to test the heterogeneity-diversity hypothesis in a low productivity grassland where we directly manipulated the spatial heterogeneity of soil nutrients. In addition to unfertilized controls, we created heterogeneous and homogeneous nutrient enrichment treatments by applying fertilizer to field plots in a patchy or uniform manner. Fertilization treatments were crossed with seed addition and cover reduction treatments, to allow examination of the effects of recruitment limitation and competition from established vegetation on diversity responses to nutrient heterogeneity. 3 In contrast to predictions, we found that spatially heterogeneous soil nutrient supply did not promote increased species richness or greater species sorting relative to a uniform supply of soil nutrients. Instead, both nutrient enrichment treatments depressed plant species richness (especially at small spatial scales) and, depending on cover reduction and on spatial location in the field, increased the abundance of two spreading clonal species. 4 Our results do not support the heterogeneity-diversity hypothesis, and suggest that the response of clonal species to nutrient enrichment may constrain how plant diversity responds to resource heterogeneity. Future tests of the heterogeneity-diversity hypothesis should explicitly consider the scale of resource heterogeneity and foraging capability of the plant species in the community.

2005. The influence of consumer diversity and indirect facilitation on trophic level biomass and stability. Á/ Oikos 110: 556 Á/566.The relationship between species diversity and the stability and production of trophic levels continues to receive intense scientific interest. Though facilitation is commonly cited as an essential underlying mechanism, few studies have provided evidence of the impact that indirect facilitation may have on diversity Á/ecosystem functioning relationships. In this laboratory study, we examined the effect of zooplankton species diversity on trophic structure (total algal and zooplankton biomass) and temporal stability of total zooplankton biomass. We utilized four species of pond zooplankton grown in either monoculture or in polyculture. When comparing responses in polycultures with responses averaged across monocultures, a positive effect of diversity on total zooplankton biomass was observed. This occurred as a result of positive facilitative effects among competing zooplankton. Daphnia pulex , a biomass dominant in monoculture, was negatively affected by the presence of interspecific competitors. In contrast, Diaphanosoma brachyurum , a species that performed poorly in monoculture, was strongly and positively affected by the presence of interspecific competitors, driving positive diversity effects on total zooplankton biomass. Positive temporal covariances among zooplankton were detected in several polyculture replicates, increasing temporal variability of total zooplankton biomass. However, this destabilizing effect was weak relative to effects of high biomass yields in polyculture which caused temporal biomass variability (as measured by the coefficient of variation) to be lower in polyculture relative to monocultures. Zooplankton diversity effects on total algal biomass were not detected. However, increased zooplankton diversity significantly altered the size structure of algae, increasing the relative abundance of large, grazer-resistant algae.

Short-term responses of producers highlight that key nutrients (e.g., N, P)-or combinations of these nutrients-limit primary production in aquatic and terrestrial ecosystems. These discoveries continue to provide highly valuable insights, but it remains important to ask whether nutrients always predominantly limit producers despite wide variation in nutrient supply and herbivory among systems. After all, predictions from simple food chain models (derived here) readily predict that limitation by grazers can exceed that by nutrients, given sufficient enrichment. However, shifts in composition of producers and/or increasing dominance of invulnerable stages of a producer can, in theory, reduce grazer limitation and retain primacy of nutrient limitation along nutrient supply gradients. We observed both mechanisms (inter- and intra-species variation in vulnerability to herbivory) working in a two-part mesocosm experiment. We incubated diverse benthic algal assemblages for several months either in the presence or absence of benthic macro-grazers in mesocosms that spread a broad range of nutrient supply. We then conducted short-term assays of nutrient and grazer limitation on these communities. In the "historically grazed" assemblages, we found shifts from more edible, better competitors to more resistant producers over enrichment gradients (as anticipated by the food web model built with a tradeoff in resistance vs. competitive abilities). However, contrary to our expectations, "historically ungrazed" assemblages became dominated by producers with vulnerable juvenile forms but inedible adult forms (long filaments). Consequently, we observed higher resource limitation rather than grazer limitation over this nutrient supply gradient in both "historically grazed" (expected) and "historically ungrazed" (not initially expected). Thus, via multiple, general mechanisms involving resistance to grazing (changes in species composition or variation in stage-structured vulnerability), producer assemblages should remain more strongly or as strongly limited by nutrients than grazers, even over large enrichment gradients.

Wojdak, J. M. 2004. The impact of density-independent mortality on species coexistence: an experimental test with zooplankton. Á/ Oikos 107: 415 Á/421.Mortality (e.g. predation, disturbance) is often thought to lower the intensity of interspecific competition and thereby promote the coexistence of competing species. However, surprisingly few tests of this idea exist, especially for metazoans feeding on a self-renewing resource. Here we examined the effect of density-independent mortality on the coexistence of four species of pond zooplankton (microcrustacean grazers) in a series of laboratory microcosms. Across the experimental mortality gradient, consumer biomass decreased and resource abundance increased with an increase in mortality. Thus, the treatments resulted in an increase in resource availability per consumer (one measure of reduced competitive intensity). There was no significant effect of mortality treatment on species relative abundances or species evenness, and the zooplankter Diaphanosoma dominated community biomass at all mortality levels. Mortality rate did have a marginally significant effect on species richness (p B/0.07), but richness did not increase monotonically with mortality level. Instead, richness tended to be highest in the low-and no-mortality treatments.

The relative effects of nutrients and herbivores on primary producers are rarely compared across ecosystems that vary in potential primary productivity. Furthermore, proposed mechanisms to explain such patterns remain understudied. Here, I examine the strength of nutrient and grazer (herbivore) limitation (i.e., the extent to which producers' growth is limited by insufficient nutrient supply or herbivory) of benthic algae across 13 southwest Michigan lakes that vary widely in productivity (i.e., resource supply). I compare the observed patterns of algal limitation and species composition to those predicted by two simple models: one that includes multiple species and species' traits (the food-web model) and one that includes no variation in species or traits (the food-chain model). Species in the food-web model are assumed to display a tradeoff between resource competitive ability and resistance to herbivory (the "keystone predator" tradeoff). Among these lakes, benthic algal nutrient limitation was positive (x=0.083 day-1) and declined significantly along a lake N:P gradient. In contrast, grazer limitation was negative (x=-0.019 day-1) and was not significantly related to any of the measured lake productivity variables. Negative grazer limitation indicated that the removal of grazers caused unexpected declines in algal biomass, which were potentially due to indirect, positive effects (e.g., nutrient recycling) of grazers. Nutrient limitation was significantly stronger than grazer limitation across lakes, which was more consistent with the food-web versus food-chain model. Changes in algal composition were also broadly consistent with predictions of the food-web model in that vulnerable, superior nutrient competitors dominated in low productivity lakes and more grazer-resistant species were observed in high productivity lakes. In general, these results point to the importance of examining limiting factors across systems and the consideration of key species' traits when predicting and interpreting patterns.