Species sorting and biomass partitioning along light:nutrient:predation risk gradients in planktonic pond ecosystems.

摘要

Proponents of the stoichiometric approach to consumer-resource interactions argue that traditional models may mislead ecologists because they ignore nutrient-limitation of grazers. I have explored these claims by developing stoichiometric food web theory and testing it in planktonic plant-grazer systems along experimental gradients of resource supply and predation risk. I found that, like other mechanistic theories, the stoichiometric “light: nutrient” hypothesis can predict changes in species composition over environmental gradients. In the experiment, the phosphorus-rich grazer Daphnia was constrained by insufficient phosphorus sequestered in algal producers at low nutrient supply. This taxa could dominate grazer assemblages at high nutrient supply (without predators). However, other phosphorus-rich grazers did not respond similarly—which is problematic for stoichiometric theory. I offer one resolution of this problem with development of stoichiometric competition theory among grazers. This model predicts changes in composition and coexistence of grazers along light:nutrient gradients: superior nutrient competitors should dominate low nutrient ecosystems, superior carbon competitors should dominate high nutrient ecosystems, and species can coexist at intermediate nutrient supply. The theory also predicts that grazers with very high phosphorus content can outcompete grazers like Daphnia at low nutrient supply. Thus, the experimental results are consistent with stoichiometric principles.; While apparently useful at a community level, the stoichiometric approach seems less powerful at the ecosystem level. I compared and contrasted theoretical food chains and food webs with either a carbon-limited or nutrient-limited grazer. A consistent prediction emerging from these models is that these two grazing scenarios can be differentiated by response of carbon and phosphorus sequestered in edible plants to variation in resource supply. In an experimental test of these two “signatures”, I found that entire grazer assemblages were carbon-limited at low and high nutrient supply. Thus, nutrient-related effects on Daphnia did not scale up to the entire grazer assemblage. Furthermore, I show that a more detailed, traditional food web model can readily predict allocation of biomass among edible and inedible producers and grazers along resource gradients. Consequently, I conclude that a food web perspective must be rigorously developed before the power—and limitations—of the stoichiometric perspective can be fully realized.