Homeostasis of element composition is one of the central concepts of ecological stoichiometry. In this context, homeostasis is the resistance to change of consumer body composition in response to the chemical composition of consumer's food. To simplify theoretical analysis, it has generally been assumed that autotrophs exhibit flexibility in their composition, while heterotrophs are confined to a constant (strictly homeostatic) body composition. Yet, recent studies suggest that heterotrophs are not universally strictly homeostatic. We examined the degree to which autotrophs and heterotrophs regulate stoichiometric homeostasis (P:C, N:C, N:P, or %P and %N). We conducted a quantitative review and meta‐analysis using 132 datasets extracted from 57 literature sources which examined the dependence of organismal stoichiometry on resource stoichiometry. Among individual datasets, there was a wide range of responses from strictly homeostatic to non‐homeostatic. Even within heterotrophic organisms, varying levels of homeostasis were observed. Comparing the degree of homeostasis between organisms based on large‐scale habitat types using meta‐analysis indicated some significant differences between groups. For example, aquatic macroinvertebrates were significantly more homeostatic in terms of P:C than terrestrial invertebrates. Our meta‐analysis also confirmed that, with regard to N:P, heterotrophs are significantly more homeostatic than autotrophs. Furthermore, our analysis indicated that the homeostasis parameter 1/H, despite being a potentially useful predictive metric, has to be utilized with caution since it oversimplifies some important aspects of the responses of organisms to elemental imbalances. This critical evaluation of stoichiometric homeostasis contributes to a better understanding of many food‐web interactions, which are commonly driven by elemental imbalances between consumers and their resources.

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2006. Physiological responses to stoichiometric constraints: nutrient limitation and compensatory feeding in a freshwater snail. Á Oikos 115: 484 Á494.Nitrogen (N) and phosphorus (P) are considered to be essential nutrients that control secondary production in various ecosystems; insufficient availability of N and P can limit herbivore growth. Here, data are presented from field samplings and from a laboratory experiment on the potential of primary producers low in P, N, or P and N to constrain growth of the freshwater gastropod Radix ovata . The filamentous green alga Ulothrix fimbriata was cultured under different nutrient regimes, resulting in algae with different C:N:P ratios. The pure algae were fed in high and low quantities to juvenile R. ovata . Low availability of N and especially P in the algae strongly constrained the biomass accrual of the herbivore. In accordance with theoretical predictions, these food quality differences were highly dependent on the food quantity. The snails' growth rate was significantly related to their body C:P ratio, thereby supporting the growth rate hypothesis. R. ovata displayed a pronounced compensatory feeding response to lownutrient food that could partly dampen but not fully compensate the food quality effects on snail growth. Increased feeding of gastropods at low P and/or N availability leads to depletion of periphyton biomass; hence compensatory feeding would shift the benthic herbivore community from a P or N limitation to a C limitation and thus have whole-ecosystem effects. P. Fink (fink@limno.net) and E.To understand the energy transfer at the plant Áherbivore interface in food webs, it is essential to determine the factors that control the efficiency of the transfer of matter and energy. The efficiency of this energy transfer is highly variable, often because of the nutritional quality of the food for the consumers. The nutritional quality can be measured as the resulting growth or reproductive output per unit of consumed resource e.g. carbon (C). However, effects of food quality and food quantity on herbivore fitness are often difficult to separate (Sterner 1997, Sterner andSchulz 1998).The factors determining food quality of freshwater zooplankton have been extensively studied (Sterner and Schulz 1998). The mineral nutrients nitrogen (N) and phosphorus (P) have gained attention because their limited availability leads to highly variable C:N:P ratios in primary producers. High C:N and C:P ratios in primary producers in a broad range of ecosystems (terrestrial insect Áherbivore interactions, and marine and especially freshwater plankton) could result in insufficient availability of N and P, which can limit herbivore growth (Sterner 1993, Elser et al. 2000b).

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