Metabolic theory proposes that individual growth is governed through the mass‐ and temperature‐dependence of metabolism, and ecological stoichiometry posits that growth is maximized at consumer‐specific optima of resource elemental composition. A given consumer's optimum, the threshold elemental ratio (TER), is proportional to the ratio of its maximum elemental gross growth efficiencies (GGEs). GGE is defined by the ratio of metabolism‐dependent processes such that GGEs should be independent of body mass and temperature. Understanding the metabolic‐dependencies of GGEs and TERs may open the path towards a theoretical framework integrating the flow of energy and chemical elements through ecosystems. However, the mass and temperature scaling of GGEs and TERs have not been broadly evaluated. Here, we use data from 95 published studies to evaluate these metabolic‐dependencies for C, N and P from unicells to vertebrates. We show that maximum GGEs commonly decline as power functions of asymptotic body mass and exponential functions of temperature. The rates of change in maximum GGEs with mass and temperature are relatively slow, however, suggesting that metabolism may not causally influence maximum GGEs. We additionally derived the theoretical expectation that the TER for C:P should not vary with body mass and this was supported empirically. A strong linear relationship between carbon and nitrogen GGEs further suggests that variation in the TER for C:N should be due to variation in consumer C:N. In general we show that GGEs may scale with metabolic rate, but it is unclear if there is a causal link between metabolism and GGEs. Further integrating stoichiometry and metabolism will provide better understanding of the processes governing the flow of energy and elements from organisms to ecosystems.

Citation: Iwabuchi, T., and J. Urabe. 2012. Food quality and food threshold: implications of food stoichiometry to competitive ability of herbivore plankton. Ecosphere 3(6):51. http://dx.doi.org/10.1890/ES12-00098.1Abstract. Exploitative competition for food resources is considered to be an important factor determining the dominant species in communities. In such a competition, a threshold food concentration (TFC), where the growth rate becomes zero, is crucial in determination of competitive outcomes. In addition, recent studies on ecological stoichiometry have suggested that the TFC of a species would differ due to changes in food elements that limit growth. If the magnitude of change in TFC differs between species, competitive superiority among species may also change. The experiments showed that the rank order of the species in TFC changed according to the P content of food, suggesting that competitively superior species can vary with elemental composition of food. These results imply that as well as quantity, the elemental composition of food is important in exploitative competition, and the changes in elemental composition of food may be critical to shape an entire community structure.

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Contrary to an expectation from the size-efficiency hypothesis, small herbivore zooplankton such as Ceriodaphnia often competitively predominate against large species such as Daphnia. However, little is known about critical feeding conditions favoring Ceriodaphnia over Daphnia. To elucidate these conditions, a series of growth experiments was performed with various types of foods in terms of phosphorus (P) contents and composition (algae and bacteria). An experiment with P-rich algae showed that the threshold food level, at which an individual's growth rate equals zero, was not significantly different between the two species. However, the food P:C ratio, at which the growth rate becomes zero, was lower for Daphnia than for Ceriodaphnia, suggesting that the latter species is rather disfavored by P-poor algae. Ceriodaphnia showed a higher growth rate than Daphnia only when a substantial amount of bacteria was supplied together with a low amount of P-poor algae as food. These results suggest that an abundance of bacteria relative to algae plays a crucial role in favoring Ceriodaphnia over Daphnia because these are an important food resource for the former species but not for the latter.

Citation: Iwabuchi, T., and J. Urabe. 2012. Competitive outcomes between herbivorous consumers can be predicted from their stoichiometric demands. Ecosphere 3(1):7. http://dx.doi.org/10.1890/ES11-00253.1Abstract. Exploitative competition for food resources is one of the crucial biological interactions in nature. However, few studies have experimentally tested if competitive ability of consumer species changes depending on elemental contents of the food, although it has been suggested by the theories of resource ratio for competition and ecological stoichiometry. To verify if competitive superiority actually changes according to elemental contents of the food, competition experiments were conducted using high and low P algal food and three Daphnia species (D. galeata, D. pulicaria, and D. tanakai ) with different threshold levels of carbon (TFC) and phosphorus (TFP) necessary for individual growth. Since the TFCs of D. tanakai and D. pulicaria were similar to each other but lower than that of D. galeata, we specifically predicted that when fed high P food (i.e., limited by C), D. tanakai and D. pulicaria would be competitively equal to each other but superior to D. galeata. We also predicted from their TFPs that when fed low P food, D. pulicaria would outcompete D. tanakai and D. galeata, while neither of the latter two would be competitively superior to the other. The results showed that when one of the two competitors was predicted to be competitively superior, the food level under competition was similar to the TFC (or TFP) of the superior species but lower than that of the other inferior species, and that the biomass of the former was much less affected by the competition. Also, when the two competing species were predicted to be competitively equal, the food levels under competition were generally similar to the TFC (or TFP) of both species and their biomasses decreased by the same magnitude. The results thus accorded well with the predictions from the TFC and TFP, and indicate that competitive superiority between the same two Daphnia species changes depending on P:C ratio of algal food. This study provides firm evidence for the first time that resource ratio theory can be applied to competition between animal consumers for essential substances packaged within a single food resource.