Competitive outcomes between herbivorous consumers can change with food stoichiometry and be predicted from their stoichiometric demands

Tsubasa Iwabuchi

(Tohoku University)

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. First, we thus conducted experiments with seven Daphnia species to examine if and how threshold food level (TFL), a useful parameter to predict competitive outcome, changes according to the elemental composition of food. The experiments showed that the rank order of the species in TFL changed according to the phosphorus (P) content of food, suggesting that competitively superior species can vary with elemental composition of food. Second, to verify if competitive superiority actually changes according to elemental contents of the food, pairwise competition experiments were conducted using high and low P algal food and three Daphnia species with different threshold levels of carbon (TFC) and phosphorus (TFP). The results accorded well with the prediction based on their rank order in TFC (or TFP), supporting that competitive superiority between the same two Daphnia species can change depending on P:C ratio of algal food. This study provides firm evidence for the first time that Tilman’s resource ratio theory can be applied to competition between animal consumers for essential substances packaged within a single food resource.