Parasitism affects consumer-driven nutrient recycling: Disease induced changes in consumer nutrient release
Parasites are increasingly recognized as important components of food webs, especially in productive, nutrient rich ecosystems. Little is known, however, about how parasites contribute to ecosystem nutrient availability and productivity through their effects on host life-history and physiology. Here we examine whether bacterial infection alters nutrient release from its host and if such effects depend upon host diet quality. To do so, we compared the elemental composition and mass specific nutrient (i.e., nitrogen and phosphorus) release rates of uninfected Daphnia with those infected with the bacterial parasite P. ramosa. To examine interactions between infection and host diet quality, we determined whether the release rates of infected and uninfected Daphnia vary with the phosphorus content of their diet.
We found that both diet and infection status affected the elemental composition and nutrient release rates of Daphnia. Daphnia body C:P and N:P ratios were elevated by low quality diets and bacterial infection. Daphnia body C:N ratios were elevated by infection in individuals fed P-rich diets but reduced by infection in individuals fed P-poor diets. Despite the decrease in body P of infected Daphnia, we found that they released less P than uninfected Daphnia fed the same diet. In contrast, neither diet quality nor infection status affected nitrogen release rates. Based on mass balance principles, our results indicate that infected animals with reduced body P content must alter pre-digestion processes (feeding rates or digestive efficiencies) to account for reduced P release associated with infection. Indeed, initial results show reduced feeding rates in infected animals. Furthermore, reduced P release of infected individuals as found here would have important feedbacks on lower trophic levels through CNR especially in ecosystems having high disease prevalence.