Control of CNP homeostasis in herbivore consumers through differential assimilation

Abstract

Stoichiometric analysis recognizes that a herbivore is a mixture of multiple chemical elements, especially C, N, and P, that are fixed in various proportions. In the face of a variable quality food supply, herbivores must regulate ingested nutrients to maintain a homeostatic state. We develop a dynamic mathematical model, based on differential assimilation, that controls the C : N and C : P ratios in a herbivore within given tolerance ranges; the actual mathematical mechanism is to define the absorption coefficients to be dependent on these elemental ratios. The model inputs variable, time-dependent food and grazing rates and calculates the net dynamic consumer production (growth) of C, N, and P. It shows that total growth is equally sensitive to variations in both food quality and in assimilation rates, and it shows quantitatively how C-, N-, or P-limited growth could occur at different periods during the development period. The analysis generalizes earlier work limited to just two elements.

Regulation of phosphorus stoichiometry and growth rate of consumers: theoretical and experimental analyses with Daphnia

“…Based on animal growth responses to food P content, Boersma and Kreutzer (2002) suggested that Daphnia may not perfectly retain P taken up from food even at low levels of food with high C:P ratios. Nonetheless, since the P release rate tended to decrease with increasing food C:P ratio (e.g., Olsen et al 1986;DeMott et al 1998;Elser and Urabe 1999), it has been often assumed that there is little P release at very high food C:P ratios (e.g., Urabe and Watanabe 1992;Sterner and Elser 2002;Logan et al 2004;Vrede et al 2004;Frost et al 2006). However, according to the model we have presented, the positive relationship between the S P and the growth rate of D. pulicaria cannot be explained without P losses from the internal pool even if P uptake is highly limited.…”

“…At least some of these are probably not exclusive to each other (Plath and Boersma 2001;Ravet and Brett 2006). Recently, Boersma and Elser (2006) stressed that excessively P-rich food (low C:P) also had adverse effects on animal growth rates by reducing feeding activities or increasing the metabolic costs for C. In our model, net C gain was assumed to be independent of P gain as long as S P was within an allowable range, and fixed values were applied to a, b, and c for P and C. In reality, these parameters would be functions of absolute or relative elements (Logan et al 2004;He and Wang 2007;Mitra and Flynn 2007) or some biochemical content (growth-essential materials or digestion-resistant substances) in food. This possibility would also explain another discrepancy between the model's output and the experimental observation.…”

“…The present model is basically the same in the sense that it incorporates gains (assimilation) and losses of multiple elements to examine a consumer's performance. However, the model differs from these studies in that it explicitly incorporates both changes in the elemental ratio of consumers within a restricted range, as in Logan et al (2004), and loss factors that are differently related to food gain and body mass, as in Anderson et al (2005). Moreover, unlike the previous studies, we formulated a dual mass balance model specifically to examine the relationship between elemental ratios and growth rates of consumer species under various food conditions.…”

“…A number of studies have applied this approach to investigate the effects of food quantity and quality (in terms of elemental ratios) on consumer activities, such as release rates of inorganic and organic elements and growth rate (Hessen 1992;Urabe and Watanabe 1992;Sterner 1997;Sterner and Elser 2002;Frost et al 2004;Logan et al 2004;Anderson et al 2005;Mitra and Flynn 2007). The present model is basically the same in the sense that it incorporates gains (assimilation) and losses of multiple elements to examine a consumer's performance.…”

Ecological stoichiometry of indirect grazer effects on periphyton nutrient content

“…This implies both the inclusion of Xexible consumers in stoichiometric models (Grover 2003) as well as evaluating why consumers can maintain lower Xexibility in nutrient content than their plant prey (Logan et al 2004). …”