Predation risk, stoichiometric plasticity and ecosystem elemental cycling

Abstract

It is widely held that herbivore growth and production is limited by dietary nitrogen (N) that in turn constrains ecosystem elemental cycling. Yet, emerging evidence suggests that this conception of limitation may be incomplete, because chronic predation risk heightens herbivore metabolic rate and shifts demand from N-rich proteins to soluble carbohydrate-carbon (C). Because soluble C can be limiting, predation risk may cause ecosystem elemental cycling rates and stoichiometric balance to depend on herbivore physiological plasticity. We report on a stoichiometrically explicit ecosystem model that investigates this problem. The model tracks N, and soluble and recalcitrant C through ecosystem compartments. We evaluate how soluble plant C influences C and N stocks and flows in the presence and absence of predation risk. Without risk, herbivores are limited by N and respire excess C so that plant-soluble C has small effects only on elemental stocks and flows. With predation risk, herbivores are limited by soluble C and release excess N, so plant-soluble C critically influences ecosystem elemental stocks flows. Our results emphasize that expressing ecosystem stoichiometric balance using customary C:N ratios that do not distinguish between soluble and recalcitrant C may not adequately describe limitations on elemental cycling.

Figure 1.

Stoichiometrically explicit model of a terrestrial herbivore–plant–soil nutrient ecosystem. The model tracks the pools of nitrogen (N), soluble carbon (C_S) and recalcitrant carbon (C_R). Under predation risk, herbivores have increased metabolism (higher C : N ratio) and therefore, actively seek plants with higher ratio of soluble carbon (e.g. sugars). We investigate stoichiometric plasticity in plants and herbivores by analysing models with and without predation risk and variable quantities of plant-soluble C. Solid lines are flows of N, dashed lines are flows of C_S and dotted lines are flows of C_R. See main text for full model description and parameter definitions.

Figure 2.

Stocks of nitrogen (circles), soluble carbon (squares) and recalcitrant carbon (triangles) in (a) soils, (b) plants, and (c) herbivores for an increasing proportion of plant carbon that is soluble (ψ). Results are for models with predation risk ((i), β = 0.85) and for models without predation risk ((ii), β = 0.5). Note different scales on y-axis. Other parameter values are a_P = a_H = 1.15, I = 0.5, r_P = r_H = 0.3, q_S = q_R = k = 0.5, π = 0.5, α = 1.15. Qualitative results are not sensitive to particular parameter values. See the electronic supplementary material, appendix S3 for analytic results.

Figure 3.

Herbivore stocks (measured as nitrogen in herbivore trophic level) under predation risk for increasing proportion of plant C that is soluble (ψ) and increasing herbivore respiration rate (π). Parameter values are β = 0.85, α = 1.15 and all other parameters are as defined in figure 2.

Figure 4.

(a) Total flux of nitrogen, (b) soluble carbon and (c) recalcitrant carbon recycled to the soil nutrient pools. Results are for models with predation risk (open symbols, β = 0.85) and without predation risk (filled symbols, β = 0.5). We show results under predation risk for three levels of herbivore respiration (π = 0.5 (solid lines), 0.6 (dashed lines), 0.7 (dotted lines)). All other parameters are as defined in figure 2. See the electronic supplementary material, appendix S3 and S5 for analytical results.