Abstract
It is widely recognized that the release of nutrients by herbivores via their waste products strongly impacts nutrient availability for autotrophs. The ratios of nitrogen (N) and phosphorus (P) recycled through herbivore release (i.e., waste N:P) are mainly determined by the stoichiometric composition of the herbivore’s food (food N:P) and its body nutrient content (body N:P). Waste N:P can in turn impact autotroph nutrient limitation and productivity. Herbivore-driven nutrient recycling based on stoichiometric principles is dominated by theoretical and experimental research in freshwater systems, in particular interactions between algae and invertebrate herbivores. In terrestrial ecosystems, the impact of herbivores on nutrient cycling and availability is often limited to studying carbon (C ):N and C:P ratios, while the role of terrestrial herbivores in mediating N:P ratios is also likely to influence herbivore-driven nutrient recycling. In this review, we use rules and predictions on the stoichiometry of nutrient release originating from algal-based aquatic systems to identify the factors that determine the stoichiometry of nutrient release by herbivores. We then explore how these rules can be used to understand the stoichiometry of nutrient release by terrestrial herbivores, ranging from invertebrates to mammals, and its impact on plant nutrient limitation and productivity. Future studies should focus on measuring both N and P when investigating herbivore-driven nutrient recycling in terrestrial ecosystems, while also taking the form of waste product (urine or feces) and other pathways by which herbivores change nutrients into account, to be able to quantify the impact of waste stoichiometry on plant communities.
Highlights
Herbivores are a major component of most ecosystems, ranging in size from zooplankton to elephants
We explore how we can apply these stoichiometric rules to terrestrial ecosystems, focusing on N:P ratios
Prediction 1: There is a positive relationship between food N:P and waste N:P
Summary
Herbivores are a major component of most ecosystems, ranging in size from zooplankton to elephants. Two basic “stoichiometric rules” have been formulated, one based on how food and consumer body N:P determine waste N:P (rule 1), and the other on how waste N:P affects autotroph nutrient limitation and productivity (rule 2). Both rules allow for explicit predictions about the N:P stoichiometry of nutrient release and its ecosystem consequence (Table 1). Waste N:P is predicted to be negatively related to body N:P, if food N:P is constant (Figure 1) These predictions have been supported in lab studies using aquatic invertebrate herbivores (Daphnia) feeding on phytoplankton Prediction 1: There is a positive relationship between food N:P and waste N:P (assuming body N:P is constant)
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