Plant-herbivore interactions and ecological stoichiometry: when do herbivores determine plant nutrient limitation?

Recent studies on plant-herbivore indirect interactions via nutrient recycl ing have led to the hypothesis that herbivores with a low nitrogen: phospho rus ratio, feeding on plants with a higher nitrogen: phosphorus ratio, recy cle relatively more nitrogen, driving plants into phosphorus limitation. We demonstrate in this paper that such a hypothesis is valid only under restr icted conditions, i.e. the nitrogen: phosphorus ratio of inorganic nutrient s supplied to the system must be neither too high nor too low compared with the nitrogen: phosphorus ratio of the whole plant + herbivore biomass. If plants have a greater affinity for phosphorus than for nitrogen, low herbiv ore nitrogen: phosphorus ratio can even promote nitrogen limitation. These results are qualitatively robust, whether grazing functions are donor-contr olled or recipient-controlled. We present a graphical analysis of these con ditions based on the Zero Net Growth Isocline method.