Too Much of a Good Thing: On Stoichiometrically Balanced Diets and Maximal Growth

Abstract

Nutritional imbalances are of great interest in the ecological stoichiometry literature, in which researchers have focused almost exclusively on cases where nutrients are available in low amounts relative to energy (carbon), and animal growth is impaired due to insufficient nutrient intake. Little attention has been given to situations where food elemental content is higher than the level that satisfies animal requirements. However, most animals are strongly homeostatic with respect to the elemental composition of their body; hence they must excrete the excess of elements that are not in short supply. To date, stoichiometric theory has assumed that excretion of superfluous elements does not come with a cost and, thus, that consumption of food with surplus nutrients does not impair performance. Here we challenge this assumption, based on a compilation of several examples involving food phosphorus content that show that the performance of a wide variety of animals decreases when supplied with food containing high concentrations of (potentially) limiting nutrients. We discuss possible mechanisms for this phenomenon, and suggest that animals most vulnerable to effects of high food nutrient content are those that normally feed on low- quality (low-nutrient: C) food, and have a relatively low body nutrient content themselves, such as herbivores and detritivores.

Wheat Nitrogen Fertilisation Effects on the Performance of the Cereal Aphid Metopolophium dirhodum

“…The aphids were confined by clip cages to their feeding site and therefore had to ingest what was available to them and their reduced performance at the lower rates of fertilisation supports the hypothesis [30] that the phloem sap composition had fallen below a nutritional 'threshold'. In a similar manner, the reduction in fecundity at the highest rate of fertilisation supports the view [60] that an upper TER may also exist for M. dirhodum. Possible underlying physiological mechanisms to explain these effects have been highlighted in a recent review of the literature [44]; they include the influence of fertiliser-induced plant defensive compounds and effects on aphid gut symbionts.…”

“…Another study with populations of Aphis nerii, begun from apterae placed on milkweed (Asclepisa syriaca) and three rates of nitrogen fertiliser (0, 0.071 and 0.143g NH 4 NO 3 ) [42], found that the highest populations were on plants with the intermediate levels of fertilisation. Here it was suggested the results supported the hypothesis that high levels of applied nitrogen increase plant N concentrations above an aphid threshold elemental ratio (TER) [60] such that performance is impaired, leading to a decrease in aphid population growth.…”

The nature of nutrition: a unifying framework

“…That there are indeed measurable costs to both excesses and deficits of macronutrients is now experimentally verified in several instances ; and see previous section). We suspect that Bertrand's rule is at least as prevalent in nutrition as is hormesis in toxicology (see also Boersma and Elser 2006).…”

Trophic flexibility in larvae of two fish species (lesser sandeel, <i>Ammodytes marinus</i> and dab, <i>Limanda limanda</i>)

“…This large variability in primary production quality for higher trophic levels is caused by a large flexibility in the biochemical composition of algal cells, which depends on growth conditions (Aberle and Malzahn, 2007). The variability of food quality decreases with increasing trophic level (Boersma, 2000;Boersma and Elser, 2006;Boersma et al, in press;Malzahn et al, in press) due to the tendency (or the constraint) of consumers to keep their chemical and thus biochemical body composition relatively constant (Elser et al, 2000).…”