Ecological theory often incorporates the effects of food supply on consumer growth and reproduction by a hyperbolic functional response between animal growth and reproductive rates and the density of food. This is referred to as a 'numerical response'. In addition, in the growth of an individual, a minimal quantity of food, alternatively known as the 'threshold food concentration' or the 'subsistence food level', is defined as that needed to offset respiratory losses. At the population level, a higher threshold is needed for growth to be sufficient to offset various sources of mortality. These effects of food quantity have been studied for a long time; however, incorporating the effects of the quality of food on consumer performance and dynamics has been much more difficult yet is done where ecological stoichiometry becomes very relevant.
While there are many known aspects of food quality (digestibility, palatability, toxicity, content of nutritive bio-chemicals such as essential amino acids and fatty acids), ecological stoichiometry measures food quality as the content of potentially important nutrient elements, especially N and P. This measure is in keeping with the extensive literature on the importance of N in affecting consumers in terrestrial and marine environments. It is also consistent with the increasing knowledge of the role of dietary P in freshwaters and perhaps in terrestrial ecosystems as well. In general, the quality of a food item is inversely related to its C:nutrient ratio, where the nutrient can be N or P. This effect can be understood to be an outcome of the fact that the nutrient element is increasingly diluted in the dominant C-biomass, making it difficult for the animal to extract sufficient limiting element from the food.
Stoichiometric theory gauges the onset of such limiting effects via calculation of the threshold elemental ratio (TER). The TER is defined as the food C:nutrient ratio above which the animal's growth becomes limited by nutrient X rather than C (carbon), where X might be N, P, or another element. In the simplest models, the TER is based on only three traits of the consumer: its own C:X ratio and its maximal gross growth efficiencies for C (Ec) and the nutrient (EX). Gross growth efficiency is defined as the rate at which mass accumulates in production (or new growth) divided by the rate at which mass is ingested by the consumer. The maximum gross growth efficiency for an element is assumed to occur when that element is limiting growth. Assuming strict stoichiometric homeos-tasis of the consumer, the TER is calculated as terc:x = (£c/£x)c : X
Noting again that the TER is the C:X ratio in the food above which the consumer's growth should become limited by element X, a consumer with low TER is more likely to be limited by element X than a consumer with a high TER. Thus, an animal becomes less likely to be limited by nutrient X (its TER increases) as the consumer's C:X ratio increases (its nutrient content declines), as its ability to sequester element X increases (EX increases), or as its carbon-assimilation efficiency decreases (Ec decreases).
The dependence of the TER on net C-growth efficiency implicitly means that the TER increases as food concentration decreases. This is because under such strong food-limited conditions, the respiratory demands of maintenance metabolism dominate the energy or C-balance, and C-growth efficiency is reduced. Thus, TER theory predicts that stoichiometric food quality should be of greatest importance under conditions of high food abundance. Figure 6 illustrates this in a general case, in which the line labeled 'A' is the minimum TER for the consumer in question, asymptotically achieved at maximum Ec at infinitely high food quantity. The line labeled 'B' is the minimum amount of food that the consumer needs in order to maintain a constant body mass.
The effects of stoichiometric food quality have been extensively tested in the laboratory and the field, especially for freshwater zooplankton under conditions of potential P-limitation. For example, Daphnia growth commonly declines when food C:P ratio is above ^250. P-assimilation efficiency increases to high values as the
Figure 6 Boundaries for limitation by food quantity (energy) vs. food quality (stoichiometry) for a homeostatic consumer as a function of food quantity. At high food abundance and high food C:nutrient ratios (low food quality), the consumer should be limited by the nutrients in its food. At low food abundance or low food C:nutrient, the consumer should be limited by total food quantity, or total energy content.
TER is approached. Lab experiments involving short-term manipulation of food P-content or direct P-supply to the Daphnia have confirmed that this growth decline is at least partially due to a direct P-limitation of the animal. Furthermore, field studies have shown that Daphnia abundance is low under lake conditions in which seston C:P ratios are above 250. Further, short-term amendment of seston P-content increases Daphnia growth when seston C:P is above the TER, showing that the predictions of stoichiometric theory are confirmed not only in the lab but also under natural conditions. Also consistent with TER theory, animals with low body P-content, such as the crustacean Bosmina, appear to be relatively insensitive to food P-content, both in the lab and in lakes. Recent studies outside of the plankton have also provided evidence of dietary P-limitation in benthic (stream insects, snails) and terrestrial animals (caterpillars, weevils), adding to previous evidence of dietary N-limitation from diverse habitats.
In sum, these studies show that stoichiometry is an important axis of food quality affecting basal consumers in diverse food webs. Relatively simple physiological processes and species traits can be used to predict their operation and impact.
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