Growth rate and other life-history attributes have been a focal area of theoretical and empirical research in evolutionary biology for more than 30 years. This work has shown that the evolution of life histories is guided by tradeoffs, in that costs associated with the expression of one trait limit the expression of another. Examples of life-history tradeoffs include those between current reproduction and future reproduction, reproduction and growth, or growth and defense.
Evolutionary tradeoffs will often be mediated by the availability of resources that must be allocated between traits. Life-history theory has often been based on the assumption that a single substance underlies such resource-based tradeoffs. For example, a model might be based on the assumption that a consumer has access to a pool of food energy that can be used equally well to meet its demands for growth or defense. Under this assumption, an increase in one trait (e.g., growth) leads to a proportional decrease in the expression of the other (defense). However, as outlined above, there are key elemental differences among biomolecules that should lead to differences in resource requirements between traits. In fact, resource requirements of traits such as growth, reproduction, and defense may be similar to the elemental composition of those traits because most of the substrate is used for structure rather than the metabolic requirements of trait assembly.
The existence of stoichiometrically distinct demands leads to an important prediction for the evolution of life histories: an optimal investment strategy will depend on the stoichiometry of each trait and the relative supply of different elements in the environment. Investment in a trait that requires an element that is difficult to acquire will be more costly than a trait requiring the same amount of another element that is relatively available. As a result, selection may favor traits (e.g., rapid growth) that rely heavily on particular substances (e.g., P) only when those substances are readily available in the environment.
Several empirical patterns suggest that economic considerations have influenced the evolution of organismal stoichiometry. For example, leaf nutrient concentrations are generally lower in plant species that dominate nutrient-poor sites than they are in species from fertile areas. Similarly, insect herbivores tend to contain less N than insect predators; one explanation for this pattern is that N scarcity in plant tissue has selected for lower N dependence in herbivores. Economic constraints may also have influenced the evolution of stoichiometry at finer scales. For example, enzymes used by the bacterium E. coli and the eukaryote Sacharomyces cerevisiae to assimilate C contain significantly less C than do typical proteins in these organisms. This result is intriguing because C assimilatory enzymes are probably most valuable to the organisms when C is scarce in the environment. The low C content in these enzymes may thus reflect selection for higher enzyme production in the face of a particular scarcity. Similar patterns have also been documented for the S content of S-assimilating enzymes of E. coli and S. cerevisiae.
Economic constraints can also be integrated with functional considerations to predict the evolution of organismal composition. Consider the evolutionary tradeoff between growth rate and competitive ability in marine phytoplankton. Such a tradeoff is likely in many organisms because the high resource demands associated with rapid growth will often put an organism at a competitive disadvantage when resources are scarce. Ecological stoichiometry provides a basis for determining the mechanisms underlying this tradeoff. In phytoplankton, response to this tradeoff can be characterized by investments in assembly machinery for biosynthesis and in resource-acquisition machinery. Assembly machinery consists of P- and N-rich ribosomes, while resource-acquisition machinery consists of chloroplasts and nutrient-uptake proteins that are rich in N but contain little P. When resources are abundant, selection favors greater investment in ribosomes for supporting rapid growth, leading to lower optimal N:P ratios. In resource-poor environments, phytoplankton with low N:P ratios are less successful because of their limited investment in resource acquisition and their high P demands for assembly. The N:P ratio in phytoplankton that is predicted to evolve by natural selection will thus depend on the availability of resources in the environment because of the different functions of P- and N-rich structures.
See also: Animal Physiology; Biogeochemical Models; Ecological Stoichiomety: Overview; Ecophysiology; Ecosystem Patterns and Processes; Fish Growth; Grassland Models; Lake Models; Optimal Foraging Theory; Organismal Ecophysiology; Plant Competition; Plant Physiology; Population and Community Interactions; Predation; Trace Elements.
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