Optimal Defense Theory

Evolution of chemical defenses according to the optimal defense theory presumes, in addition to costly defenses, that there is genetic variation for the defensive metabolites, that herbivory is the major selective agent for such metabolites, and that the chemical trait in question is efficient in reducing herbivory (Stamp 2003). Research on macroalgal chemical defenses has strongly emphasized the last precondition, which has mainly been studied by testing the deterrence effects of secondary metabolites in bioassays. The defensive role for the trait has been assumed on the basis of deterrence it provides. Veiy little research on the first two... 

The optimal defense theory predicts that the benefit/cost ratio of defenses will be maximized by natural selection. When a plant consists of different kinds of tissues, within-plant variation in defenses is predicted to depend on the value of the plant part for fitness and on its risk of becoming grazed. The more valuable the tissue is... 

Pavia H, Toth GB, Aberg P (2002) Optimal defense theory elasticity analysis as a tool to predict intraplant variation in defenses. Ecology 83 891-897 Peckol P, Krane JM, Yates JL (1996) Interactive effects of inducible defence and resource availability on pholorotannins in the North Atlantic brown alga Fucus vesiculosus. Mar Ecol Prog Ser 138 209-217... 

Cronin G, Hay ME (1996b) Within plant variation in seaweed palatability and chemical defenses optimal defense theory versus the growth differentiation balance hypothesis. Oecologia... 

Zangerl, A. R. and Rutledge, . E. (1996). The probability of attack and patterns of constitutive and induced defense a test of optimal defense theory. American Naturalist 147 599-608. 

The optimal defense theory (ODT) asserts that organisms allocate defenses in a way that maximizes fitness, and that defenses are costly (in terms of fitness) when enemies are absent. However, in the presence of enemies, defensive secondary metabolites can become beneficial when their costs are outweighed by benefits gained with protection.7 This theory encompasses both evolutionary and ecological time scales, providing explanations for between-species, within-species, and within-individual variation in defenses. 

The spatial-variation-in-consumers model (SVICM) was proposed by marine ecologists to explain both interspecific and intraspecific patterns of chemical defenses in seaweeds.111 It states that chemically defended seaweeds will be more evolutionarily persistent than undefended seaweeds in areas subject to significant herbivore impact. Likewise, this model explains intraspecifc variation in chemical defenses, in that well-defended individuals or populations will be more persistent over ecological time scales than undefended seaweeds in areas subject to significant herbivore impact. This model borrows heavily from the optimal defense theory, in that it suggests that seaweeds that occur in areas with high herbivory pressure will be vulnerable to herbivore attack and should thus be heavily defended. 

The production of toxins is only one aspect of plant defense strategy. As a result of the persistent battle of plants and herbivores, many optimized phenotypes have evolved, such as the preferential accumulation of alkaloids in tissues with a pattern that is consistent with predictions of optimal defense theory,65 i.e., the defense metabolites are allocated preferentially to tissues with a high probability of attack.66 The inducibility of pathways leading to plant secondary compounds as a strategy to minimize the costs of plant defense is a result of permanent optimization. One of a few examples of inducible alkaloid biosynthesis is the different Nicotiana species that exhibit dramatic wound-induced increases of nicotine, nomicotine, or anabasine.67... 

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