Abstract
We constructed a model to investigate conditions under which intraspecific competition amplifies or diminishes the selective advantage to resistance. The growth trajectories of competing individual plants were depicted by logistic difference equations that incorporated basic costs (lowered growth rate) and benefits (lowered damage) of defense. Analytical results showed that when competition is absent, resistance is favored by high damage, low cost, and slow growth rate. Competition makes selection more complex. When herbivore damage reduces the size of a susceptible plant, resistant neighbors can usurp its resources and thus suppress its regrowth. This competitive interaction amplifies the relative fitness of the resistants. Numerical simulations explored a broader range of conditions. Three factors were varied: competition mode (symmetric vs. asymmetric), resistance type (damage avoidance vs. damage reduction), and timing of attack (early, mid, or late season). We found that competition mode had drastic effects on outcomes. Under symmetric competition, increased plant density intensified the selective advantage of resistance, damage avoidance was more strongly favored than damage reduction, and resistance to late attack was more favored than to early attack. Asymmetric competition had opposite effects: selection acted against resistance at high density, damage reduction was more strongly favored, and resistance against early attack was more favored. Interestingly, the two competition modes induced opposite patterns of density-dependent selection. The difference between the symmetric and asymmetric cases is explained by the fact that resistance costs during the preattack phase are more strongly amplified by asymmetric competition. When resistance is induced, so that preattack costs of resistance are zero, asymmetric competition more strongly amplified the benefits during the postattack phase. The prediction that selection on resistance will be plant density-dependent has complex implications for the evolutionary dynamics of defense evolution.
Highlights
We constructed a model to investigate conditions under which intraspecific competition amplifies or diminishes the selective advantage to resistance
A competitive cost of plant resistance can be inferred from studies that show increased defense costs under resource limitation (Vrieling and van Wijk 1994), as well as by analogy to cases where competition amplifies the cost of herbicide resistance (Williams et al 1995) and the intensity of inbreeding depression (Schmitt and Ehrhardt 1990)
Analytical solutions to the model were possible only when several restrictive assumptions were made, such as resistance comes through damage reduction and competition is symmetric
Summary
We present our model in terms of selection acting on a genotype that codes for defense structures such as secondary chemicals or leaf hairs. These defenses are assumed to impose a metabolic cost that reduces growth rate. The model can be applied to cases in which herbivores preferentially feed on plants with high growth rate (because of their superior nutritional value), as is posited by the plant vigor hypothesis of Price (1991)
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