The diverse effects of intraspecific competition on the selective advantage to resistance: A model and its predictions

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 pre-attack costs of resistance are zero, asymmetric competition more strongly amplified the benefits during the postattack phase. The prediction that selection on resistance will he plant density-dependent has complex implications for the evolutionary dynamics of defense evolution.