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 log istic difference equations that incorporated basic costs (lowered growth ra te) and benefits (lowered damage) of defense. Analytical results showed tha t when competition is absent, resistance is favored by high damage, low cos t, and slow growth rate. Competition makes selection more complex. When her bivore damage reduces the size of a susceptible plant, resistant neighbors can usurp its resources and thus suppress its regrowth. This competitive in teraction amplifies the relative fitness of the resistants. Numerical simul ations explored a broader range of conditions. Three factors were varied: c ompetition mode (symmetric vs. asymmetric), resistance type (damage avoidan ce vs, damage reduction), and timing of attack (early, mid, or late season) . We found that competition mode had drastic effects on outcomes. Under sym metric competition, increased plant density intensified the selective advan tage of resistance, damage avoidance was more strongly favored than damage reduction, and resistance to late attack was more favored than to early att ack. Asymmetric competition had opposite effects: selection acted against r esistance at high density, damage reduction was more strongly favored, and resistance against early attack was more favored. Interestingly, the two co mpetition modes induced opposite patterns of density-dependent selection. T he difference between the symmetric and asymmetric cases is explained by th e fact that resistance costs during the preattack phase are more strongly a mplified 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 sel ection on resistance will he plant density-dependent has complex implicatio ns for the evolutionary dynamics of defense evolution.