Competitive exclusion between contest and scramble strategists in Callosobruchus seed-beetle modeling

Interspecific competition between an intermediate contest strain of Callosobruchus maculatus and a scramble strain of C. chinensis was investigated on two types of resource beans: the azuki (Vigna anguralis, small seed) and the black-eye bean (Vigna unguiculata, large seed). We conducted both single-generation competition experiments and multiple-generation experiments and then analyzed the system with a discrete version of the Lotka-Volterra competition model. The estimated competition coefficients showed the competitive dominance of the C. maculatus strain over the C. chinensis strain in both types of bean. Zero-growth isocline analysis based on the predicted model showed that an overall dominance of the contest species, C. maculatus, in azuki beans, whereas the outcome of competition was dependent on the initial population size of each species in the black-eye beans. In the multiple-generation experiments, C. maculatus was the successful competitor irrespective of the ratio of the two beans, whereas C. chinensis overcame C. maculatus in one of five replicates with the black-eye beans system. When we incorporated the cost of scramble competition, or density-dependent reduction in body mass into the predicted model, the model alteration decreased the survival region of C. chinensis in phase space on the black-eye bean. Thus, the competitive equations, which do not consider the density-dependent body size reduction, tend to overestimate the advantages of scramble strategists.