Graph (a): The parameter plane spanned by the preference strength and the CI level consists of two areas. For low levels of CI, P
2 cannot spread no matter how strong the mating preference is. For CI levels above a certain threshold, however, P
2 spreads on the island. Black squares indicate simulation results, gray circles values determined by formula (17). Graph (b): The parameter space spanned by the CI level and the migration rate consists of four areas. First, if l
CI<f then mainland and island are uninfected and P
2 cannot spread. Second, if m>mc then both mainland and island are infected and P
2 cannot spread. Third and fourth, if l
CI>f and m<mc then P
2 can spread if the CI level is above a threshold but cannot if the CI level is below. Parameters are f = 0.1 in both graphs, m = 0.001, s = 0.1 in (a), and a
2 = 10, s = 1 in (b).