Figure 5.

Demographic declines of a native species. Simulations are based on the colonization history of mallards (Anas platyrhynchos) in New Zealand where they seriously threaten the native grey duck (Anas superciliosa). We evaluated different values of carrying capacity (K), interspecific competition exerted by the native species (α_native) and interbreeding success rate (γ) to minimize the error when fitting observed abundance values. There are seven documented phenotypes due to hybridization between mallards and grey ducks. We assume these phenotypes are the product of three additive genes (n_L = 3, see Methods). (a) Relative carrying capacities of mallards to explain the colonization time in New Zealand (red point minimize the error with observed values); (b) mean absolute error when fixing the best estimated carrying capacity of mallards (two times larger) and varying the value of competition (α_native) and interbreeding success rate (γ) (the dark green values minimize the error: α_native = 0 and γ = 0.7); we explored various levels of interbreeding success rate, from rare hybridization success (γ = 0.01) to a complete panmictic mating system (γ = 1); (c) forward simulation using the best combination of genetic and demographic parameters that explain the invasion of mallards in New Zealand. Simulations with alternative carrying capacities (green points in Fig. 5a) are presented in Supplementary Fig. S5.