From indirect to direct taxis by fast reaction limit
Many ecological population models consider taxis as the directed movement of animals in response to a stimulus. The taxis is named direct if the animals are guided by the density gradient of some other population or indirect if they are guided by the density of a chemical secreted by individuals of the other population. Let $u$ and $v$ denote the densities of two populations and $w$ the density of the chemical secreted by individuals in the $v$ population. We consider a bounded, open set $Ω\subset \mathbb{R}^N$ with regular boundary and prove that for the space dimension $N\leq 2$ the solution to the Lotka-Volterra competition model with repulsive indirect taxis and homogeneous Neumann boundary conditions $$u_t - d_uΔu = χ\nabla \cdot u \nabla w +μ_1u(1-u-a_1v)\,,$$ $$ v_t - d_vΔv = μ_2v(1-v-a_2u)\,,$$ $$\varepsilon ( w_t - d_wΔw )= v- w\, , $$ converges to the solution of repulsive direct-taxis model: $$ u_t - d_uΔu = χ\nabla \cdot u \nabla v +μ_1u(1-u-a_1v)\,,$$ $$ v_t - d_vΔv = μ_2v(1-v-a_2u)\,$$ when $\varepsilon\longrightarrow 0$. For space dimension $N\geq 3$ we use the compactness argument to show that the result holds in some weak sense. A similar result is also proved for a typical prey-predator model with prey taxis and logistic growth of predators.