Fig. 4.

Endemicity boundaries in a spatially heterogeneous metacommunity with two human groups living in separate locations, each with one preferential water source (model (2) with $G=2$ and $S=2$). The groups may differ for their relative abundance (h₁ and h₂, with $h_1+h_2=1$) and intrinsic transmission risk (ϵ₁ and ϵ₂), while the two sources may differ for the relative abundance of snails they host (n₁ and n₂, with $n_1+n_2=1$) and the frequency of human-water contacts (for each community, the fraction of contacts at the farthest water point is m, while $1-m$ is the fraction of contacts at the home site). Parasite establishment is possible (${\mathcal{R}}_0^{22}>1$) on the right of the bifurcation curves, which correspond to ${\mathcal{R}}_0^{22}=1$ and are obtained for different spatial distributions of the human host population (legend). A) Spatially homogeneous transmission risk (${ϵ}_1/{ϵ}_2=1$) and snail population distribution ($n_1=n_2=1/2$). B) As in panel A, with spatially heterogeneous transmission risk (${ϵ}_1/{ϵ}_2=10$). C) As in panel A, with spatially heterogeneous snail abundance ($n_1=h_1$). D) Spatially heterogeneous transmission risk (${ϵ}_1/{ϵ}_2=10$) and snail population distribution ($n_1=h_1$).