Oppressed species can form a winning pair in a multi-species ecosystem

The self-protection of alliances against external invaders is a key concept behind the maintenance of biodiversity in the face of natural selection. But since these alliances, which can be formed by different numbers of competitors, can also compete against each other, it is important to identify their strengths and weaknesses. Here, we therefore compare the vitalities of two two-species alliances whose members either beat each other mutually via a bidirectional invasion or they exchange their positions during an inner dynamics. The resulting four-species model shows rich behavior in dependence on the model parameter p, which characterizes the inner invasions, and β, which determines the intensity of site exchanges. In the low p and the large p limit, when the inner invasion becomes biased, three-member rock-scissors-paper-type solutions emerge, where one of the members is oppressed by having the smallest average concentration due to heterogeneous inner invasion rates. Interestingly, however, if we allow a more intensive site exchange between the oppressed species, they can morph into a winning pair and dominate the full parameter plane. We show that their victory utilizes the vulnerability of the rival alliance based on cyclic dominance, where a species can easily fixate a limited-size domain.