An Ecological Theory of Democracy

We present a model of competition between political parties inspired by the ecological theory of competition between species. We show that parties are incentivized to differentiate from one other to maintain a viable voter base, similar to how competing species evolve to occupy different ecological niches. The propensity of voters to reconsider their party support, i.e., voter loyalty, emerges as key to how far apart in the political spectrum parties evolve in a two-party system. Contrary to conventional wisdom, parties evolve more extreme platforms as voters become less loyal. However, partisanship, measured as the average time a voter spends supporting a party, increases with decreased voter loyalty. Furthermore, when voters are not loyal and incumbent parties move away from the center, centrist third parties cannot outcompete the incumbents. Our results highlight the paradoxical moderating force of voter loyalty and the promise of an ecological approach to model political competition.