Fixation and Fluctuations in Two-Species Cooperation

Abstract

Cooperative interactions pervade in a broad range of many-body populations, such as ecological communities, social organizations, and economic webs. We investigate the dynamics of a population of two equivalent species A and B that are driven by cooperative and symmetric interactions between these species. For an isolated population, we determine the probability to reach fixation, where only one species remains, as a function of the initial concentrations of the two species, as well as the time to reach fixation. The latter scales exponentially with the population size. When members of each species migrate into the population at rate $\lambda$ and replace a randomly selected individual, surprisingly rich dynamics ensues. Ostensibly, the population reaches a steady state, but the steady-state population distribution undergoes a unimodal to trimodal transition as the migration rate decreases below a critical value ${\lambda }_c$. In the low-migration regime, $\lambda <{\lambda }_c$, the steady state is not truly steady, but instead strongly fluctuates between near-fixation states, where the population consists of mostly A's or of mostly B's. The characteristic time scale of these fluctuations diverges as ${\lambda }^{-1}$. Thus in spite of the cooperative interaction, a typical snapshot of the population will contain almost all A's or almost all B's.

arXiv comment12 pages, 6 figures. Version 2 is almost completely rewritten, with various new results. Now 22 pages and 7 figures in IOP format. Version 3 has some additional minor changes in response to referee comments and now extends to 23 pages. For publication in J Phys Complexity

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