Fixation and Fluctuations in Two-Species Cooperation

TL;DR

This paper studies the dynamics of two cooperative species, revealing how migration rates influence fixation probabilities, population fluctuations, and the transition from steady to fluctuating states in a two-species system.

Contribution

It introduces a model analyzing how migration affects fixation and fluctuations in two-species cooperation, uncovering a critical migration rate for population behavior transitions.

Findings

Fixation probability depends on initial concentrations.

Steady-state distribution transitions from unimodal to trimodal at a critical migration rate.

Low migration induces strong population fluctuations between near-fixation states.

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.