Figure 1
The degree to which each mutualist (red line = symbiont; blue line = host) utilizes the resources provided by the other mutualist over time.
An increase in resource usage is assumed to translate directly to an increase in the fitness of the exploiting mutualist. (a) In systems that lack spatial heterogeneity, the fitness of both symbionts and hosts will increase bilaterally before any common resource becomes exhausted. However, the fitness increase of symbionts will be at the expense of host fitness at the point resources are fully exploited. (b) In systems in which spatial heterogeneity occurs, the fitness of both hosts and symbionts will each increase until an asymptote is achieved and the system will become stable.
