• Allee effect;
  • coexistence;
  • emigration;
  • immigration;
  • metacommunity;
  • mutualism;
  • spatial heterogeneity;
  • source–sink dynamics


  • 1
    I present a model of mutualistic interactions in a patchy landscape. The interaction is between two species that differ in their mobility. The landscape is spatially structured, with several localities interconnected by dispersal of the mobile mutualist.
  • 2
    Within a given locality, an Allee effect can occur such that the per capita growth rate of the non-mobile mutualist declines with its own abundance. The Allee effect arises in response to low abundances of the mobile mutualist, and causes extinction of both species once their numbers fall below a critical threshold.
  • 3
    Dispersal of the mobile mutualist can rescue such sink communities from extinction, provided there is at least one source community in which both species have abundances above the extinction threshold. Dispersal itself is density-independent, but induces negative density-dependence that counteracts the positive density-dependence due to the Allee effect. This negative feedback effect of dispersal has not previously been demonstrated in mutualism models.
  • 4
    Rescue of sink communities, however, depends on how dispersal influences the local dynamics of source communities. If dispersal involves surplus individuals the loss of whom does not affect the reproductive output of the source community, persistence of sink communities is guaranteed as long as the survivorship of long-distance dispersers exceeds a lower threshold. In contrast, if dispersal involves emigrants that constitute a fraction of the source community's reproductive output, persistence of sink communities additionally requires that emigration does not exceed an upper threshold. Too much emigration can cause a net loss of the mobile mutualist from the source community, resulting in landscape-wide extinction of the mutualistic interaction.
  • 5
    The effect of dispersal mode on the dynamics of mutualistic interactions has not been appreciated previously. If the benefit of the rescue effect to sinks is outweighed by the cost of dispersal to sources, mutualistic communities linked by dispersal may experience a greater loss of diversity than communities that are isolated.