Minimum Viable Metapopulation Size

We define the minimum viable metapopulation (MVM) size as the minimum number of interacting local populations necessary for long-term persistence of a metapopulation in a balance between local extinctions and recolonizations. The minimum amount of suitable habitat (MASH) is defined as the minimum density (or number) of suitable habitat patches necessary for metapopulation persistence. Levins's metapopulation model suggests that MASH can be estimated by the fraction of empty patches in a network in which the metapopulation occurs at a stochastic steady state. We discuss three reasons why this rule of thumb is likely to give an underestimate, and possibly a severe underestimate, of MASH: the rescue effect, colonization-extinction stochasticity, and nonequilibrium (transient) metapopulation dynamics. The assumption that metapopulations occur at a steady state, common to many models, may be frequently violated because of the high rate of habitat loss and fragmentation in many landscapes. Scores of rare and endangered species may already be "living dead," committed to extinction because extinction is the equilibrium toward which their metapopulations are moving in the present fragmented landscapes. To conserve these species we should reverse the process of habitat loss and fragmentation.