MINIMUM VIABLE METAPOPULATION SIZE

We define the minimum viable metapopulation (MVM) size as the minimum number of interacting local populations necessary for long-term persis tence of a metapopulation in a balance between local extinctions and r ecolonizations. The minimum amount of suitable habitat (MASH) is defin ed as the minimum density (or number) of suitable habitat patches nece ssary for metapopulation persistence. Levins's metapopulation model su ggests that MASH can be estimated by the fraction of empty patches in a network in which the metapopulation occurs at a stochastic steady st ate. We discuss three reasons why this rule of thumb is likely to give an underestimate, and possibly a severe underestimate, of MASH: the r escue effect, colonization-extinction stochasticity, and nonequilibriu m (transient) metapopulation dynamics. The assumption that metapopulat ions occur at a steady state, common to many models, may be frequently violated because of the high rate of habitat loss and fragmentation i n many landscapes. Scores of rare and endangered species may already b e ''living dead,'' committed to extinction because extinction is the e quilibrium toward which their metapopulations are moving in the presen t fragmented landscapes. To conserve these species we should reverse t he process of habitat loss and fragmentation.