BOX 3-1 Minimal Viable Population
Theoretical development associated with the concept of minimal viable population (MVP) for rare taxa (Soulé 1987, Lande 1988, Menges 1991) has demonstrated the profound demographic implications of low population numbers in the extinction process. Although descriptions of what is implied by MVP have varied (Shaffer 1981), most definitions focus on estimating the minimal population density for a given probability of population persistence over a specified period. In applications of the idea of MVP for rare species, the time frames are often thousands of years (Shaffer 1987). As a result, the recommended population sizes in conservation biology are often large to reduce the likelihood of chance extinction over the long term by various stochastic processes that can differ in their frequency and their potential demographic and evolutionary impacts.
Nonindigenous species are often rare during the initial phases of their colonization. By analogy, an application of the concept of MVP to a newly introduced species might be useful in predicting the likelihood of its establishment. Needed here is knowledge of the acceptable maximal persistence time for the newly introduced species. Precisely because they are at low population densities, new introductions are typically not detected, so it is difficult to know how long they persisted without obvious population growth. However, one would expect that the periods for the establishment phase of currently widespread nonindigenous plants were much less than the 1000-year persistence times sometimes recommended for rare species. Actual periods for the establishment phase in invasive species on which data are available indicate that the initial “lag phase” in the population growth trajectory is often less than 100 years. For example, after initial introduction into northern Australia around 1900, Mimosa pigra populations remained small and confined to environs around Darwin until the 1950s. After that short lag phase, this native of Mexico and South America began to spread rapidly throughout Australia’s Northern Territory; it is now a major threat to wetland areas in Kakadu National Park (Lonsdale et al. 1989, Cousens and Mortimer 1995). The population required to ensure persistence for this much shorter period could be very small and thus much more difficult to detect and eradicate. Difficulties in detection because of low population densities might be exacerbated in invasive species that can reproduce vegetatively or by uniparental forms of reproduction. The theory predicts that the population required for persistence of clonal or parthenogenic species is even smaller than those for species that reproduce sexually (MacArthur and Wilson 1967, Shaffer 1981).
Efforts to predict the likelihood of establishment of a recently introduced species will need to consider how an Allee effect may influence the MVP. Life-history traits related to dispersal, reproduction, host finding, predator defense, or other factors can affect the critical density threshold, below which a population that is subject to inverse density dependence cannot recover. Characteristics of the habitat colonized by the founding population can further interact with an Allee effect; the MVP may be higher or lower, depending on the size of the founding colony and the extent of environmental stochasticity.