Critical minimum population sizes in modified habitats

Effective population size is an important element of population management, because of the intimate correlation assumed to exist between population size and genetic fitness. As a population is reduced by size it becomes more prone to both deterministic and stochastic effects which can lead to extinction. It seems that there is no 'magic number' below which populations are doomed and above which they are safe. Rather, as effective size reduces, persistance times reduce and risk of extinction becomes greater. Many factors contribute to each situation, including breeding system, individual and meta-population structure, critical habitat conditions, genetic architecture and past genetic history. Reduction of habitat areas, edge effects and patterns of patch dynamics in semi-natural areas, leads to reduced persistance times for many species. Long term maintenance of biodiversity requires decisive management, and not preservation (and loss) by default.