Minimum viable population size of capercaillie Tetrao urogallus: results from a stochastic model

© WILDLIFE BIOLOGY · 6:4 (2000) Even if the mean rate of population growth is positive, populations are in danger of becoming extinct if they are too small to buffer environmental and demographic fluctuations. But how small is 'too small'? Answering this question is important to guide management decisions in fragmented landscapes. Therefore, a major task of conservation biology is to determine "minimum viable populations and their area requirements" (Shaffer 1981: 100). Shaffer defined viability as an extinction risk that does not exceed a certain threshold within a certain time interval; typically 1 or 5% in 100 years are used. Then, a minimum viable population (MVP) in a given habitat is defined as the smallest isolated population which is viable. Although rough assessments of MVP may be inferred from biogeographic patterns (Shaffer 1981), the main tool to determine MVP quantitatively are stochastic population models (Shaffer 1981). For capercaillie Tetrao urogallus in central Europe, Storch (1995) proposed a preliminary estimate of an MVP of 500 birds based on biogeographic patterns. The first model-based assessment of MVP for capercaillie has recently been presented by Marshall & EdwardsJones (1998); the aim of their study was to evaluate the suitability of potential sites for reintroducing capercaillie in Scotland. Based on the generic population simulation model VORTEX (Lacy, Hughes & Miller 1995) they conclude that under certain conditions, a population of 60 individuals in a habitat with a capacity (i.e. ceiling population size) of 150 individuals would be viable. Reliable estimates of MVP and minimum area reMinimum viable population size of capercaillie Tetrao urogallus: results from a stochastic model