Population viability analyses on a cycling population: a cautionary tale

Population viability analysis (PVA) packages do not always allow realistic simulation of particular life cycle features, so they may produce unrealist ic predictions of extinction risk. This was suspected for a cycling Soay sh eep population, Ovis aries L., that grows, overeats its habitat and then su ffers high winter mortality. We compared projections of PVA models for the sheep that incorporated either an unrealistic ceiling carrying capacity usi ng INMAT (the only choice) and VORTEX (the default)? or realistic density d ependence for survival (VORTEX). At year 50, the ceiling models predicted e xtinction probabilities of 60.4% (INMAT) and 87.4% (VORTEX), compared to on ly 4.6% for the density-dependent model. Small populations were equally lik ely to increase or decrease with the ceiling models, while they had high pr obabilities of increase with the density-dependent model, as found in reali ty. PVA cannot be relied upon to produce realistic projections if inappropr iate mechanisms of population regulation are used. (C) 2000 Elsevier Scienc e Ltd. All rights reserved.