Long-term trends in great bustard ( Otis tarda) populations in Portugal suggest concentration in single high quality area

Abstract

Areas of occurrence, population trends and extinction patterns for great bustard Otis tarda populations in Portugal are described for a 22-year period (1980–2002). The major population trends were a large decline in the 1984–1995 period, followed by a fast post-1995 increase. Most of this variation was explained by the trend observed in a single site, Castro Verde, where population has been increasing, in contrast with all other areas where populations have been declining. Eight local extinctions were documented, and probability of extinction increased fast below a threshold of 30 individuals in the initial (1980) population. Agricultural intensification, illegal hunting, road and power line building, and afforestations were the main causes of population decline and extinctions. The exceptional character of the Castro Verde population is explained by the maintenance of good habitat quality, the existence of a bustard conservation project and of an agri-environmental scheme promoting farm management compatible with bustard conservation. Stochastic computer simulations, using VORTEX, suggested that migration of individuals from other sites into Castro Verde was likely, as local productivity alone could not explain the observed population increase. The Portuguese population was estimated at 1150 birds in 2002, of which 912 were concentrated in Castro Verde. The present trend for the increasing concentration of the Portuguese population of great bustards in a single site might lead to increased probability of extinction, particularly due to environmental stochasticity.