ENVIRONMENTAL STOCHASTICITY AND EXTINCTION RISK IN A POPULATION OF A SMALL SONGBIRD, THE GREAT TIT

Using a long-term demographic data set, we estimated the separate effe cts of demographic and environmental stochasticity in the growth rate of the great tit population in Wytham Wood, United Kingdom. Assuming l ogistic density regulation, both the demographic (sigma(d)(2) = 0.569) and environmental (sigma(e)(2) = 0.0793) variance, with interactions included, were significantly greater than zero. The estimates of the d emographic variance seemed to be relatively insensitive to the length of the study period, whereas reliable estimates of the environmental v ariance required long time series (at least 15 yr data). The demograph ic variance decreased significantly with increasing population density . These estimates are used in a quantitative analysis of the demograph ic factors affecting the risk of extinction of this population. The ve ry long expected time to extinction of this population (approximate-to 10(19) yr) was related to a relatively large population size (greater -than-or-equal-to 120 pairs during the study period). However, for a g iven population size, the expected time to extinction was sensitive to both variation in population growth rate and environmental stochastic ity. Furthermore, the form of the density regulation strongly affected the expected time to extinction. Time to extinction decreased when th e maximum density regulation approached K. This suggests that estimate s of viability of small populations should be given both with and with out inclusion of density dependence.