FACTORS INFLUENCING DETECTION OF DENSITY-DEPENDENCE IN BRITISH BIRDS .2. LONGEVITY AND POPULATION VARIABILITY

If censuses are taken at less than generation intervals, the number of successive censuses ill which a given individual is recorded will dep end on longevity Repeatedly recording the same individuals could produ ce under-estimates of population variability and influence detection o f density dependence. We investigated this possibility in 60 time seri es of abundances of British birds compiled from the Common Birds Censu s data and then used simple population models to illustrate the propos ed mechanism, Species had average lifespans of 2-10 years and were cen sused annually. Density dependence was detected (at P < 0.05) much mor e frequently in bird species with long lifespans than in those with sh ort lifespans; 75% of the 12 longest-lived species showed density depe ndence compared to 46% of all species. Population variability measured in annual censuses (termed ''annual variability'') was lower in bird species with longer lifespans. We used discrete time models based on d ifference equations to demonstrate how longevity influences population variability and detection of density dependence in series of annual c ensuses. A model in which only first-year birds experienced density de pendence was rejected because annual variability was greater and detec tion of density dependence was less likely when longevity was greater, the opposite of the observed effects of longevity in birds. A model i n which all age classes experienced density dependence gave time serie s with lower annual variability and in which density dependence was de tected more frequently when longevity was greater, which is the patter n observed in British birds. Analysis of data from this model showed t hat the amount of density dependence actually present caused only smal l changes in annual variability, whereas detection of density dependen ce from simulated series was strongly influenced by annual variability . The high annual variability of series from short-lived bird species could mask any density dependence that was present. Correcting for tre nds lead us to detect density dependence in 75% of the 12 longest live d bird species. There is no reason to believe that this rate is not al so representative of short-lived species.