When individuals senesce: The 'Florida effect' on stable populations of territorial, long-lived birds

Abstract

Senescence can be defined as the entire set of age-related changes that affect both vitality and function, one of which is within-individual age-related decline in reproduction. This factor is crucial for population persistence, because the senescence of individuals of a population can increase the likelihood of local extinction. Using simulations based on long-term information on a small metapopulation of a long-lived bird species, we highlight two mechanisms able to engender senescence in both breeders and floaters (i.e. non-breeding individuals) of the same population. We define 'floater shortage senescence' as breeder senescence due to low juvenile replacement rates because of high floater mortality during dispersal. Less obviously, senescence can also occur with very low floater mortality rates, and when breeding populations are remarkably free from factors that could cause catastrophic population decimation. In this scenario, low mortality in reproductive areas results in territory owners in breeding populations being characterised by progressively older breeding individuals, and old individuals waiting for a breeding opportunity: a phenomenon we refer to as the 'Florida effect'. Consistent with current views that adaptive death plays a crucial role in population dynamics, it seems reasonable to suppose that, under stable environmental conditions, the evolution of some mating mechanisms could limit the senescence of breeding individuals in a population, allowing the pool of breeding individuals to be refreshed by selection of younger breeders.