Rates of increase in age-structured populations:: a lesson from the European harbour seals

Behavioural differences among population segments coupled with the transient dynamics of perturbed population structures lead to severely biased estimates of the intrinsic rates of increase in natural populations. This phenomenon is expected to occur in most populations that are structured by age, sex, state, or rank. The 1988 epizootic in European harbour seals (Phoca vitulina) perturbed the population composition radically. Detailed documentation of mass mortality, 20 years of population surveys, and data on age- and sex-specific behaviour were used to quantify biases in the observed rate of increase (lambda(obs.)), which in many areas substantially exceeded the maximum rate of increase. This is serious, since lambda(obs.) is a key parameter, for example, in estimating potential biological removal or modelling population dynamics. For populations where the underlying age and sex composition is unknown, we suggest that data on fecundity and survival rates be used to find the upper theoretical rate of population increase. We found that the intrinsic rates of increase (lambda(1)) in populations of true seals with even sex ratios and stable age structures cannot exceed 13% per year (lambda(1max.) = 1.13). Frequently reported larger values are indicative of nonstable population structures or populations affected by migrations.