Population Projections

Abstract

Population projections based on observed growth trends are important in captive animal management. Several methods, which require different levels of detail in the data, are available. A projection based on the geometric mean of population growth (λ) estimated from recent census data, optionally per sex, is the simplest model. Ecological growth models, e.g. logistic growth that fits the population data, can be used to project future development of the population. Census data from the blesbok studbook are used to illustrate these census–based methods. The Leslie matrix is applied to project population development of age– and stage–structured populations. Application of this method is described for birth–pulse and birth–flow reproduction. Damping of λ and convergence to a stable age distribution are also described. Since studbook software handles projections, managers are generally not exposed to matrix algebra, so a brief introduction to matrix algebra is presented as this method is often applied in ecological studies. Matrix calculus provides methods for testing the effects of changing individual survival and fecundity rates on λ. Changes can be estimated as absolute values (sensitivity) or proportional values (elasticity). The standard Leslie matrix is a deterministic model. A stochastic version of this matrix model is used to show the effects of demographic uncertainty on population development. The differences between projection and prediction are discussed. A retrospective study in which projections are compared with actual population development is presented. The advantages and disadvantages of applying stage–based models to studbook populations are discussed.