11 - Demography

Summary

Introduction

In this chapter I cover those ecological attributes that are features of populations rather than of individual animals. Population ecology is generally framed in terms of the logistic model of population growth. The parameters of this model include (i) the maximum or ‘intrinsic’ rate of population growth shown when population density is very low, labelled r_max; (ii) the equilibrium density or ‘carrying capacity’ eventually attained, labelled K.

However, rate of population growth is the difference between recruitment, determined by processes of birth and immigration, and losses, the outcome of deaths and emigration. Since natality and mortality rates vary with age and sex, the realized rate of population increase is influenced by population structure. Strictly, r_max and K are defined only for populations that have attained an equilibrium age and sex composition. However, real populations seldom remain at any equilibrium for long, due to environmental fluctuations. Furthermore, density varies spatially over the population range in relation to habitat suitability.

The ecological features to be considered in this chapter include (i) population composition, in terms of age structure and sex ratio; (ii) rates of population change with time; (iii) population densities attained.

Population structure

In considering age structure, the functional age classes include (i) adults, i.e. animals that have passed the age of socio-sexual maturity and are reproductively active, or at least potentially so; (ii) juveniles, i.e. animals that have not attained the age of independence from their mothers; (iii) subadults, i.e. animals that are intermediate between the above two categories.