Chapter 11 - The Construction, Description and Analysis of Age-specific Life-tables

The construction of a number of life-tables is an important component in the understanding of the population dynamics of a species. Although some animal ecologists, such as Richards (1940), had expressed their results showing the successive reductions in the population of an insect throughout a single generation, Deevey (1947) was really the first to focus attention on the importance of this approach. Life-tables have long been used by actuaries for determining the expectation of life of an applicant for insurance and thus the column indicating the expectation of life at a given age (the ex column) is an essential feature of human life-tables. However, the fundamental interests of the ecologist and, even more so, of the applied biologist are essentially different from those of the actuary and it is a mistake to believe that the approaches and parameters of primary interest in the study of human populations are also those of greatest significance to the animal ecologist. Because many insects have discrete generations and their populations are not stationary, the age-specific life-table is more widely applicable than the time specific life-table. The differences between these two types are as follows:

An age-specific (or horizontal) life-table is based on the fate of a real cohort; conveniently the members of a population belonging to a single generation. The population may be stationary or fluctuating.

A time-specific (or vertical) life-table is based on the fate of an imaginary cohort found by determining the age structure, at one instant in time, of a sample of individuals from what is assumed to be a stationary population with considerable overlapping of generations, i.e. a multi-stage population. Age determination is a prerequisite for time-specific life-tables (Chapter 12). A modification of this approach is the variable life-table of Gilbert et al. (1976), which is an inductive strategic computer model of the population: this is varied until it provides a reasonable description of the population (see Chapter 12).

11.1 Types of life-table and the budget. 11.2 The construction of a budget 11.2.1 Southwood's Graphical method 11.2.2 Richards & Waloff's first method 11.2.3 Manly's method 11.2.4 Ruesink's method 11.2.5 Dempster's method 11.2.6 Richards & Waloff's Second Method 11.2.7 Kiritani, Nakasuji & Manly's method 11.2.8 Kempton's method 11.2.9 The Bellows and Birley Method 11.3 The description of budgets and life-tables 11.3.1 Survivorship curves 11.3.2 Stock-recruitment (Moran-Ricker) curves 11.3.3 The life-table and life expectancy 11.3.4 Life and fertility tables and the net reproductive rate 11.3.5 Population growth rates 11.3.6 The calculation of r 11.4 The analysis of life-table data 11.4.1 The comparison of mortality factors within a generation 11.4.2 The simple statistical relationship of population size to a factor