Ecologists are interested in the numbers of individuals, the distributions of individuals, the demographic processes (birth, death and migration) that influence these, and the ways in which these demographic processes are themselves influenced by environmental factors.

Not all individuals are alike, especially amongst modular, as opposed to unitary, organisms. The growth forms of modular organisms are described, as well the nature and ecological importance of senescence and physiological integration in modular organisms. Ecology necessarily involves counting individuals or modules. A population is a group of individuals of one species, though what constitutes a population will vary from study to study. It is often most convenient to consider the density as opposed to the size of a population. Methods of estimating population size or density are described briefly.

We explain the variety of patterns of life cycle, including the distinction between semelparous and iteroparous species. Basic methods of quantification of these include life tables, survivorship curves and fecundity schedules. For annual species, cohort life tables can be constructed, the elements of which are described. A summary term of this and a fecundity schedule is the basic reproductive rate, R0. The survivorship curves that emerge from a life table can be classified into three broad types. However, a variety of features, including seed banks, mean that there are many not-quite-annual species.

For individuals with repeated breeding seasons, it may also be possible to construct cohort life tables; a static life table is an imperfect alternative that must be interpreted with caution.

We explain how basic reproductive rates, R0, generation lengths and population rates of increase are interrelated when generations overlap, leading to definitions of the fundamental net reproductive rate, R, and the intrinsic rate of natural increase, r (= ln R). We explain, too, how these may be estimated from life tables and fecundity schedules, and move on to describe the population projection matrix, a more powerful method of analyzing and interpreting fecundity and survival schedules when generations overlap.

Three different types of question that are commonly asked about the evolution of life histories are described. Most answers to these questions have been based on the idea of optimization. The components of life histories, and their ecological importance, are also described: size, development rate, semel- or iteroparity, clutch size, offspring size and some composite measures -reproductive allocation and especially reproductive value.

Trade-offs are central to an understanding of life history evolution, though they may be difficult to observe in practice. Key trade-offs are those that reveal an apparent 'cost of reproduction' in terms of a decrease in residual reproductive value. Another is that between the number and fitness of offspring.

To address the question of whether there are patterns linking particular types of life history to particular types of habitat, the concepts of options sets and fitness contours are introduced, leading to a general, comparative classification of habitats. Armed with this, light is thrown on patterns in reproductive allocation and its timing, the optimal size and number of offspring. We explain the concept of r and K selection, its limitations and the evidence for it. We explain, too, that patterns in the phenotypic plasticity of life histories may equally be governed by natural selection.

Finally, the effects of phylogenetic and allometric constraints on the evolution of life histories are discussed - especially the effects of size - but end with the conclusion that the essentially ecological task of relating life histories to habitats remains the most fundamental challenge.

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