Summary

Intraspecific competition is defined and explained. Exploitation and interference are distinguished, and the commonly one-sided nature of competition is emphasized.

We describe the effects of intraspecific competition on rates of mortality and fecundity, distinguishing under-, over- and exactly compensating density dependence. We explain, however, that density itself is usually just a convenient expression of crowding or shortage of resources.

Log density (individuals m-2)

Figure 5.34 (a) Self-thinning in the gregarious tunicate, Pyura praeputialis, where density has been modified to include an 'effective area' which incorporates the number of layers in the tunicate colonies. The estimated slope is -1.49 (95% CI -1.59 to -1.39, P < 0.001). (After Guinez & Castilla, 2001.) (b) Dynamic thinning lines for 23 year-classes of sea trout, Salmo trutta, from an English Lake District stream, with the position of the mean regression line (slope = -1.35) indicated by the arrows (After Elliott, 1993.)

Figure 5.34 (a) Self-thinning in the gregarious tunicate, Pyura praeputialis, where density has been modified to include an 'effective area' which incorporates the number of layers in the tunicate colonies. The estimated slope is -1.49 (95% CI -1.59 to -1.39, P < 0.001). (After Guinez & Castilla, 2001.) (b) Dynamic thinning lines for 23 year-classes of sea trout, Salmo trutta, from an English Lake District stream, with the position of the mean regression line (slope = -1.35) indicated by the arrows (After Elliott, 1993.)

These effects at the individual level lead in turn to patterns, and regulatory tendencies, at the population level. The carrying capacity is defined and its limitations are explained, along with the domed nature of net recruitment curves and the sigmoidal nature of population growth curves.

We describe the effects of intraspecific competition on rates of growth, explaining the 'law of constant final yield', especially in modular organisms.

The use of k values in quantifying intraspecific competition is described, and scramble and contest competition are distinguished.

We introduce the use of mathematical models in ecology generally, then go on to develop a model of a population with discrete breeding seasons subject to intraspecific competition. The model illustrates the tendency of time lags to provoke population fluctuations and that different types of competition may lead to different types of population dynamics, including patterns of deterministic chaos - the nature and importance of which are themselves explained. A model with continuous breeding is also developed, leading to the logistic equation.

The importance of individual differences in generating asymmetries in competition is explained, as is the importance of competition in generating individual differences. Asymmetries tend to enhance regulation; territoriality is a particularly important example of this.

The progressive effects of competition on growth and mortality may often be interlinked in the process of self-thinning, which has been a particular focus in plant populations. We explain the nature of dynamic thinning lines and the -3/2 power law when single cohorts are followed, and also species and population boundary lines when a series of crowded populations is observed at different densities. We address the question of whether there is a single boundary line for all species.

We explain how two broad types of explanation for the consistent trend amongst species have been proposed: those based on geometry and those based on resource allocation in plants of different sizes.

Finally we examine self-thinning in animal populations and conclude that plants are not so consistent in their pattern of self-thinning as was once thought, while animals are not much less bound than plants by 'general' self-thinning rules.

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