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Organisms grow, reproduce and die (Chapter 4). They are affected by the conditions in which they live (Chapter 2) and by the resources that they obtain (Chapter 3). But no organism lives alone. Each, for at least part of its life, is part of a population of its own species.

a definition of competition

Competition can be defined as an interaction between individuals brought about by a shared requirement for a resource, leading to a reduction in the survivorship, growth and/or reproduction of at least some of the individuals concerned. Individuals of the same species have very similar requirements for survival, growth and reproduction, but their combined demand for a resource may exceed the immediate supply. The individuals then compete for the resource and at least some of them will become deprived. In this chapter we examine the nature of such intraspecific competition, its effects on the competing individuals and on populations of competing individuals.

Consider, initially, a simple hypothetical community: a thriving population of grasshoppers (all of one species) feeding on a field of grass (also of one species). To provide themselves with energy and material for growth and reproduction, grasshoppers must find and eat grass, but use energy in doing so. A grasshopper that finds itself at a spot where there is no grass because another grasshopper has eaten it must move on and expend more energy before it takes in food. The more grasshoppers there are, the more this will happen, increasing energy expenditure, decreasing the rate of food intake, and hence potentially decreasing its chances of survival and leaving it less energy for development and reproduction. Survival and reproduction determine a grasshopper’s contribution to the next generation. Hence, the more intraspecific competitors for food a grasshopper experiences, the less its likely contribution will be.

As far as the grass itself is concerned, an isolated seedling in fertile soil may have a very high chance of surviving to reproductive maturity. It will probably exhibit extensive modular growth and probably, therefore, eventually produce many seeds. However, a seedling that is closely surrounded by neighbours (shading it with their leaves and depleting the water and nutrients of its soil with their roots) will be very unlikely to survive, and if it does, will almost certainly form few modules and set few seeds.

We see immediately that the ultimate effect of competition on an individual is a decreased contribution to the next generation compared with what would have happened had there been no competitors. Intraspecific competition typically leads to decreased rates of resource intake per individual, and thus to decreased rates of individual growth or development, or perhaps to decreases in the amounts of stored reserves or to increased risks of predation. These may lead, in turn, to decreases in survivorship and/or decreases in fecundity, which together determine an individual’s reproductive output.