Competition is a major factor in determining the distributions of species. It also contributes to the biodiversity we see around us today. It can be seen all around us in the natural world, from the begging behaviour of chicks in a nest as they compete for the food offered by their parents, to bacteria fighting for space to live on our skin. Competition can even start before conception as sperm race to reach and fertilize an egg. Scientists can measure the effects of competition by looking at how well living things grow, how long they live, or how many offspring they produce in the presence or absence of competitors.

What is competition?

In order to survive, organisms need resources. Where a resource is of limited availability, such as the amount of a nutrient in soil or the space available on a rock, individuals have to compete to gain the share they need. Some organisms will get the resources they require whilst others miss out and are less likely to survive and reproduce.

Individuals of the same species have similar needs so will usually have to compete for resources. This could be for a mate, for territory, or for food. But competition can also occur between different species, for food, shelter or resources, such as light and water.

When different individuals or different species compete directly for resources it is known as resource competition. They may also compete indirectly, by preventing other organisms accessing resources, an interaction known as interference competition. One example of this is territoriality, seen in a huge variety of species including species of mammals, fish and birds. The ‘owner’ of each territory excludes other individuals from accessing the resources within it.

Win or change…?

Where species have had more time to co-evolve together similar species often live in the same habitat. However, when you examine their life histories, they do not usually use exactly the same resources. Blue tits, marsh tits and great tits are closely related bird species although they differ in body size and the size of their beaks. These birds often live in the same habitat and they eat similar things, insects such as caterpillars and seeds.

When scientists studied the birds in areas where they co-exist, they found that there were subtle differences between them. Great tits tend look for food on the ground, marsh tits mainly feed on bushes and low in trees, and blue tits mostly feed higher in the trees. The great tit, the largest bird of the three birds, feeds on larger caterpillars and seeds such as acorns and hazel. The smallest of the three, the blue tit, feeds on smaller caterpillars and seeds and the marsh tit tends to feed on intermediate sized prey. This may reflect the effects of competition between these species in the past, which could have led to changes in their morphology and behaviour.

Competition in context

In isolation the best competitors could always outcompete the weaker ones. But in nature these competitors have a host of other things to contend with in their local environment. This includes variability in the environment, natural catastrophes, predators and parasites. The effects of these other interactions often allow weaker competitors to survive in areas alongside their more competitive rivals.

Next: Working together

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