Together, the habitat and the community of populations make up an ecosystem – e.g. the tropical rainforest.

Why do Living Things suit their habitat?
Plants and animals are adapted to life in a particular ecosystem. Features of their bodies or behaviour seem to have been ‘designed’ with this particular habitat in mind. It is important to understand how these adaptations have developed as a result of natural selection.

The powerful theory of natural selection explains how over very long periods of time, small, inherited differences can accumulate in the population. This process results in dramatic adaptations such as eyespots on a butterfly’s wing. The motor that drives this process is the struggle for existence. Life is a struggle against disease, predators, starvation and harsh weather conditions. Most individuals do not survive to an age where they can reproduce. Individuals which have characteristics better suited to life in a particular habitat will survive and reproduce successfully.

If these characteristics are determined by genes, more individuals in the next generation will inherit this useful feature from their parents. When the habitat does not change for a long period of time, many small differences can accumulate. This allows dramatic adaptations to develop, e.g. the camouflage of rainforest insects.

Abiotic and Biotic Factors
In the process of natural selection the environment must ‘judge’ the most suitable individuals. These individuals can withstand the physical conditions found in a particular habitat and can compete with other plants and animals from the community. The features that define an ecosystem are divided into abiotic and biotic factors.

A particular habitat has certain physical features called abiotic factors, for example:

• temperature
• humidity
• light
• day length
• nesting sites

Biotic factors are the interactions that an individual has with the community of plants and animals, such as:

• source of food
• predators
• disease-causing organisms
• competition for food or other resources

Japanese Knotweed and Exponential Population Growth
Observing any population of plants or animals, it quickly becomes apparent that during reproduction many more offspring are created than survive to adulthood. If all these individuals did survive, the population would grow at an increasingly rapid rate. This is known as exponential growth. Exponential growth is not often seen in real life situations but it can be seen in the early stages of population growth in a colony of bacteria.

In an ecosystem, population growth is limited by a number of biotic and abiotic factors:

• the food supply is exhausted
• predators can find prey very easily and increase in numbers
• disease spreads quickly in crowded conditions where waste builds up
• there is a shortage of nesting or breeding sites

Mink and Vole – Predator-Prey Relationships
Predators and prey have a special effect on each other’s population size. Each population depends on the size of the other. This relationship can be most easily understood where one predator relies almost exclusively on one prey animal.

As a simple example, consider rabbits and foxes. Over a number of years the rabbit population grows quickly because the climate encourages plant growth. Foxes will find it easier to catch rabbits as the population increases. The foxes will raise more cubs and their population will increase over a number of years. As the fox population increases the rabbit population begins to be affected by predation. The growth in the rabbit population will slow and the population size may fall. Some time later the fox population will fall as rabbits become hard to find. The two populations may cycle in this way for long periods of time.

This situation is rarely found in a real life ecosystem for a number of reasons:

• When prey are hard to find, predators will switch to different sources of food, e.g. when their usual food is scarce, foxes will pursue mice, eat blackberries and scavenge carcasses or rubbish bins.
• Other predators, such as birds of prey and weasels, may also eat rabbits.
• Changes in the climate or damage to the habitat could lead to the extinction of one or more populations.

Grey and Red Squirrels and Competitive Exclusion
The introduction of grey squirrels from North America has had a dramatic and negative effect on the native red squirrel. Why can’t the two species coexist?

In the community that lives in a habitat, each species has become adapted to a particular pattern of life, known as a niche. It is a general rule in ecology that no two species occupy the same niche. This is called competitive exclusion.

To understand why this rule exists we need to think about competition. In ecosystems, individuals compete with members of their species and members of other species. Those who compete successfully survive, those who do not die. It is easy to see how competitiveness as a characteristic will be an advantage. Ecologists have studied apparent cooperation between individuals of the same or different species. In general they have concluded that in some way, apparently selfless behaviour has some benefit to the individual or to its relatives. Relatives carry some of the same genes which may influence competitive behaviour.

Consider two species eating the same food and living in the same part of the habitat at the same time. They will compete with each other as food becomes scarce. Because the species are different, one of the two species must be more successful than the other. The less successful species might become extinct, or the range of its population might be restricted, or it might be forced to modify its niche – it might alter its diet.

Habitat Destruction and Conservation
If habitats are destroyed, communities of plants and animals will be lost forever. Habitat destruction is the result of unsustainable human development. Around the world, political and economic decisions are taken every day which directly result in habitat destruction. These decisions include the building of new motorways in Europe, the destruction of rainforest to release land for ranching in Brazil, and deciding to take the car to the supermarket, rather than walk to the corner shop. We all share the responsibility for making human life more sustainable.