There is a special branch of ecology – called population ecology where one of the areas of research focuses on the life cycle of a population.

Basically a population goes through three main stages; population growth, stability and population decrease. In this article I will look at the different factors that affect these life stages so we can see that tiny changes can determine if a species becomes extinct or remains in an area. Now when a species goes extinct, let's be honest, there is some uproar about protecting biodiversity which dies down after a while. Well human beings – Homo sapiens - are a species too. So what if we were to go extinct? It has happened before, where huge, technologically advanced civilisations like the Aztecs have just disappeared.

The first life stage – growth occurs when the available resources exceed the number of individuals present, allowing the population to expand as there are enough resources to feed new members. Usually in this phase the reproductive rate is fast and there is a low death rate. For instance in the first stage – population growth, a population can grow in size due to births as a result of matings within the population, or by members of other populations immigrating and joining this one. At this stage the factors that influence decrease in population size are also important. These factors include the death of individuals in the population or by members leaving or emigrating to join other populations.

But what must be kept in mind is that resources are not infinite, but limited so that only a certain number of individuals in the population that can be maintained, so that at this point the population reaches the next stage - stability. One way a population is maintained is through interaction.

The main one is competition; this could be competition for food or for mates or for the best spot to live. At the end of the day, the strongest or smartest individual gains the best resources and survives to reproduce while the loser dies as a result of the fighting or are forced to leave the population and fend for themselves.

But competition usually for food, water and habitat can also occur between individuals of different species.

This had led to some species becoming quite specialised, for instance, only occupying habitats of a certain temperature or humidity. It is these species that are most vulnerable to extinction in the final stage of a population life cycle.

Another type of interaction that needed to taken into account is predation, usually between individuals of different species. Sounds simple enough, a large carnivore eats a smaller animal. But what if the number of large carnivores in an area was to decrease due to death or migration, then the number of smaller animals would increase since the pressure from this carnivore has disappeared and naturally stay at a steady level as there is only so much resource to support them. However, it this pressure might still be present with the arrival of another type of large predator. So the species composition of the area has changed but the relationships between the species present are relatively the same.

In the final stage, the number of individuals in the population decreases and the population may eventually go extinct. This stage can be as a result of a variety of factors, such as, environmental factors, increase in predators, illness, and low genetic diversity so that members are susceptible to diseases, over hunting and pollution by humans.

Another factor that can cause or increase death rate include disease which can kill or make surviving members too weak to fight off predators. The availability of food and water can also weaken individuals against disease and predators.

Both disease and lack of food can result in older members of the population dying and leaving to younger members unable to care and get food for themselves or are defenceless against predators.

Also the young members of the population might die or be so undernourished that they die before reaching reproductive age or their bodies put the available energy to surviving and not towards production of hormones that cause the development of reproductive organs.

So far, we have dealt with animals, but the population cycle of plants is also important. The main reason is that plants gather energy from the sun and pass it along the food chain when they are consumed by herbivores, which are then consumed by carnivores and so on.

Therefore, when looking at a population one also has to consider the abiotic or non-living components like soil and the nutrients they contain.

Also, plants are the first colonisers of disturbed or areas that have been cleared. But as the amount of organic matter increases more species with more complex structures inhabit the area. So it goes from grassland to forest with many layers of vegetation and in turn more microhabitats for organisms to inhabit.

Therefore it can be seen that populations change naturally over time.

However, many factors are involved in this change, most of which are due to the activity of humans making it a more comfortable place for themselves, but we do share this planet with millions of other species. We seem to be playing a game of tic tac toe with our environment.

Every time nature finds a solution to a human-made obstacle, we put another obstacle in its way. But when the game is over will any side have their Xs or Os all in a row or will both come out the loser.