Question: How is the population of animals changing? Is it dropping or rising?

Edward Codling answered on 15 Jun 2011:

Another great question!

This really depends on what animal population you are interested in. Some animal populations have declined recently, often because of humans catching or killing too many or building on land which would have been the animal’s habitat.

However, many animal populations naturally fluctuate up and down over time – there is never a ‘stable’ or fixed level of animals. A famous example is the simple ‘predator-prey’ system:

Suppose we have a small field with a population of rabbits (who eat the grass in the field) and foxes (who eat the rabbits). Suppose also that we start off with only a few foxes but lots of rabbits. In the first few months, the foxes have lots of rabbits to eat and they breed and produce more foxes, but the rabbits decrease because they are being eaten by the foxes. This continues for a while until we have a lot of foxes and not many rabbits. Now there isn’t much food for the foxes and they start to die and their population decreases. After a while most of the foxes have died because of starvation but then that means there are not many foxes left to eat the rabbits. The rabbit population then quickly increases because there are only a few foxes and we are back at the start again. This then continues over many years – it is known as ‘cyclical’ or ‘periodic’ behaviour because the populations change in cycles over time.

An explanation with a great video is here:
http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel/environment/populationsandpyramidsrev5.shtml

Because of this we have to be careful when looking at wildlife populations – we have to be sure that any decline in a population is due to humans before we take action. Sometimes it might simply be the cyclical ‘up and down’ behaviour I described above.

Of course this example only had 2 species. In the real world there are loads of species all interacting with each other – this is a much harder problem to study but quite exciting because of this!