### 4.1.1 Background

In the general area of mathematical modelling, populations are very important for a variety of reasons. Governments, locally and centrally need to know the population profiles of their communities in order to plan, for example:

1. Pension arrangements;
2. Urban Development;
3. Rail Transport;
4. Road Transport.

In the last two cases, it is obvious that national boundaries do not contain, in any sense, the totality of the transport problems of a nation since we all need to use international transport systems for items such as travel, trade, education and leisure. Population models can have international importance as well as national importance. At the present time, many countries round the world are becoming more and more concerned about the long term effects of:

1. Industrial pollution;
2. The use of pesticides;
3. nuclear accidents;
4. rain forest destruction;
5. The extinction of species;

on the environment as a whole. Pressure groups exist, such as Friends of the Earth in the United Kingdom, who take it upon themselves to try to defend both the environment and (particularly) animals living in the environment from damage caused by man.

It is well-known that species are interdependent and that as a result we cannot consider the well-being of any one species and at the same time allow other species to become extinct. Population graphs of species can be very revealing in terms of the observed behaviour of a population.