Despite the high public interest in the environment in many parts of the world, conservationists are generally very short of funds. It is therefore essential to target the money that is available towards species or habitats where it will do the most good, and indeed to choose between an emphasis on species or on habitats. This may mean spending money in ways that might surprise people, for example on areas that are not the best of their type, or on species which are not the most highly endangered. Conservation requires money for research, for monitoring, for management, for education and for compensation. Many of the decisions in conservation relate to the availability and use of limited funds.

All decisions about priorities are subjective. The values attached to things, such as species or habitats, depend on the personal values of the people doing the evaluation. Just as beauty is in the eye of the beholder, different people with different motives for conservation (Chapter 1) will assign values in different ways. Faced with a diversity of values, there may be a diversity of aims in conservation. Choosing one aim may require a democratic decision, or expert opinion – and so debates and controversies are likely.

Before any evaluation it is therefore essential to define the aims. It is also necessary to decide the weightings which are to be given to the different reasons for conservation (such as rarity or diversity) in a particular case.