For over a decade the mail has brought a steady stream of flyers announcing new books and conferences with the terms ‘biological diversity’ or ‘biodiversity’ featured prominently. Apparently ‘biodiversity’ is here to stay, holding an important position in the vocabulary of natural resource management. It arrived just as people were becoming sensitive to the entire spectrum of life with which we share the planet and the myriad threats facing it. Despite this prominence the term still confuses many people. In large part this is because biodiversity seems like something that we should be able to quantify, like biomass or population density, but in practice only limited components of biodiversity can be measured easily. This constraint pushes biodiversity toward being more of a conceptual entity, analogous to aesthetics or ecosystem integrity, than a tangible thing. In this chapter we will first try to sort through the confusion to provide a clear idea of what biodiversity is. In the second part we will summarize the many reasons why it is important to maintain biodiversity. Finally, in the third part we will discuss the relationship between biodiversity and related concepts such as ecosystem integrity and sustainability.

What is biodiversity?

Biodiversity could be defined simply as ‘the diversity of life’, but a fuller definition is generally preferable. A definition such as ‘the diversity of life in all its forms and at all its levels of organization’ (Hunter 1990) reminds us that biodiversity includes the microbes and fungi that are often overshadowed by plants and animals. It also compels us to look beyond species to the genetic and ecosystem components of life on earth (Figure 1.1).