Introduction

Diversity is the very essence of life on Earth -- a vast array of microorganisms, algae, fungi, plants and animals occupies every terrestrial and aquatic habitat, showing endless variation in size, shape, colour, form, mode of nutrition, type of habitat, reproduction, motility and length of life cycle. This variation has arisen because living things have continually had to adjust to different climatic, edaphic, topographic and geographic conditions in order to survive and reproduce, and these adaptations have, over time, become built into each organism's life cycle.

By definition, biodiversity is the totality of genes, species and ecosystems present in a region -- encompassing the genetic differences among individuals of a species, the variety and richness of plant and animal species across local, national and global scales, and the range of terrestrial and aquatic ecosystem types found within a defined area, as understood by bodies such as IUCN, UNEP and WRI. The word 'biodiversity' was coined by Walter Rosen in 1982 and was later popularised by the sociologist Edward Wilson to capture the combined diversity seen at every level of biological organisation. The biodiversity we observe today is the product of over 3.5 billion years of evolutionary history, shaped primarily by natural processes and, increasingly in recent times, by human activity. This chapter examines the basic concepts of biodiversity, including its levels and patterns, its current extent, its importance, the causes and consequences of its loss, and the strategies used to conserve it -- before turning to the wider environmental issues, from pollution to climate change, that threaten it.