This chapter examines some of the main threats to biodiversity. No book can cover all of them. As we will see, extinctions and habitat alterations have been occurring for many thousands of years. We will examine the importance of human population growth, and our consumption of resources such as energy, food, water and land. We will also examine the problem of pollution, which occurs once the limited capacity of the environment to deal with waste is exceeded. We then turn to one of the oldest reasons for extinctions, introduced species, and relate this to the controversy over one of the potential new threats – genetically modified organisms (GMOs). Most species are threatened by several factors, some of which are not known. Threats may cause extinctions directly, or may cause ‘secondary extinctions’ of dependent specialist species.

One way to examine the threats is to look at the known causes of extinctions of species over the past 400 years (Fig. 2.1), and another is to look at the known threats to the species that are currently identified as officially threatened by the IUCN in their ‘Red Lists’ (Section 3.1.3 and Fig. 2.2). About 12% of vascular plant species, 25% of mammals, and 12% of birds, are listed as threatened. However, N. C. A. Pitman and P. M. Jørgansen have given more detailed consideration to the less well-studied tropical regions, and suggest that between a third and a half of the world's plant species are threatened.