The world has been transformed by human activities, and mankind is centre stage in the evolutionary drama now being played out in the world's ecosystems. In this chapter, different groups of plant species in the evolutionary play are introduced – native, wild, crop, weedy, introduced, invasive, feral and endangered species. On the face of it there would appear to be little difficulty in assigning plants to species, and deciding into which category particular species fall. But as we shall see, in confronting the following major questions, there are many complications. How are species to be defined? How many species of plants are there in the world? How are native and introduced species distinguished? What do conservationists mean by wildlife? How many species are cultivated, weeds or invasive? In response to different kinds of environmental change and disturbance, how many species are presently at risk of extinction, and are current extinction rates greater than those of the past? Is there any agreement on the numbers of species that are endangered? Is there any evidence that conservationists might have exaggerated the situation?

Species

The concept of the species is at the heart of any consideration of evolution, and both the theory and the practice of conservation are to a large extent based on these concerns. Different groups of plants – wild, cultivated etc. – are widely discussed in terms of the numbers of species they contain. In considering these estimates, however, a major concern must be faced.