This book is about the distribution and abundance of different types of organism, and about the physical, chemical but especially the biological features and interactions that determine these distributions and abundances. Unlike some other sciences, the subject matter of ecology is apparent to everybody: most people have observed and pondered nature, and in this sense most people are ecologists of sorts. But ecology is not an easy science. It must deal explicitly with three levels of the biological hierarchy – the organisms, the populations of organisms, and the communities of populations – and, as we shall see, it ignores at its peril the details of the biology of individuals, or the pervading influences of historical, evolutionary and geological events. It feeds on advances in our knowledge of biochemistry, behavior, climatology, plate tectonics and so on, but it feeds back to our understanding of vast areas of biology too. If, as T. H. Dobzhansky said, ‘Nothing in biology makes sense, except in the light of evolution’, then, equally, very little in evolution, and hence in biology as a whole, makes sense except in the light of ecology.