Most ecological and evolutionary problems have their basis in changes in the number of organisms. This is one reason why mathematics is an important tool for ecologists and evolutionary biologists. Another one is the overwhelming complexity of ecological systems. An ecosystem often contains hundreds or thousands of species that interact in a complex food web. All species are different and they change continuously - evolution never stops! By formulating mathematical models of ecological and evolutionary processes we can to some extent study the behaviour and even understand systems that are this complex.