Populations interact, some decline to extinction, and some expand. In an ice age the tundra advances on the forest, but in no sense is there a "struggle" between them. As the temperature gets colder, the forest species decline and the tundra species increase; that is all. There is neither effort nor fight. As we move toward the more complex animals, of course, both effort and fighting become more common, but even in the predator-prey relationship, there is catching and eating rather than fighting.

p. 110 - Ecodynamics: A New Theory Of Societal Evolution, 1978