Response by living creatures, in the form of novel or intensified efforts, to their environmental difficulties and limitations. This process is characterized by continuing, and often painful or fatal, clashes both between the living creatures and with their environments. The struggle is aggravated by the tendency of any species towards over-population, by the dependency of members of the population on one another and on other species, and by unpredictable changes in the environment. The struggle is thus not limited to intraspecific competition. It takes place between both fellow-organisms of the same species and between organisms of very different natures.