Struggle for existence. Natural selection

Ch. Darwin used the phrase “struggle for existence” in a broad sense: the relations of organisms with each other and with unfavourable lifeless nature, and also the effort to leave offspring. The reason is that organisms produce many offspring while food, space and light are limited, so most offspring die before maturity. There are three forms. The struggle within a species is the most intense, because the needs of individuals of one species are the same: competition for nesting sites, food and mates; among plants densely sown seedlings compete for light and water. Struggle between species includes competition (for example two herbivores for one pasture), predator–prey and parasite–host relations; since different species often have different food, this struggle is often less intense. Struggle against unfavourable abiotic conditions means adaptation to cold, heat, drought and salinity: in autumn annual plants die after leaving seeds, birds migrate, some mammals hibernate. In the struggle, advantage goes to adapted individuals. People take these relations into account: they do not sow seedlings too densely, they remove weeds, they attract natural enemies of pests (entomophages, insect-eating birds) – biological control – and use bees to increase pollination. Predators are also needed for a healthy ecosystem – they remove sick and weak individuals and control the number of prey.