Struggle for existence
Organisms produce more offspring than - given the limited amounts of resources - can ever survive, and organisms therefore compete for survival. The Atlantic cod for instance lays around five million eggs a year while Darwin calculated that even the low reproducing elephant (opposite) produces more young than will ever live to maturity. Only the successful competitors will reproduce themselves.
Darwin referred to this competition as the "struggle for existence".
The struggle for existence takes place within a web of ecological relations. Above an organism in the ecological food chain, there will be predators and parasites, seeking to feed off it; and below it are the food resources it must in turn consume in order to stay alive. At the same level in the chain are competitors, which may be competing for the same limited resources of food, or space.
An organism competes most closely with other members of its own species, because they have the most similar ecological needs to its own; other species, in decreasing order of ecological similarity, also compete and exert a negative influence on the organism's chance of survival.
Why aren't the oceans overrun by Atlantic cod?