(a) : Organisms that are $r$ ­selected ($r$ ­- strategists) able to colonise a habitat rapidly, utilising the food and other resources before other organisms are established and begin to compete. The r­strategists tend to be relatively small organisms with short life spans ($e.g.$, bacteria) and often live in temporary or unstable environments; characteristically their survival depends on their ability to produce large numbers of offspring rather than on their ability to compete.