R-selected and K-selected species are terms that biologist use to describe the reproductive strategy of animals. Either an animal produces a large number of offspring and it basically is a numbers game for those offspring or the animals have few offspring and invest a large amount of time in those offspring. R-selected species are those that favor a large number of offspring. This includes animals like insects, amphibians, many fish, and reptiles. They tend to be smaller organisms so the energy used to make each individual is low and live in environments that are unstable. They also have shorter lifespans and reach sexual maturity quickly. They have type III survivorship pattern which means that earlier in life more organisms will die than later on in their life. In these species, the number of offspring is important because it directly impacts the population size.
Other species are K-selected where the population size is determined by the carrying capacity. In ecology carrying capacity or the maximum number of individuals in a species that the environment, is represented by a capital K. Humans, elephants, non-human primates, horses, cows and the like are all K-selected species. These species have few offspring but invest a lot of energy into ensuring that the offspring survive. In these species, the number of offspring is less important but survivorship becomes more important. K-selected species are characterized by type I or II survivorship pattern in which most organisms survive to the life expectancy. The young often have a long period of time before they reach maturity, which is why they are so energetically demanding to the parents.
While a species may not be necessary either strictly K-selected or r-selected they fall somewhere on a spectrum. Members of squamate (large lizards and snakes) and Cricetidae (a rodent family) tend to fall more towards the r-selected side of the scale, therefore, the number of offspring plays a large role in the population size. It is also important to note that evolution is what caused species to be r-selected or K-selected. Unstable environments favored a large number of offspring, as investing a large amount of energy into the offspring produced little return. However, in a stable environment offspring are more likely to survive with large parental investment. Since there are not unlimited resources organisms must make trade-offs between reproduction and survival. For example, an organism can grow to a larger mass and then reproduce or it can reproduce at a smaller size and produce less viable offspring or fewer offspring. Predation also plays a role in this, as larger organism tend to have higher survival but they must use more energy to grow and not reproduce.