The types of trade-offs illustrated in the previous section result in many species exhibiting particular groupings of life history traits that have complementary trade-offs. In this section, we will consider two examples of these groupings, r-selected species and K-selected species. The terms r and K are components of the population growth equation, which is discussed in detail in the Population Growth and Dynamics section. Table 6.3.1 shows traits, characteristics, and representative examples of species that are highly r-selected and species that are highly K-selected. These two categories represent two ends of a continuum, and so not all species are considered either r-selected or K-selected; instead some species (as we will discuss below) may fall in the middle of the continuum (and be neither r-selected nor K-selected) or have traits at each of the continuum (and so, in a sense be both r-selected and K-selected).
Recall the trade-off related to offspring size and number. We discussed that a larger individual offspring has a higher likelihood of succeeding, but that more numerous (and therefore smaller) offspring represent more 'chances' at successful maturation. Offspring size (especially in traits such as egg or seed size) directly relates to parental investment in offspring, though parental investment can also involve care as the offspring matures. Consider the bison (Bison bison) and European grass frog (Rana temporaria) discussed previously, and included again below. A female bison may produce one (or, rarely, two) calves in each annual reproductive event. The female carries the offspring within its own body, nourishing and protecting it, for a gestation period of ~283 days. Calves are nourished solely by milk produced by the female bison for approximately 7 months, and even after weaning remain near their mother for protection and care for an additional five months. Because of this care and support, a large proportion of calves born in a given year generally survive to maturity. Conversely, a female Rana temporaria, lays up to 2,000 eggs per reproductive event and clusters them together in a mass (Figure 6.3.1C), which she does not tend or protect or nourish. The only parental care provided by the female in this case, is the nourishment contained within the egg when it is laid. Consequently, the eggs are vulnerable to predators, pathogens, and environmental stressors (such as the pool drying out) and experience a high degree of mortality before hatching into tadpoles. These two species illustrate the differences in reproductive strategies between r-selected (Rana temporaria) and K-selected (Bison bison) species.
They differences in offspring number and size and degree of parental care result in different survival patterns throughout the developmental stages of individuals. Species with Type I survivorship curves have high offspring survival and a large proportion of individuals survive to adulthood and reproductive maturity. The highest degree of mortality occurs in advanced adult age groups. Type I survivorship curves are therefore characteristic of K-selected species.Type III survivorship curves exhibit significant juvenile mortality such that the majority of offspring never reach sexual maturity. Type III survivorship curves are characteristic of r-selected species. Type II survivorship curves exhibit relatively constant survivorship and mortality throughout different age groups and therefore are not r-selected or K-selected, but instead somewhat in the middle of the continuum between the two.
As the types of survivorship curves illustrate, r-selection and K-selection exist on a continuum, not as discrete groupings. Some organisms are somewhat in the middle. For example, many medium-sized mammals and birds might fall in this area. Organisms in the middle of the continuum are neither quite large nor small, have middling life spans and rates of growth, produce moderate numbers offspring in which they invest a moderate amount of resources and parental care. Many also exhibit a Type II survivorship curve.
Additionally, other organisms seem to exist at both ends of the continuum, exhibiting some highly r-selected traits, and other highly K-selected traits. For example, leatherback sea turtles (Fig 6.3.3) are among the largest and longest-lived reptiles, with average weight in excess of 1,000 pounds (454 kg) and average life spans estimated at 50-100 years, both of which would indicate K-selection. In reproduction, however, female leatherbacks lay around 110 eggs, do not tend or protect the nest and do not return to care for nestlings once they hatch. The small size of hatchlings makes them highly vulnerable to predation until they reach a sufficiently safe size, producing a Type III survivorship curve. These traits indicate r-selection. Species such as leatherback sea turtles are therefore difficult to categorize on the r to K continuum.