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In the preceding sections, we discussed population size and growth curves, and we outlined age structure and survivorship. These topics converge when we consider patterns of reproduction. Some species produce large numbers of offspring but provide few resources for their support. Such species are described as r-strategists (Chapter 42). In contrast, some species produce relatively few young but invest considerable resources in their support. These are termed K-strategists. How do these concepts relate to r and K in population growth?

In general, species that live in unpredictable environments produce many young. When the environment is full of predators watching for vulnerable prey, such as gulls that watch for young turtles on a beach, or when resources are patchily distributed and abundant for only a short time, the chances of a young organism’s surviving to adulthood are low. Laying an egg, then, is like buying a lottery ticket—there is a limited chance of success. It is more advantageous to buy many tickets cheaply (that is, to have many offspring and make little parental investment in any of them) than to pay a lot for one. Thus, salmon and other fish produce many thousands of young, each with an extremely small chance of survival.

As we saw earlier, organisms like insects and plants that display Type III survivor curves also experience high mortality in the first stages of life. Because their reproductive success depends on the small chance that a few of their young will survive all the hazards they encounter, parents produce many offspring and invest relatively little in any of them. We refer to species like most fish as r-strategists because they reproduce at rates approaching r, the intrinsic rate of growth.

In contrast, species living where resources are predictable often produce a lower number of offspring and invest their reproductive resources in just a few, often larger and better cared for, offspring. These offspring are therefore better protected from predators, able to compete with others when they are young, and more likely to survive periods of bad weather. Large animals, such as eagles and bears, produce only two or three young, each of which is much more likely to survive to reproductive age than the offspring of small animals like mice, which have many predators and are easily killed by cold weather. Animals like eagles and bears are called K-strategists because their population densities commonly lie near the carrying capacity K, when resources become scarce.

These two strategies reflect an evolutionary trade-off. A reproductive individual has access to a finite amount of resources and can invest either in many inexpensive young or few well-provisioned offspring. Depending on the combination of physiology and environment, natural selection in a given set of circumstances may favor one strategy over the other. However, r- and K-strategists represent the opposite ends of a continuum in reproductive pattern. Many species lie between these extremes.