Some species’ numbers grow slowly and steadily and have stable populations over long periods of time, whereas others have populations that fluctuate wildly in response to changes in environmental conditions. Such differences are due to variation in life history among species, which includes such variables as the age at which individuals begin reproducing, the number of offspring they produce, and the rate at which the young survive.

Whales, bears, wolves, and gorillas all have populations that can stabilize close to their carrying capacity, which is why we call them K-selected species. Ecologists propose that living near the carrying capacity favors individuals that excel at competing for limited resources in a crowded environment. These species have a long life span, tend to reproduce later in life, and have a small number of large offspring that receive intensive parental care. In general, density-dependent regulating factors, such as disease and competition, exert significant controls on populations of K-selected species.

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Mice, rabbits, dandelions, and cockroaches are examples of r-selected species, which grow rapidly when the environment is favorable. You might sum up their philosophy as “live fast, die young.” Unlike large, enduring animals such as whales, r-selected species are small and subject to catastrophic mortality from harsh weather, fires, and other density-independent factors. They have undergone strong natural selection to reproduce prolifically when the time is right. Table 3.2 contrasts the major characteristics of K-selected and r-selected life species.

Let’s look closely at some animal and plant examples and see how the life history concept applies. Mountain gorillas, which are large and have low reproductive rates, are a perfect example of a K-selected species. Female mountain gorillas may weigh up to 100 kilograms (220 pounds) and give birth every fourth year. In comparison, pygmy marmosets, which weigh only 0.1 kilogram (3.5 ounces) and may have two sets of twins per year, appear to be r-selected (Figure 3.14). As a result of their higher reproductive rates, a pygmy marmoset can produce 16 offspring in the time that a female gorilla produces one. Not surprisingly, while mountain gorillas live at population densities of about one per km² (2.6 per mi²), pygmy marmoset population densities may be as high as 200 per km² (518 per mi²).

The concepts of r- and K-selection apply to plants as well. For example, the American chestnut, which produces large seed-bearing fruits that number in the thousands, contrasts sharply with the plains cottonwood, which produces millions of tiny wind-dispersed seeds each spring (Figure 3.15).

The life history of an organism influences its capacity to recover from environmental disturbances. Ecologists define disturbance as a discrete event, such as a fire, earthquake, or flood, that disrupts a population, ecosystem, or other natural system by changing the resources, such as food, nutrients, or space, available to organisms or by altering the physical environment. In general, populations of r-selected species are faster to recover from disturbance than are K-selected species. For instance, a population of mountain gorillas decimated by massive mortality during a volcanic eruption would take a longer time to recover compared with a population of pygmy marmosets subjected to a similar disturbance. It should not be surprising, then, that endangered species often have K-selected life histories.

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