Humans and Earths Carrying Capacity

One way to determine if the human population is reaching Earth's carrying capacity is to examine whether, and how rapidly, the growth rate is declining. As we saw above, the logistic growth curve is a result of a gradually declining growth rate as the population approaches the carrying capacity of the environment.

Human population growth rates were at their highest in the early 1960s, about 2.1% per year, but have since declined to the current rate of 1.2%—this steady decline is one indication that the population is nearing a stable number. Uncertainty about the future rate of decline leads to differing estimates of this number and how soon stability will be reached (Figure 15.4). However, the unique characteristics of humanity make it difficult to determine exactly what number represents Earth's carrying capacity for humans.

The growth rate of fruit flies and water fleas in laboratory populations slowed as these populations neared the limit of their resources, but the growth rates of fruit flies and water fleas are essentially forced down by environmental factors—low levels of resources cause increased death rates or decreased birth rates. However, this is not the case in human populations. Even as the number of people has rapidly increased, death rates continue to decline—an indication that people are not limited by food resources. Growth rates are declining because rates of birth are falling faster than death rates. Unlike the water fleas and white-tailed deer, where females are unable to have offspring when their populations are near carrying capacity, birth rates in human populations are falling because women and families are choosing to have fewer children, not because they do not have the resources to support a pregnancy. In fact, birth rates are generally lowest in regions where resources are most abundant and where the education of women is encouraged (Figure 15.5). Although the growth rate is slowing, rising living standards around the globe appear to indicate that humans are not nearing Earth's environmental carrying capacity.

Another way to determine if the human population is nearing Earth's carrying capacity is to estimate the amount of resources that are currently being used by humans and to use that estimate to calculate the theoretical limit to

Figure 15.4 Projected human population growth. The United Nations' report predicting the eventual size of the human population is based on a number of uncertainties leading to three projections—from a low-growth scenario of 7.9 billion people, to medium growth resulting in 9.3 billion, or even a high-growth estimate of 10.9 billion.

High growth rate -Medium growth rate

Low growth rate

Figure 15.4 Projected human population growth. The United Nations' report predicting the eventual size of the human population is based on a number of uncertainties leading to three projections—from a low-growth scenario of 7.9 billion people, to medium growth resulting in 9.3 billion, or even a high-growth estimate of 10.9 billion.

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