Scientists Disagree on Earth’s Carrying

Scientists disagree on Earth’s carrying capacity Every 5 days, the global human population increases by roughly a million lives: 1.8 million infants are born and 800,000 people die. The human population has not always grown at this rate, however. As FIGURE 7.1shows, until a few hundred years ago, the human population was stable:deaths and births occurred in roughly equal numbers. This situation changed about 400 years ago, when agricultural output and sanitation began to improve. Better living conditions caused death rates to fall, but birth rates remained relatively high. This was the beginning of a period of rapid population growth resulting in the current human population of 6.8 billion people. Human population growth. The global human population has grown more rapidly in the last 400 years than at any other time in history. As we saw in Chapter 6, under ideal conditions, all populations grow exponentially. In most cases, exponential growth halts when an environmental limit is reached. The limiting factor can be a scarcity of resources such as food or water or an increase in predators, parasites, or diseases. Limiting factors determine the carrying capacity of a habitat. Are human populations constrained by limiting factors, as other populations are? A theoretical model of food supply and population size. (a) In a theoretical 100­year periodwithout significant improvements technology, the human population grows exponentially, while the food supply grows linearly. Consequently, a food surplus is followed theoretical 100­year period with significant improvements in agricultural technology, the food supply increases suddenly. Consequen food surplus. Environmental scientists have differing opinions on Earth’s carrying capacity for humans. Some scientists believe we have outgrown, or eventually will outgrow, the available supply of food, water,timber, fuel, and other resources. One of the first proponents of the notion that the human population could exceed Earth’s carrying capacity was English clergyman and professor Thomas Malthus. In 1798, Malthus observed that the human population was growing exponentially,while the food supply we rely on was growing linearly. In other words, the food supply increases by a fixed amount each year, while the human population increases in proportion to its own increasing size. Malthus concluded that eventually the human population size would exceed the food supply. Shows this projection graphically. A number of environmental scientists today subscribe to Malthus’s view that humans will eventually reach their carrying capacity, after which the rate of population growth will decline. Other scientists do not believe that Earth has a fixed carrying capacity for humans. They argue that the growing population of humans provides an increasing supply of intellect that leads to increasing amounts of innovation. By employing creativity, humans can alter Earth’s carrying capacity. This is one of the fundamental ways in which humans differ from most other species on Earth. For example, in the past, whenever the food supply seemed small enough to limit the human population, major technological advances increased food production. This progression began thousands of years ago. The development of arrows made hunting more efficient, which allowed hunters to feed a larger number of people. Early farmers increased crop yields by plowing by hand and later with oxen­ or horse­driven plows. More recently, mechanical harvesters made farming even more efficient. Each of these inventions increased the planet’s carrying capacity for humans, asshows. The ability of humans to innovate in the face of challenges has led some scientists to expect that we will continue to make technological advances indefinitely into the future. This expectation is reasonable, but questions remain. Based on our history, should we assume that humans will continue to find ways to feed a growing population? Are there other limits to human population growth? How do we know if we have exceeded Earth’s carrying capacity?