As of the year 2005, the human population exceeds 6.4 billion people, versus 3.5 billion in 1950 and 1.6 billion at the start of the twentieth century. The growth rate in 2005 is 74 million per year (almost 1.2% per year). However, growth in food production is not keeping pace.
The amount of fish caught has increased 4.6 times from the 1950s to the late 1980s. Over the same period, world grain production has tripled. But since then, fishing has reached the sustainable-yield limit and has stopped increasing, and grain production has slowed its increase to 0.5% per year, far less than world population growth. Thus, the world's annual per capita grain production has dropped almost 10% from its highest value of 346 kg in 1984 to 313 kg in 1996.
Agricultural food production can be increased by either increasing the area under cultivation or by increasing the yield. The oldest means of increasing yield is to initiate irrigation. In the twentieth century, fertilizer use provided the most dramatic improvement. But fertilizer use is linked to water use. The more rain or irrigation that is applied, the more fertilizer the crops can utilize. World fertilizer use has actually dropped recently, as farmers have learned to apply it more precisely. A final way to improve yields was to develop better varieties of crops, but the high-yielding varieties require higher fertilizer doses.
Some of these practices cannot be extended or even be sustained. The per capita area under cultivation has been falling since the 1950s, and the total area has been dropping since 1981. Increasing the area now means bringing marginal lands under cultivation and/ or destroying sensitive ecosystems such as tropical rainforests or wetlands. In the former Soviet Union the marginal land was subject to soil erosion. Over the 18 years leading to 1995, 26% of the land used for grain production had to be abandoned for this reason. Agricultural practices tend to increase erosion and do not contribute much organic matter by which natural systems rejuvenate the soil. Since 1950 some 20% of the world's topsoil has been lost. Industrial and urban development is also taking much prime farmland out of production, even in China.
The per capita amount of land under irrigation has been dropping since 1979. A principal reason is the depletion of underground aquifers. Twenty-one percent of U.S. cropland under irrigation gets its water from aquifers that are being drawn down. Many of the world's great rivers are pumped dry before they reach the sea. In the United States, these include the Colorado River and the Arkansas River. The river feeding the Aral Sea in Uzbekistan has run dry, and the sea itself is disappearing. China's Yellow River first ran dry in 1972, and does so now each year for longer and longer periods.
These dismal statistics call for an evaluation of the carrying capacity of Earth. The total world grain production is currently about 1.82 billion tons per year. This might be increased to 2 billion tons. Americans use about 800 kg per person per year, much of it indirectly in the form of meat and dairy products. Indians on average use only 200 kg per person per year, in a diet consisting mostly of grain only. Some think that the healthiest diet is what is known as the Mediterranean diet, which is lower in fat and protein than the American diet. It requires about 400 kg per person per year. Thus, the world's grain supply could support 2.5 billion people on the American diet, 5 billion on the Mediterranean diet, or 10 billion on the Indian diet.
The world population growth rate is 1.2% per year. The good news is that this is a slowing, down from a peak of 2.5% per year in the 1960s. Fertility has fallen significantly in most of the large developing countries. Modern contraceptives are becoming more accepted by these countries. For example, they are used by 66% of married women in Brazil. Indonesia is a model for population control in developing countries because it has linked it to health care and education for women and children, reducing infant mortality from 133 per 1000 births in the 1960s to 57 in the 1990s. Some think that feelings of economic security are an even more potent factor in controlling population than birth control education.
However, we are already beyond the number that could be supported on the Mediterranean diet. Even if population control is pursued aggressively by developing countries, the population will continue to grow because of the high proportion that is yet to reach childbearing age. Under this scenario the population is expected to level off at about 10 to 16 billion during the twenty-first century.
492 ECOLOGY: THE GLOBAL VIEW OF LIFE 14.6 CONCLUSION
In the next chapter we study particular types of ecosystems. Based on what we have learned from this chapter, we can formulate a few questions to think about when examining any ecosystem:
• What is its primary productivity? Which organisms are responsible for most of its productivity?
• How much energy is stored and transferred at each trophic level? How much biomass?
• What do the biogeochemical cycles look like for this ecosystem? What are the amounts of nutrient storage and the flux rates between compartments?
• What are the important organisms and groups of organisms in this ecosystem? What adaptations make them abundant in this ecosystem?
• What is the niche of each of these organisms (habitat; food source; is it food for another organism; in what types of species interactions does it participate)?
• What limits the population of each species?
• What stage of development is the ecosystem in?
• How do human activities affect all of these factors?
In the next several chapters we travel from the land to the sea, examining several types of ecosystems with these questions in mind.
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