entially arable land is now under cultivation.
In Global Change I, we saw that the ecosystems of the world varied greatly in their net primary production (NPP). Isn't it surprising that some parts of the Earth's land surface are much more productive than others. Most of the "good bits" have been in agriculture for a long time. Some of the remaining land surface, such as tropical rain forests, is highly productive under native vegetation (hence not much use for feeding people), and, at least with the farming methods of today, not very productive when converted to agriculture. In fact, cropland per capita is declining world-wide, as agriculture land is degraded, or urbanized. Increasing the yields from available farmland appears to be the key to increased food production.
The Green Revolution
The overall consequence is that per capita food production has contradicted the doom-sayers of the 1960s: during the greatest episode of population growth in human history, food supply per capita grew. Note that Asia did better than the world average, while Africa did worse. The Green Revolution's great success with rice explains the former; lack of success with breeding new arid-land crop varieties, combined with a large dose of political instability, explain Africa's worsening condition. Per capita grain production in Africa is down 12% since 1981 and down 22% since 1967. Some 20 years ago, Africa produced food equal to what it consumed; today it produces only 80% of what it consumes.
From the perspectives of feeding a growing population, the Green Revolution was a smashing success. Lurking behind this success story, however, are some disturbing issues:
Continued improvements in agricultural practices and land management, hopefully will allow us to both increase yields and minimize some of the negative effects of agriculture. Some areas that hold much promise include: better pest management, water-conserving irrigation, conservation tillage, and development of new crops through breeding or genetic modification.
The use of pesticides multiplied by a factor of 32 between 1950 and 1986, with developing countries now accounting for a quarter of the world's pesticide use. However, inappropriate and excessive use can cause contamination of both food and environment and, in some cases, damage the health of farmers. Pesticides also kill the natural enemies of pests, allowing them to multiply; meanwhile the number of pest species with resistance to pesticides has increased from a handful 50 years ago to over 700 now.
Crops and foods can produced using recombinant DNA techniques which enhance their agronomic potential, nutritional characteristics, or one or more features of pest protection (insect and viruses) and tolerance to herbicides. More than 40 such transgenic crop varieties have been cleared through the federal review process. New varieties of plants are being developed using genes from wild varieties with inbuilt disease resistance. Genes from the wild have been used to protect Brazil's coffee plantations; while a Mexican wild maize confers resistance to seven major diseases. According to the American Medical Association, these foods are "substantially equivalent to their conventional counterparts," and no long-term side effects have yet been detected.
While it is difficult to estimate the gains in food production that we may obtain from exploring new crop species, this could make a large contribution. Ninety percent of the world's food is derived from just 15 plant and 8 animal species. Wheat, rice, maize (corn), millet, and sorghum provide nearly all (70%) the food energy (calories) and up to 90% of all protein consumed by the world's people. Cereal grains are humankind's major food, contributing more than two-thirds of the world production of edible dry matter and half of the world's protein.
What Fraction of the World's NPP do Humans Appropriate
In order to determine how much food the world can produce, and what fraction of that production is captured by humans, we will re-visit a topic discussed in Global Change I: The Flow of E