The Green Revolution in agriculture began in the 1940s with the introduction of high-yield, disease-resistant varieties of wheat to Mexico (Case 6: Agriculture). In subsequent decades, new strains of corn, wheat, and rice, widespread application of industrial fertilizer, and the spread of farm machinery increased global crop production to levels that could scarcely have been imagined at the beginning of the twentieth century. As population growth has accelerated, however, the need to grow ever more food remains strong. And other factors, notably rising meat consumption in many countries and the increasing use of corn to generate biofuel, now place still greater demands on global agriculture. It is estimated that, over the next decade alone, global corn and wheat production will have to increase by 15% to meet demand.

How can we meet the needs of a growing population? Conventionally, agronomists see two options: We can devote more land to crops, or we can increase the yield of fields already in place. Fig. 49.17 shows that globally both the amount of land devoted to growing corn and corn yield have increased steadily over the past 50 years. It is possible to increase yield further, as the yield per hectare in the United States, Canada, China, and western Europe far exceeds that of most other regions. Increasing yields, however, will require increasing the use of fertilizer and the use of fossil fuels to power farm machinery, with all the attendant problems of climate change and eutrophication. Increasing crop area also comes at a cost, as forests and natural grasslands go under the plow, decreasing the biological storage of carbon and threatening biodiversity.

1084

For more than a century, biologists have developed improved varieties of crop plants by breeding, the highly effective practice that Darwin called “variation under domestication” and used to frame his arguments about natural selection. Today, we have another tool available: genetic engineering. Biologists can now introduce genes for desirable traits into crop plants, improving their yield, disease resistance, or nutritional value (Chapter 32). The potential benefits of genetically modified crops are obvious, but some biologists see risks as well, many of which are detailed in Chapter 12.

Conventional breeding and genetic modification may improve crop yields, but, in a world of changing climate and evolving pests, these gains may have something of a “Red Queen” flavor to them (Chapter 42). In Lewis Carroll’s Through the Looking-Glass, the Red Queen tells Alice, “Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!” Much of evolution appears to work this way: As the physical or biological environment changes, continued adaptation is needed just to maintain fitness. Strains of wheat, modified to improve drought tolerance, may simply maintain high yields in a changing environment, not increase them. Increases in yields through genetic modification may be possible, but limited. Thus, massive fertilization by nitrate and phosphate fertilizers will continue to be a key component of agriculture.

Another type of biological intervention, the reduction of pests, may also help increase crop yields. We are not the only species with a nutritional interest in croplands. Fungi, bacteria, protists, nematodes, insects, birds, and mice all target cornfields, sharply reducing yield. In the United States alone, such unwanted guests cause about $1.5 billion worth of damage per year. Conventional breeding and genetic modification have produced improved resistance to pests, but fungus-resistant plants provide strong selective pressure for fungi that can circumvent the mechanisms of resistance, gaining access to otherwise unavailable food.

Food spoilage after harvest also reduces the effective yield of agricultural lands. It is estimated that one-third of the food grown in the United States is never eaten, and figures for Europe are similar. Fungi and bacteria spoil food during storage and transportation, on supermarket shelves, and in refrigerators. The problem also exists in less developed countries. The United Nations estimates that in some parts of Africa a quarter of all crops spoil before they ever reach the market. More efficient storage, transportation, and packaging could sharply increase the effective yield of fields and pastures.

As with energy, a sustainable future for agriculture requires creative biologists, ingenious engineers, and wise citizens. Geneticists and physiologists can help to develop plant breeds that improve crop yield while using less fertilizer. Working with microbiologists, engineers can discover new ways of storing and transporting food that increase the likelihood that it will be eaten before it spoils. As individuals, we make daily choices about the food we eat. We can choose to eat more plant products, for example. The basic ecological logic of the trophic pyramid (Chapter 47) tells us that a hectare of grain fed directly to humans feeds more people than the meat of cattle fed on the same crops. We can also choose to eat locally grown products where possible, saving energy while decreasing the likelihood of spoilage.

The entire carbon cycle, including its interactions with the nitrogen and phosphorus cycles and human impacts, is summarized in Fig. 49.18.