The human species has had a greater impact on the biosphere than any other single species. It is now poised to cause more changes in the future of the biosphere than even photosynthetic bacteria caused when they first filled the atmosphere with oxygen. While human impacts are as old as the human species itself, their pace and extent have grown rapidly, and recent changes have begun to dwarf the consequences of even the most profound change ever brought about by our species, the development of agriculture.
The Coming of Agriculture
Until the development of agriculture, the human species did not affect the biosphere any more significantly than other highly efficient predators. While small nomadic groups could, and did, deplete local game populations, and could, and did, drive some species to the edge of extinction through over-hunting, human impacts were for the most part small, local, and short-lived.
Agriculture changed all that. By cultivating and harvesting grains, humans set in motion a series of changes with deep effects on both the natural world and their own culture that have continued, and intensified, to this day. First and most profoundly, grains gave humans a source of surplus food that allowed population growth. While a surplus of meat would rot, a surplus of grain could be stored for months, even years, without losing its nutritional value. With a steady source of food supplied by plants, the human population began the extraordinary growth that continues exponentially in the twenty-first century.
Changes in the Landscape
The inexorable growth of the human population has caused significant impacts on the landscape everywhere humans have settled. For instance, before the coming of the Europeans in America, it is said that the eastern forests were so thick that a squirrel could travel from the Atlantic coast to the Mississippi River without ever setting foot on the ground. Less than two centuries later—a blink of the eye in evolutionary time—more than two-thirds of that forest had been cleared for pasture or plowing. While the earliest settlers feared the bears and the wolves that haunted their forests, by the nineteenth century, not even deer or beavers were found in central Massachusetts. (Remarkably, much of this has changed yet again. With the western movement of agriculture in the late 1800s and the general decline of farming in the northeast, much of the forest has returned, and that squirrel has a better chance of making its journey now than it did at any time in the last 150 years.)
But while part of the country has reverted somewhat to its forested past, much of the rest remains significantly altered by agriculture, especially in the Midwest. Here, the flat terrain and deep, rich soils combine to form an ideal region for growing grain. The ancient grasslands have mostly long since disappeared, and with them went the herds of bison and other animals that formed the food chain of the prairie. While eastern forests may have returned with the shift of agriculture to the Midwest, it is unlikely that the prairie will ever regain its predominance in turn—there simply is nowhere else for agriculture to move to in this country.
For Better or Worse
The Midwest is not the only region in which ancient food chains have been altered. In fact, it is estimated that almost 50 percent of the terrestrial net primary productivity of the Earth—almost one-half of all the photosynthesis carried out over the entire surface of the land—is consumed, wasted, or diverted by humans. In a very real sense, our species farms the entire planet. As the population expands in the twenty-first century, this number is expected to grow.
This harnessing of Earth's potential for our own purposes is not necessarily a bad thing, and how we view such transformations depends quite a lot on our own preconceptions about nature and the place of humans in it. Are buffalo better than cows? Are forests better than pastures? Throughout much of our history, most people have decided, consciously or not, that human need, and sometimes greed, is sufficient reason for wreaking change on the natural world. It is unarguably true that more people live in better conditions as a result of agriculture and all it has brought. Agricultural changes are, in any event, a fait accompli—the human species is simply not going to return to its hunting and gathering ways.
While agriculture has wrought slow, pervasive changes on human culture and the landscape over more than ten millennia, other human endeavors have had much faster impacts on Earth and its biota . The greatest of them all, and second only to agriculture in its overall impact, has been the Industrial Revolution. Beginning in the late 1700s with the invention of the steam engine and continuing through the twenty-first century, humans have harnessed increasing amounts of stored energy to drive larger, faster, and more powerful machines.
The effects on the biosphere have been pervasive. Fuel-powered machines have allowed humans to cultivate more land, consume more resources, and sustain larger populations than was conceivable before the beginnings of this most important revolution. In addition to these effects, the use of fuel has had far-reaching consequences by itself. Wood-fired boilers soon gave way to coal, but not before deforestation of thousands of acres of virgin forests in the rapidly industrializing regions of Europe. Coal mining is a dirty business, and leaves in its wake scars on the landscape that can take generations to heal. More significantly, coal and its replacement, oil, are fossil fuels, the geologic remains of ancient plants that contain carbon removed from the carbon cycle millions of years ago. Burning fossil fuels releases carbon dioxide into the atmosphere, and records show the atmospheric level of CO2 has risen steadily since the beginning of the Industrial Revolution. Carbon dioxide is a greenhouse gas, which traps heat in the atmosphere, preventing it from escaping into space. (Other greenhouse gases include methane and water vapor.) Deforestation raises CO2 levels even more, since forests remove CO2 from the air and lock it up in their woody tissues.
Global Climate Change
While the chemistry and physics of the atmosphere are highly complex, and although there is still some debate about the pace and ultimate extent of global warming, most atmospheric scientists agree that the average temperature of the planet is likely to rise by at least a few degrees over the next several centuries. What portion of this effect is attributable to human activity is still under debate, although many scientists think the human contribution is very significant.
The scope of the possible effects of global warming is hard to forecast accurately, but some examples may provide a glimpse of potential outcomes. The current distribution of plant species is determined in large part by their climatic requirements. Boreal forest, or taiga, circles the Earth just below the arctic circle. Its coniferous trees require cold winters and mild summers, with moderate but not excessive rainfall. As surface temperatures rise, as much as 40 percent of the world's boreal forests could be lost, according to estimates published by the Intergovernmental Panel on Climate Change. At the same time, deserts and other arid lands may experience more water stress, increasing the rate of desertification in these regions. In contrast, some areas will become milder and wetter. All of these changes will shift plant geographic distribution, and with this, alter wildlife and plant predator distributions.
Changes in climate are likely to accelerate another trend, one already begun by humans in their global travels. The distribution of plants changes over time, but in most natural migrations, predators move along with the plant, providing checks on the potential for otherwise explosive growth into a new habitat. When humans deliberately introduce a foreign plant into a new habitat, however, the system of ecological controls is not often transplanted at the same time. In these situations, a new species may have a significant impact on local ecosystems , driving out indigenous species and altering balances in place for many years. Such has been the case, for instance, with purple loosestrife in eastern wetlands, melaleuca in the Everglades, and kudzu throughout the southern United States.
As temperature and rainfall patterns change, global climate change is likely to provoke the large-scale introduction of new species into ecosystems where they have never existed before. A significant unanswered question is whether these changes are likely to be slow, allowing time for species to migrate gradually and for communities to slowly adapt to new constellations of species, or whether change will come rapidly, causing extinction of some species too slow to migrate, and population explosions of others that outpace their predators in a new environment. The rate of climate change, as well as its extent, will have a significant impact on the characteristics of plant communities in the twenty-first century.
The Sixth Extinction
While species have become extinct at a small, steady rate throughout evolutionary time, the three- and-a-half-billion-year history of life has been punctuated by only five great extinction events, most caused by heavenly cataclysms, such as an asteroid colliding with Earth. We are now in the early stages of the sixth great extinction, but one with a difference—this cataclysm is entirely of human origins.
This wave of extinction is perhaps the most alarming, and most grim, of the impact humans have had on the biosphere. By some estimates, one in eight species of plants is on the edge of extinction, and most of these were not expected to survive into the twenty-first century. Similar predictions have been made for other life forms. Human activities have increased the extinction rate by a thousand-fold, so that for every new species created by evolution, one thousand become extinct through the effects of human activities. Expanding populations, pollution, and atmospheric ozone depletion all have played their part, but the most dramatic effect has come from the clearing of tropical rain forests for agricultural and lumber activities. Forest clearing in tropical areas destroys 86,000 acres of forest per day, and an area the size of Kansas every year. With this land go thousands of species, many of them never identified.
The loss of these species is significant for practical as well as bio-ethical reasons—the unique biochemistry of each species makes each a potential source for new drugs or raw materials with unique and valuable properties. Plants are especially valuable in this regard, since their inability to run away from predators has led to the evolution of many types of bioactive compounds , only now being discovered by plant prospectors and ethnobotanists . Destroying this inventory before even cataloging potentially throws away our future.
The enormity of human impacts on the biosphere—increasing global temperatures, decreasing biodiversity , higher populations—is sometimes enough to make one despair of changing anything. While it is true that the major outlines of the future are unlikely to be reversed in the next several decades, it is most definitely not true that inaction is the only sensible course. Many important steps have already been taken to steer a course toward a more sustainable environmental future. While political differences and short-sighted economic interests will continue to prevent the full range of international actions needed, heartening agreements are already in place to decrease ozone destruction, limit greenhouse gas emissions, and protect bio-diversity. The world's people and its political leaders are slowly understanding that the future health and prosperity of the human species depends critically on the health of the world's environments.
Despite these promising beginnings, a great deal remains to be done, and the doing of it will depend on the commitment and foresight of people like the readers of this book, who are willing to learn, get involved, and try to make a difference. In the twenty-first century, that commitment to make a difference may have the greatest impact of all.
see also Acid Rain; Agriculture, History of; Agriculture, Modern; Atmosphere and Plants; Biogeochemical Cycles; Bioremediation; Boreal Forest; Carbon Cycle; Deforestation; Ecology, Fire; Genetic Engineering; Global Warming; Grasslands; Green Revolution; Invasive Species; Rain Forests.
Eldridge, Niles. Life in the Balance: Humanity and the Biodiversity Crisis. Princeton, NJ: Princeton University Press, 1998.
Erlich, Paul R., and Anne H. Erlich. The Population Explosion. New York: Simon & Schuster, 1990.
Primack, Richard. A Primer of Conservation Biology. Sunderland, MA: Sinauer Associates, 2000.
Terborgh, John. Requiem for Nature. Washington, DC: Island Press, 1999.