The Dominant Animal

Human Evolution and the Environment

Protected: Chapter 10. Ecosystems and Human Domination of Earth

Chapter Summary

Human society is now profoundly reshaping the biosphere, and the pace of change seems to be faster than at any time since the dinosaurs’ demise 65 million years ago. Ecosystems have always been subject to change, although it was not always recognized. It was once thought that all areas had a “climax community,” a condition to which the assemblage of plants and animals would return after disturbance, in a process known as succession. More recently, ecologists have understood that ecosystems are in a state of constant flux. Succession, a sequence of replacement primarily of plants, is a common phenomenon following any disturbance, but one climax may be followed by another sequence of succession, leading to a different climax as local climate changes or some other factor influences a new trend.

While many communities have appeared to last for centuries with little change, they are not unchanging. Now, under the impact of anthropogenic (human-generated) climate change as well as other human disturbances, change may be accelerating in many ecosystems. Human beings, in constructing their ecological niches, are reshaping the planet to suit their needs: changing the climate, the land surface, the ocean depths, the distribution of organisms, and the biosphere’s chemical composition. Human use of the land surface includes about 2 percent for habitation, roads, and the like; 12 percent for crops; 25 percent for livestock grazing; and exploitation one way or another of 30 percent more in forests or tree farms. The remaining 20 percent is mostly deserts, mountaintops, and other places with inhospitable climates. People also mobilize minerals at rates far above those of natural processes and are changing the composition of the atmosphere in potentially dangerous ways. A study in 1986 found that humanity had already destroyed or was co-opting almost half of the planet’s NPP, thus interfering in many ways with the biosphere’s present and future capacity to support the human population.

Ecosystems are important because they support human life; the economy is a wholly owned subsidiary of the ecosphere. Without ecosystem services such as the genetic variability of wild relatives of crops, pollination of crops, natural enemies of pests, and natural controls of the hydrologic cycle, there would be no economy. Ecosystem goods include timber from forests, fuelwood, seafood, and wild game and other non-cultivated foods, as well as natural ingredients for many pharmaceuticals. Scientists continue to find additional benefits to be drawn from natural capital (ecosystems)—”interest” in the form of goods and services.

Today ecosystem goods are increasingly being replaced by goods from human-managed systems such as tree plantations, fish farms, and feedlots, and by industrial products such as kerosene for fuelwood, synthetic fertilizers, and the like. Plainly, modern industry can more readily find substitutes for ecosystem goods than for services.

Freshwater supply and flood control are two such related ecosystem services. Heavily vegetated highland ecosystems help rainfall and snowmelt infiltrate into aquifers, and the groundwater slowly feeds rivers and streams. If the vegetation is lost through fire or logging, excess water runoff may cause flooding while reducing dry-season stream flow. The gradual depletion of the forest for agriculture in Rwanda’s Parc National des Volcans, home of the mountain gorilla, resulted in reduction of the stream flow needed to support agriculture elsewhere in the country. Similar problems have been seen in many areas as people convert natural ecosystems and divert water resources for their own uses. Wetlands are particularly rich ecosystems that people often convert for agriculture or habitation, thus losing their natural services of absorbing floodwaters and purifying waters. Some efforts are being made today to preserve or restore wetlands. One success story is the decision of New York City to protect its watershed in the Catskills, preserving the natural water purification service of the area at a modest cost, rather than building and operating an expensive water treatment plant.

Generation and maintenance of soils is a crucial ecosystem service without which agriculture would be impossible. The myriad organisms in the soil are indispensable for preserving soil fertility, which is essential for farming and forestry. Soils, however, can be damaged or impoverished by erosion or by the continual removal of crops (or forest products) without replenishment of nutrients through crop residues or fertilizers.

Ocean flood protection is an important service for coastal communities. Much of the damage caused by the 2004 tsunami in the Indian Ocean and Hurricane Katrina in 2005 might have been prevented if coastal mangroves in the Indian Ocean and coastal wetlands in Louisiana had been preserved.

Natural pest control is another underrated ecosystem service; an estimated 99 percent of potential crop pests are controlled by climate (cold or dry seasons) and natural enemies. Plant-eating insects have long evolutionary experience in dealing with poisons that plants synthesize to protect themselves, so they readily develop resistance to human-made pesticides. The poisons people use are limited by the risks of poisoning themselves. Biological pest control, however, can be a potent remedy, as was learned when rampant introduced ##Opuntia cactus was controlled in Australia by a natural enemy, a tiny moth, introduced from Argentina. Not all such efforts are successful, though: the cane toad, introduced in Queensland, Australia, to control cane beetles, became an even bigger pest itself.

Amelioration of the weather is an ecosystem service largely effected through moderation of the hydrologic cycle by land-based vegetation, which provides windbreaks and shade and recycles water, preventing drought. Transpiration of the Amazon forest recycles the region’s rainfall many times over; without the forest, the Amazon basin would be desertified.

Cycling of nutrients, with the help of decomposers in soil or water bodies, is a vital service that makes agriculture possible and at the same time provides a waste disposal service. And both nutrient cycling and the hydrologic cycle are key factors in regulating the gaseous composition of the atmosphere and thus the greenhouse effect.

Pollination of plants is another ecosystem service that is essential for agriculture as well as for natural vegetation. While pollination for some plants is carried out by wind, numerous plants depend on the services of animals such as bees, butterflies, moths, hummingbirds, and bats to reproduce.

Finally, entertainment and tranquility are services provided by natural ecosystems to millions of people who enjoy bird-watching, hiking, gardening, scuba diving and snorkeling, hunting and fishing, and seeing wild animals such as gorillas and elephants. Many just appreciate nature’s beauty and visit scenic places for vacations. Such activities are a large but often overlooked share of the global economy.

Ecosystem services thus are more than just amenities—they are absolutely essential underpinnings of the human economy, operating on a scale far greater than human substitutes could match.

A three-year study, the Millennium Ecosystem Assessment, published in 2005, was a collaborative effort by more than 1,300 scientists from ninety-five nations. Its objective was to assess the condition of world’s ecosystems and to project alternative future trends and related policy choices. Among the findings of the study:

  • Many ecosystem services have been lost as ecosystems have been turned to directly providing food, timber, fiber, and seafood to society, resulting in losses of habitat and biodiversity.
  • Fishery stocks have been seriously depleted and fish harvests are declining, although offset by fish farming.
  • Supplies of fresh water are inadequate in many areas, and the gap will continue widening unless patterns of water use are changed. About a fifth of the world’s population has no access to dependable supplies today. Climate change may worsen the situation through drought and desertification.
  • The capacity of ecosystems to detoxify pollutants, maintain nutrient balances, provide protection from natural disasters, and control pests, diseases, and invasive organisms is declining.

The effects of human activities on our life-support systems are summarized in the I = PAT identity, as the product of three factors. The overall Impact equals the product of Population size times per capita Affluence (consumption) times the Technologies and socioeconomic-political systems used to generate the affluence. The equation was developed by John Holdren and us to aid analysis and is useful for making comparisons between societies. Thus we can claim that the United States is more overpopulated than many poorer countries with large populations because of our high level of consumption. Scientists have warned for some time that with present and increasing levels of population size and consumption, humanity cannot be supported for long on our planet.

Key Terms

  • Aquifers
  • Bush meat
  • Chaparral
  • Climax community
  • Ecosystem goods
  • Ecosystem services
  • Fish farms
  • Habitat
  • I = PAT identity
  • Mangroves
  • Millennium Ecosystem Assessment
  • Nutrient cycling
  • Pyrethrum
  • Ramsar Convention on Wetlands
  • Serpentine soil
  • Soil ecosystem
  • Succession
  • Synthetic fertilizers
  • Tree farms
  • Tsunami
  • Wetlands

Discussion Questions

  1. In what main ways do organisms influence Earth’s climate?
  2. If you wanted to explain human impacts on the environment to a twelve-year-old, how would you summarize them?
  3. What does the term “ecosystem service” refer to, and what do you think are the most important such services?
  4. Describe the range of ecosystems in which you are embedded.