• These are feedback loops in which an increase in output leads to an increase in input.

 What is a negative feedback loop?

• These are feedback loops in which an increase in output leads to a decrease in input.

• Yes, although ecosystems are being displaced from a steady state on a regular basis by ecological disturbance. Ecosystems can change from one steady state to another, giving rise to the ecological concept of multiple stable states. A shift in the communities and ecosystems can occur if the shift is large enough and a new stable state is reached.

• Some ecologists think that ecosystems never reach equilibrium because of frequent disturbances. For example, a high surf zone (like on North Carolina's Outer Banks beaches) is a habitat in which disturbance is frequent. In such systems, equilibrium may never be reached.

 What is the Concept of Environmental Unity?

• The concept of Environmental Unity suggests that "every thing is hitched to everything else" in an ecosystem.

 What is Uniformatarianism?

• The concept of Uniformatarianism suggests that the history of the Earth can be found in fossils trapped in the layers of rocks, with the lowest layers containing the oldest rocks and fossils.

 What is exponential growth?

• Exponential growth is a constant rate of growth in which the number of organisms (or any other quantity, such as money) at any time period is a multiple of the number present in the previous time period. It is an example of positive feedback.

 What is doubling time?

• Doubling time is the time necessary for the quantity of whatever is being measured to double. For populations, this is normally measured in years.

• Residence time is the average amount of time it takes for materials in a pool of resources to cycle through the system.

• The biota are the living things on Earth.

• The biosphere is the surface layer of the Earth in which the biota can be found, including parts of the lithosphere (rocks), the hydrosphere (water) and atmosphere (air).

 What are extinction rates?

• Extinction rates measure the number of species (kinds of organisms) that have disappeared from Earth over time. The rates have increased as humans have become more abundant on Earth, suggesting that humans cause the extinction of species.

 What is the Gaia hypothesis?

• The Gaia hypothesis is the superorganism concept for Earth ecosystems, developed by James Lovelock, and named for the Greek Goddess of Earth, Gaia. This hypothesis suggests that there is a giant positive feedback loop in which some lifeforms (like plants that produce oxygen) promote the conditions for other lifeforms (like animals that consume it). (see Frequently Asked Questions in Chapter 1)

• No, because it is a worldview rather than a hypothesis. This worldview is comprised of individual hypotheses about positive feedback loops that are testable hypotheses.

• Have you been aware of ecosystem changes in your town or region?
• What are the suggested causes of the ecosystem changes?
• Are humans blamed for the change?
• Could the changes be due to natural, long-term changes in climate?
• Are ecological disturbances to blame for the changes?
• Please respond to these questions or send your thoughtful examples and comments to:

 The best responses will be posted on the Wiley Environet Website, so check the page regularly for updates to see if your email is posted!

Hardcopy Links In The Library

• Jones, P. and T. Wigley. 1990. Global Warming Trends. Scientific American. August 1990, pages 84-91.
• Repetto, R. Deforestation in the tropics. Scientific American. April, 1990, pages 36-42.
• Consequences: The Nature and Implications of Environmental Change. - This excellent journal contains many articles on how humans are transforming the Earth. It has only been in publication for a few years, but there about sixteen articles, all of which relate to the issues in this chapter and in later chapters. Your library may not get this series, but it is distributed free of charge, so tell your librarian. These articles are also available online on the internet (See Ecolinks section below) To obtain any of these articles, write or fax the editorial office of Consequences:

• Trends in U.S. Climate during the Twentieth Century
• America's Water Supply: Status and Prospects for the Future
• Past and Present Land Use and Land Cover in the USA.

• Global and U.S. National Population Trends
• Impacts of a Projected Depletion of the Ozone Layer
• Potential Impacts of Climate Change on Agriculture

• The Environment Since 1970
• Climate Models: How Reliable are their Predictions?

• Greenhouse lessons from the Geologic Record
• How Bountiful are Ocean Fisheries?
• The Sun and Climate

• Ending Hunger: Current Status and Future Prospects
• Impacts of Introduced Species in the United States
• Population Policy: Consensus and Challenges

• The Case of the Missing Songbirds
• Do We Still Need Nature?

Note: If any of these links are not working, please see if alternative links are available at the Ecolink Update Site .

Ecotest Online

1. A (An) ______________ is any interconnected set of components that acts as a whole.

a. ecosystem