1. Human Activities Can Alter Ecosystems
36.4 & 36.5

2. Over the past few centuries, many ecosystems have been affected by the rapidly growing human population's need for resources.
The effects of human activities are sometimes felt in only a small area.
Sometimes, though, the ecological impact is more widespread or even global.

3. Impact on Chemical Cycles
Human activities can affect chemical cycling by literally moving nutrients from one place to another.
On an even larger scale, some human activities can disrupt the processes within global chemical cycles.

4. Carbon Cycle Impacts
Deforestation, the clearing of forests for agriculture, lumber, and other uses, also affects the carbon cycle by eliminating plants that absorb carbon dioxide during photosynthesis.
Sometimes after being cut down, the trees are then burned, releasing more carbon dioxide.
Burning after deforestation in the tropics accounts for about 20 percent of the carbon dioxide added to the atmosphere by human activities.
Worldwide burning of fossil fuels accounts for most of the other 80 percent.

5. The process by which atmospheric gases trap heat is called the greenhouse effect.
As the levels of carbon dioxide and other "greenhouse gases" rise, the greenhouse effect becomes stronger, trapping more heat in the atmosphere and raising Earth's average temperature.
Such an overall rise in Earth's average temperature is called global warming.

6. Nitrogen Cycle Impacts
The high levels of nitrogen, often along with phosphates, feed the rapid growth of algae in these bodies of water, a condition called eutrophication.
As the algae die, the bacteria decomposing them can use up so much of the oxygen in the water that there is no longer enough to support other organisms.
Smokestacks and automobile exhaust pipes release certain nitrogen and sulfur compounds into the atmosphere.
Precipitation that carries this acid back to Earth's surface is called acid rain.

7. Water Cycle Impacts
One human activity that can impact the water cycle is deforestation.
A primary way that fresh water returns to the atmosphere is transpiration from dense tropical forests.
As a result, tropical deforestation greatly reduces the amount of water vapor added to the atmosphere.
This changes precipitation patterns and affects ecosystems.

8. Biological Magnification
As organisms take in nutrients and water from the environment, they may also take in pollutants.
Though some pollutants may be excreted, others accumulate in an organism's tissues.
The process by which pollutants become more concentrated in successive trophic levels of a food web is called biological magnification.

9. Damage to the Ozone Shield
Some pollution in the atmosphere affects a gas called ozone (O3) that has particular importance to living things.
The ozone layer, a region of the atmosphere between 17 and 25 kilometers above Earth's surface, contains concentrations of ozone that absorb ultraviolet radiation, shielding organisms from its damaging effects.
The consequences of ozone depletion for humans may include an increase in health problems such as skin cancer and cataracts, caused by more intense ultraviolet radiation reaching Earth's surface.
The radiation may also harm crops and other producers.

10. Why Diversity Matters
The number of species in an ecosystem is one aspect of biodiversity, a term that encompasses the variety of life on Earth.
The other aspects of biodiversity are the variety of ecosystems in the biosphere and the genetic variety among individuals within a species.
One basic reason biodiversity matters is that many of the species in an ecosystem are interconnected.
Species depend on community interactions for food, shelter, and other needs.
If a key species disappears, other species—and the health of the whole ecosystem—may be affected.

11. Threats to Biodiversity
Throughout Earth's history, species have become extinct—the last members of the population died—and the species no longer exists on the planet.
But there are signs that species are disappearing at a dramatic rate:
About 11 percent of the 9,040 known bird species in the world are endangered.
Of the approximately 20,000 known plant species in the United States, at least 680 species are endangered.
Conservation biologists estimate that about 20 percent of the known freshwater fishes in the world have either become extinct during historical times or are threatened.

12. What is causing these threats to biodiversity?
Habitat Destruction
Clearing land for these uses and for obtaining natural resources such as lumber, coal, and minerals may harm or even destroy natural communities. If the organisms that require that habitat do not adapt or move to a new area, they will not survive.
Introduced Species
Introduced (non-native) species often prey on native species or compete with them for resources.
Overexploitation
The practice of harvesting or hunting to such a degree that the small number of remaining individuals may not be able to sustain the population.

13. Conservation Biology Approaches
Focusing on Hot Spots
These hot spots are small geographic areas with high concentrations of species.
Biodiversity hot spots also tend to be hot spots of extinction.

14. Conservation Biology Approaches
Planning for a Sustainable Future
A zoned reserve includes areas of land that are relatively undisturbed by humans, surrounded by areas that are minimally impacted by humans called buffer zones.
Although humans live in the buffer zones, destructive environmental practices such as massive logging, large-scale single-crop agriculture, and extensive mining are discouraged.
The zoned reserves encourage long-term ecosystem conservation through a balance of human needs and habitat preservation.
Understanding an Organism's Habitat
Understanding the habitat requirements of a species can help biologists manage its existing habitat or create new habitat areas.
Balancing Demands for Resources
A tug of war exists between efforts to save species and the economic and social needs of people.