The biosphere can be divided into large regions called biomes. Biomes are massive areas that are classified mostly on the basis of their climates and plant life. Because of the different climates and terrains on the earth, the distribution of living organisms varies.

TundraRegions—northernmost regionsPlant life—few, if any, trees; primarily grasses and wildflowers Characteristics—contains permafrost (a layer of permanently frozen soil); has a short growing seasonAnimal life—includes lemmings, arctic foxes, snowy owls, caribou, and rein deer

TaigaRegion—northern forestsPlant life—wind-blown conifers (evergreens), stunted in growth, possess modified spikes for leavesCharacteristics—very cold, long wintersAnimal life—includes caribou, wolves, moose, bear, rabbits, and lynxTemperate Deciduous ForestRegions—northeast and middle eastern United States, western Europe Plant life—deciduous trees that drop their leaves in winter Characteristics—moderate precipitation; warm summers, cold winters Animal life—includes deer, wolves, bear, small mammals, birds

Summary of Major Biomes

GrasslandsRegions—American Midwest, Eurasia, Africa, South AmericaPlant life—grassesCharacteristics—hot summers, cold winters; unpredictable rainfallAnimal life—includes prairie dogs, bison, foxes, ferrets, grouse, snakes, and lizards

DesertsRegions—western United StatesPlant life—sparse, includes cacti, drought-resistant plants Characteristics—arid, low rainfall; extreme diurnal temperature shifts Animal lift—includes jackrabbits (in North America), owls, kangaroo rats,lizards, snakes, tortoises

Tropical Rain ForestsRegions—South AmericaPlant life—high biomass; diverse typesCharacteristics—high rainfall and temperatures; impoverished soilAnimal life—includes sloths, snakes, monkeys, birds, leopards, and insects

COMMUNITY A community refers to a group of interacting plants and animals that show some degree of interdependence. All organisms within a community fill one of the following roles:

Producers, or autotrophs, have all of the raw building blocks to make their own food. From water and the gases that abound in the atmosphere, and with the aid of the sun's energy autotrophs convert light energy to chemical energy. They accomplish this through photosynthesis.

Consumers, or heterotrophs, are forced to find their energy sources in the outside world. Basically, heterotrophs digest the carbohydrates of their prey into carbon, hydrogen, and oxygen, and use these molecules to make organic substances.The bottom line is: Heterotrophs, or consumers, get their energy from the things they consume.

DecomposersAll organisms at some point must finally yield to decomposers. Decomposers are the organisms that break down organic matter into simple products. Generally, fungi and bacteria are the decomposers. They serve as the "garbage collectors" in our environment.

Niche

• An organism position or function in a community.

• Because every species oc cupies a niche, it's going to have an effect on all the other organisms.

• These connections are shown in the food chain.

Autotrophs produce all of the available food. They make up the first trophic (feeding) level. They possess the highest biomass (the total weight of all the organisms in an area) and the greatest numbers. Plants make up about 99 percent of the earth's total biomass.Primary consumers are organisms that directly feed on producers. A good example is a cow. These organisms are also known as herbivores. They make up the second trophic level.The next level consists of organisms that feed on primary consumers. They are the secondary consumers, and they make up the third trophic level. Above these are tertiary consumers.So now you have it. We've got our four complete levels of the food chain.Producers make their own food.Primary consumers (herbivores) eat producers.Secondary consumers (carnivores and omnivores) eat producers and primary con sumers.Tertiary consumers eat all of the above.

A food chain describes the way different organisms depend on one another for food. There are basically four levels to the food chain: producers, primary consumers, secondary consum ers, and tertiary consumers.

• In a food chain, only about 10 percent of the energy is transferred from one level to the next

• The other 90 percent is used for things like respiration, digestion, running away from predators—in other words, it's used to power the organism doing the eating!

• The producers have the most energy in an ecosystem; the primary consumers have less energy than producers; the secondary consumers have less energy than the primary consumers; and the tertiary consumers will have the least energy of all.

The 10% Rule

ECOLOGICAL SUCCESSION

•Communities of organisms don't just spring up on their own; they develop gradually over time. Eco logical succession refers to the predictable procession of plant communities over a relatively short period of time (decades or centuries).

•Centuries may not seem like a short time to us, but if you consider the enormous stretches of time over which evolution occurs, hundreds of thousands or even millions of years, you'll see that it is pretty short.

•The process of ecological succession where no previous organisms have existed is called primary succession.

• How does a new habitat full of bare rocks eventually turn into a forest?

• The first stage of the job usu ally falls to a community of lichens. Lichens are hardy organisms. They can invade an area, land on bare rocks and erode the rock surface, and over time turn it into soil. Lichens are considered pioneer organisms.

• Once lichens have made an area more habitable, they've set the stage for other organisms to settle in. Communities establish themselves in an orderly fashion. Lichens are replaced by mosses and ferns, which in turn are replaced by tough grasses, then low shrubs, then evergreen trees, and finally, a climax community is reached.

HUMAN IMPACT ON THE ENVIRONMENT

Unfortunately, humans have disturbed the existing ecological balance, and the results are far-reaching. Soils have been eroded and various forms of pollution have increased. The potential consequences on the environment are summarized below:

•Ozone depletion—Pollution has also led to the depletion of the atmospheric ozone layer by such chemicals as chlorofluorocarbons (CFCs), which are used in aerosol cans. Ozone (0,), forms when UV radiation reacts with 02. Ozone pro tects the earth's

surface from excessive ultraviolet radiation. Its loss could have major genetic effects and could increase the incidence of cancer.

•Desertification—When land is overgrazed by animals, it turns grasslands into deserts and reduces the available habitats for organisms.

•Deforestation—When forests are cleared (especially by the slash and burn meth od), erosion, floods, and changes in weather patterns can occur

•Greenhouse effect—The increasing atmospheric concentrations of carbon dioxide through the burning of fossil fuels and forests have contributed to the warm ing of the earth. Higher temperatures may cause the polar ice caps to melt and flooding to occur. Other potential effects of global warming include changes in precipitation patterns, changes in plant and animal populations, and detrimental changes in agriculture.

•Acid rain—The burning of fossil fuels produces pollutants such as sulfur dioxide and nitrogen dioxide. When these compounds react with droplets of atmospher ic water in clouds they form sulfuric and nitric acids, respectively. The rain that falls from these clouds is weakly acid and is called acid rain. Acid rain lowers the pHof aquatic ecosystems and soil which damages water systems, plants and soil. For example, the change in soil pH causes calcium and other nutrients to leach out, which damages plant roots and stunts their growth. Furthermore, useful microorganisms that release nutrients from decaying organic matter into the soil are also killed, resulting in less nutrients being available for the plants. Low pH also kills fish, especially those that have just hatched.