Community and Ecosystem Dynamic1

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COMMUNITY AND ECOSYSTEM DYNAMICS

Table of Contents

Definitions |Community Structure |Classification of Communities | CommunityDensity and Stability |

Change in Communities Over Time | Ecosystems and Communities |Links

Definitions | Back to Top

A community is the set of all populations that inhabit a certain area.

Communities can have different sizes and boundaries. These are often

identified with some difficulty.

An ecosystem is a higher level of organization the community plus its physicalenvironment. Ecosystems include both the biological and physical components

affecting the community/ecosystem. We can study ecosystems from a structural

view of population distribution or from a functional view ofenergy flow and

other processes.

Community Structure | Back to Top

Ecologists find that within a community many populations are not randomly

distributed. This recognition that there was a pattern and process of spatial

distribution of species was a major accomplishment of ecology. Two of themost important patterns are open community structure and the relative rarity of

species within a community.

Do species within a community have similar geographic range and density

peaks? If they do, the community is said to be a closed community, a discrete

unit with sharp boundaries known as ecotones. An open community, however,

has its populations without ecotones and distributed more or less randomly.

In a forest, where we find an open community structure, there is a gradient of

soil moisture. Plants have different tolerances to this gradient and occur atdifferent places along the continuum. Where the physical environment has

abrupt transitions, we find sharp boundaries developing between populations.

For example, an ecotone develops at a beach separating water and land.

Open structure provides some protection for the community. Lacking

boundaries, it is harder for a community to be destroyed in an all or nothing
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fashion. Species can come and go within communities over time, yet the

community as a whole persists. In general, communities are less fragile and

more flexible than some earlier concepts would suggest.

Most species in a community are far less abundant than the dominant species

that provide a community its name: for example oak-hickory, pine, etc.Populations of just a few species are dominant within a community, no matter

what community we examine. Resource partitioning is thought to be the main

cause for this distribution.

Classification of Communities | Back to Top

There are two basic categories of communities: terrestrial (land) and aquatic

(water). These two basic types of community contain eight smaller units known

asbiomes. A biome is a large-scale category containing many communities of a

similar nature, whose distribution is largely controlled by climate

Terrestrial Biomes: tundra, grassland, desert, taiga, temperate forest,

tropical forest. Terrestrial biome distribution is shown in Figure 1.

Aquatic Biomes: marine, freshwater.

Figure 1. Major terrestrial biomes.Image from Purves et al., Life: The Science

of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH

Freeman (www.whfreeman.com), used with permission.
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Terrestrial Biomes

Tundra and Desert

The tundra and desert biomes occupy the most extreme environments, with

little or no moisture and extremes of temperature acting as harsh selectiveagents on organisms that occupy these areas. These two biomes have the fewest

numbers of species due to the stringent environmental conditions. In other

words, not everyone can live there due to the specialized adaptations required

by the environment.

Tropical Rain Forests

Tropical rain forests occur in regions near the equator. The climate is always

warm (between 20 and 25 C) with plenty of rainfall (at least 190 cm/year).

The rain forest is probably the richest biome, both in diversity and in totalbiomass. The tropical rain forest has a complex structure, with many levels of

life. More than half of all terrestrial species live in this biome. While diversity

is high, dominance by a particular species is low. Typical tropical rain forest

views are shown in Figure 2.
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While some animals live on the ground, most rain forest animals live in the

trees. Many of these animals spend their entire life in the forest canopy. Insects

are so abundant in tropical rain forests that the majority have not yet been

identified. Charles Darwin noted the number of species found on a single tree,

and suggested the richness of the rain forest would stagger the future

systematist with the size of the catalogue of animal species found there.

Termites are critical in the decomposition and nutrient cycling of wood. Birds

tend to be brightly colored, often making them sought after as exotic pets.

Amphibians and reptiles are well represented. Lemurs, sloths, and monkeys

feed on fruits in tropical rain forest trees. The largest carnivores are the cats

(jaguars in South America and leopards in Africa and Asia). Encroachment and

destruction of habitat put all these animals and plants at risk.

Epiphytes are plants that grow on other plants. These epiphytes have their own

roots to absorb moisture and minerals, and use the other plant more as an aid to

grow taller. Some tropical forests in India, Southeast Asia, West Africa, Central

and South American are seasonal and have trees that shed leaves in dry season.

The warm, moist climate supports high productivity as well as rapid

decomposition of detritus.

With its yearlong growing season, tropical forests have a rapid cycling of

nutrients. Soils in tropical rain forests tend to have very little organic matter

since most of the organic carbon is tied up in the standingbiomass of the

plants. These tropical soils, termed laterites, make poor agricultural soils after

the forest has been cleared.

About 17 million hectares of rain forest are destroyed each year (an area equal

in size to Washington state). Estimates indicate the forests will be destroyed

(along with a great part of the Earth's diversity) within 100 years. Rainfall and

climate patterns could change as a result.

Figure 2.Top image: Costa Rican cloud forest. Image from the Botanical

Society of America website, https://reader009.{domain}/reader009/html5/0411/5acd8e81.Bottom image: Tropical rainforest in Puerto Rico. Image from the Botanical

Society of America website, https://reader009.{domain}/reader009/html5/0411/5acd8e81.

Temperate Forests
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The temperate forest biome occurs south of the taigain eastern North America,

eastern Asia, and much of Europe. Rainfall is abundant (30-80 inches/year; 75-

150 cm) and there is a well-defined growing season of between 140 and 300

days. The eastern United States and Canada are covered (or rather were once

covered) by this biome's natural vegetation, the eastern deciduous forest.

Dominant plants include beech, maple, oak; and

otherdeciduoushardwood trees. Trees of a deciduous forest have broad leaves,

which they lose in the fall and grow again in the spring. A scenic view of this

type of biome is shown in Figure 3.

Figure 3.Fall color in the eastern deciduous forest. Note the presence of a few

evergreens among the hardwoods. Image from the Botanical Society of America

website,https://reader009.{domain}/reader009/html5/0411/5acd8e819a21b/5acd8e856437.

Sufficient sunlight penetrates the canopy to support a well-developed

understory composed of shrubs, a layer ofherbaceous plants, and then often a

ground cover of mosses and ferns. This stratification beneath the canopy

provides a numerous habitats for a variety of insects and birds. The deciduous

forest also contains many members of the rodent family, which serve as a food

source for bobcats, wolves, and foxes. This area also is a home for deer and

black bears. Winters are not as cold as in the taiga, so many amphibian and

reptiles are able to survive.

Shrubland (Chaparral)

The shrubland biome is dominated by shrubs with small but thick evergreen

leaves that are often coated with a thick, waxy cuticle, and with thick

underground stems that survive the dry summers and frequent fires. Shrublands

occur in parts of South America, western Australia, central Chile, and around

the Mediterranean Sea. Dense shrubland in California, where the summers are

hot and very dry, is known as chaparral, shown in Figure 4. This

Mediterranean-type shrubland lacks an understory and ground litter, and is also

highly flammable. The seeds of many species require the heat and scarring

action of fire to induce germination.

Figure 4.Chaparral vegetation (predominantly Adenostema) in California.

Image from the Botanical Society of America

website, https://reader009.{domain}/reader009/html5/0411/5acd8e819a21b/5acd8e85a88.
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Grasslands

Grasslands occur in temperate and tropical areas with reduced rainfall (10-30

inches per year) or prolonged dry seasons. Grasslands occur in the Americas,

Africa, Asia, and Australia. Soils in this region are deep and rich and are

excellent for agriculture. Grasslands are almost entirely devoid of trees, and cansupport large herds of grazing animals. Natural grasslands once covered over

40 percent of the earth's land surface. In temperate areas where rainfall is

between 10 and 30 inches a year, grassland is the climax community because it

is too wet for desert and too dry for forests.

Most grasslands have now been utilized to grow crops, especially wheat and

corn. Grasses are the dominant plants, while grazing and burrowing species are

the dominant animals. The extensive root systems of grasses allows them to

recover quickly from grazing, flooding, drought, and sometimes fire.

Temperate grasslands include the Russian steppes, the South American

pampas, and North American prairies. A tall-grass prairie occurs where

moisture is not quite sufficient to support trees. A short-grass-prairie, shown in

Figure 5, survives on less moisture and occurs between a tall-grass prairie and

desert. A desert grassland is shown in Figure 6.

Figure 5.Short grass prairie, Nebraska. Image from the Botanical Society of

America website, https://reader009.{domain}/reader009/html5/0411/5acd8e819a21b/5acd.

Figure 6. A desert grassland in southeastern Arizona. Image from the Botanical

Society of America website, https://reader009.{domain}/reader009/html5/0411/5acd8e81.

Animal life includes mice, prairie dogs, rabbits, and animals that feed on them

(hawks and snakes). Prairies once contained large herds of buffalo and

pronghorn antelope, but with human activity these once great herds ahve

dwindled.

The savanna is a tropical grassland that contains some trees. The savanna

contains the greatest variety and numbers of herbivores (antelopes, zebras, and

wildebeests, among others). This environment supports a large population of

carnivores (lions, cheetahs, hyenas, and leopards). Any plant litter not

consumed by grazers is attacked by termites and other decomposers. Once
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again, human activities are threatening this biome, reducing the range for

herbivores and carnivores. Will extinction of the great cats be a result?

Deserts

Deserts are characterized by dry conditions (usually less than 10 inches peryear; 25 cm) and a wide temperature range. The dry air leads to wide daily

temperature fluctuations from freezing at night to over 120 degrees during the

day. Most deserts occur at latitudes of 30o N or Swhere descending air masses

are dry. Some deserts occur in the rainshadow of tall mountain ranges or in

coastal areas near cold offshore currents. Plants in this biome have developed a

series of adaptations (such as succulent stems, and small, spiny, or absent

leaves) to conserve water and deal with these temperature extremes.

Photosynthetic modifications (CAM) are another strategy to life in the

drylands.

The Sahara and a few other deserts have almost no vegetation.Most deserts,

however, are home to a variety of plants, all adapted to heat and lack of

abundant water (succulents and cacti).A view of the Sonoran desert vegetation

type in Arizona is shown by Figure 7. Animal life of the Sonoran desert

includes arthropods (especially insects and spiders), reptiles (lizards and

snakes), runningbirds (the roadrunner of the American southwest and Warner

Brothers cartoon fame), rodents (kangaroo rat and pack rat), and a few larger

birds and mammals (hawks, owls, and coyotes).

Figure 7.Saguaro and cholla cacti in association with palo verde trees in the

Sonoran desert, AZ. Note the lack of a canopy and the scarcity of ground cover.

Image from the Botanical Society of America

website, https://reader009.{domain}/reader009/html5/0411/5acd8e819a21b/5acd8e87d31.

Taiga (Boreal Forest)

The taiga (pronounced "tie-guh" and shown in Figure 8) is a coniferous forest

extending across most of the northern area of northern Eurasia and NorthAmerica. This forest belt also occurs in a few other areas, where it has different

names: the montane coniferous forest when near mountain tops; and the

temperate rain forest along the Pacific Coast as far south as California. The

taiga receives between 10 and 40 inches of rain per year and has a short

growing season. Winters are cold and short, while summers tend to be cool.

The taiga is noted for its great stands of spruce, fir, hemlock, and pine. These
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trees have thick protective leaves and bark, as well as needlelike (evergreen)

leaves can withstand the weight of accumulated snow. Taiga forests have a

limited understory of plants, and a forest floor covered by low-lying mosses

and lichens. Conifers, alders, birch and willow are common plants; wolves,

grizzly bears, moose, and caribou are common animals. Dominance of a few

species is pronounced, but diversity is low when compared to temperate and

tropical biomes.

Figure 8. Top image: Taiga, Glacial River in Alaska. Image from the Botanical

Society of America website, https://reader009.{domain}/reader009/html5/0411/5acd8e81;Middle image of a Larix-dominated area of the taiga biome. Image from the

Botanical Society of America website, http://images.botany.org/bsa/set-01/01-

027v.jpg; Bottom image: Temperate rain forest, Washington. Note the dense

understory of ferns and herbaceous plants.Image from the Botanical Society of

America website, https://reader009.{domain}/reader009/html5/0411/5acd8e819a21b/5acd.

Tundra

The tundra, shown in Figure 9, covers the northernmost regions of North

America and Eurasia, about 20% of the Earth's land area. This biome receives

about 20 cm (8-10 inches) of rainfall annually. Snow melt makes water

plentiful during summer months. Winters are long and dark, followed by very

short summers. Water is frozen most of the time, producing frozen soil,

permafrost. Vegetation includes no trees, but rather patches of grass and

shrubs; grazing musk ox, reindeer, and caribou exist along with wolves, lynx,

and rodents. A few animals highly adapted to cold live in the tundra year-round

(lemming, ptarmigan). During the summer the tundra hosts numerous insects

and migratory animals. The ground is nearly completely covered with sedges

and short grasses during the short summer. There are also plenty of patches of

lichens and mosses. Dwarf woody shrubs flower and produce seeds quicklyduring the short growing season. The alpine tundra occurs above the timberline

on mountain ranges, and may contain many of the same plants as the arctic

tundra.

Figure 9.Top image: View of the tundra, locality unknown. Image

from http://ths.sps.lane.edu/biomes/tundra3/tundra3.html. Bottom image:
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Caribou, an animal characteristic of the tundra.Image

from http://ths.sps.lane.edu/biomes/tundra4/tundra4a.html.

Climate, Altitude and Terrestrial Biomes

Climate controls biome distribution by an altitudinal gradient and a latitudinal

gradient. With increases of either altitude or latitude, cooler and drier

conditions occur. Cooler conditions can cause aridity since cooler air can hold

less water vapor than can warmer air. This is shown by Figure 10.

Figure 10. Effect of temperature on precipitation. Image from Purves et

al., Life: The Science of Biology, 4th Edition, by Sinauer Associates

(www.sinauer.com) and WH Freeman (www.whfreeman.com), used with

permission.
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Deserts can occur in warm areas due to a blockage of air circulation patterns

that form a rain shadow, or from atmospheric circulation patters as shown in

Figure 11. Warm air rises, producing low pressure areas. Cooler air sinks,

producing high pressure areas. The tropics tend to be atmospheric low pressure

zones the arctic areas atmospheric highs. Relative humidity is a measure of

how much water an air mass at a given temperature can hold. In short, warm air

can hold more moisture than can cold air. This basic physical feature of air

helps explain the distribution of some of the world's great deserts.

The warm, moist air masses in the tropics rise upward in the atmosphere as

they heat. The pressure of air rising forces air in the upper atmosphere to flow

away north and south. This air at higher elevations is cooler and loses much of

its moisture as rainfall. When the air masses begin to descend they heat up and

begin to draw moisture from the lands they descend upon, at 30 degrees north

and south of the equator. Many of the world's deserts are at approximately 30

degrees latitude, as shown in Figure 11..

Rain shadow deserts also form when cool, dry air masses descend after passing

over a tall mountain range, such as the Coast Range and Sierras in California.

The Sonoran desert in Arizona (shown in Figure 7) is a doubly caused desert,

being at 30 degrees latitude as well as in the rain shadow of California

mountains. The Tian Shan desert in China is a typical rain shadow desert, as

shown by Figure 12.


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Figure 11. Top image: Air circulation patterns and the global distribution of wet

and dry areas.Image from Purves et al., Life: The Science of Biology, 4th

Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman

(www.whfreeman.com), used with permission. Bottom image: Rainshadows

and deserts. Image from Purves et al., Life: The Science of Biology, 4th Edition,

by Sinauer Associates (www.sinauer.com) and WH Freeman

(www.whfreeman.com), used with permission.
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Figure 12. The Turpan Depression in the Tian Shan desert of China, as viewed

from space. In this iamge vegetation is red, and basre desert is grey-light blue.

Image fromhttp://pubs.usgs.gov/gip/deserts/types/.

Aquatic Biomes

Conditions in water are generally less harsh than those on land. Aquatic

organisms are buoyed by water support, and do not usually have to deal with

desiccation. Despite covering 71% of the Earth's surface, areas of the open

ocean are a vast aquatic desert containing few nutrients and very little life, as

shown by Figure 13. . Clearcut biome distinctions in water, like those on land,

are difficult to make. Dissolved nutrients controls many local aquatic

distributions. Aquatic communities are classified into: freshwater (inland)communities and marine (saltwater or oceanic) communities.

Figure 13. Species diversity and salt concentration.Image from Purves et



permission.
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The Marine Biome

The marine biome contains more dissolved minerals than the freshwater biome.

Over 70% of the Earth's surface is covered in water, by far the vast majority of

that being saltwater. There are two basic categories to this

biome:benthic andpelagic. Benthic communities (bottom dwellers) are

subdivided by depth: the shore/shelf and deep sea. Pelagic communities

(swimmers or floaters suspended in the water column)

includeplanktonic(floating) and nektonic (swimming) organisms. The upper

200 meters of the water column is the euphotic zoneto which light canpenetrate.

Coastal Communities

Estuaries are bays where rivers empty into the sea. Erosion brings down

nutrients and tides wash in salt water; forms nutrient trap. Estuaries have high

production for organisms that can tolerate changing salinity. Such organisms

are shown in Figure 14. Estuaries are called "nurseries of the sea" because

many young marine fish develop in this protected environment before moving

as adults into the wide open seas.

Figure 14. Brackish water coastal marsh.Image from the Botanical Society of

America website https://reader009.{domain}/reader009/html5/0411/5acd8e819a21b/5acd.

Seashores
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Rocky shorelines offer anchorage for sessile organisms. Seaweeds are main

photosynthesizers and use holdfasts to anchor. Barnacles glue themselves to

stone. Oysters and mussels attach themselves by threads. Limpets and

periwinkles either hide in crevices or fasten flat to rocks.

Sandy beaches and shores are shifting strata. Permanent residents thereforeburrow underground. Worms live permanently in tubes. Amphipods and ghost

crabs burrow above high tide and feed at night.

Coral Reefs

Areas of biological abundance in shallow, warm tropical waters. Stony corals

have calcium carbonate exoskeleton and may include algae. Most form

colonies; may associate with zooxanthellae dinoflagellates. Reef is densely

populated with animal life. The Great Barrier Reef of Australia suffers from

heavy predation by crown-of-thorns sea star, perhaps because humans haveharvested its predator, the giant triton.

Oceans

Oceans cover about three-quarters of the Earth's surface. Oceanic organisms are

placed in either pelagic (open water) or benthic (ocean floor) categories, ash

shown in Figure 15. Pelagic division is divided into neritic and three levels of

pelagic provinces. Neritic province has greater concentration of organisms

because sunlight penetrates; nutrients are found here. Epipelagic zone is

brightly lit, has much photosynthetic phytoplankton, that support zooplanktonthat are food for fish, squid, dolphins, and whales. Mesopelagic zone is semi-

dark and contains carnivores; adapted organisms tend to be translucent, red

colored, or luminescent; for example: shrimps, squids, lantern and hatchet

fishes. The bathypelagic zone is completely dark and largest in size; it has

strange-looking fish. Benthic division includes organisms on continental shelf

(sublittoral), continental slope (bathyal), and the abyssal plain.

Figure 15.Zones within the marine biome. Image from Purves et al., Life: The

Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and

WH Freeman (www.whfreeman.com), used with permission.
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Sublittoral zone harbors seaweed that becomes sparse where deeper; most

dependent on slow rain of plankton and detritus from sunlit water above.

Bathyal zone continues with thinning of sublittoral organisms. Abyssal zone is

mainly animals at soil-water interface of dark abyssal plain; in spite of high

pressure, darkness and coldness, many invertebrates thrive here among sea

urchins and tubeworms.

Thermal vents along oceanic ridges form a very unique community. Molten

magma heats seawater to 350oC, reacting with sulfate to form hydrogen sulfide

(H2S). Chemosynthetic bacteria obtain energy by oxidizing hydrogen sulfide.

The resulting food chain supports a community of tubeworms and clams.

The Freshwater Biome

The freshwater biome is subdivided into two zones: running waters andstanding waters. Larger bodies of freshwater are less prone to stratification

(where oxygen decreases with depth). The upper layers have abundant oxygen,

the lowermost layers are oxygen-poor. Mixing between upper and lower layers

in a pond or lake occurs during seasonal changes known as spring and fall

overturn.


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Lakes are larger than ponds, and are stratified in summer and winter, as shown

in Figure 16. The epilimnion is the upper surface layer. It is warm in summer.

The hypolimnion is the cold lower layer. A sudden drop in temperature occurs

at the middle of the thermocline. Layering prevents mixing between the lower

hypolimnion (rich in nutrients) and the upper epilimnion (which has oxygen

absorbed from its surface). The epilimnion warms in spring and cools in fall,

causing a temporary mixing. As a consequence, phytoplankton become more

abundant due to the increased amounts of nutrients.

Figure 16. Lake overturn.Images from Purves et al., Life: The Science of

Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH

Freeman (www.whfreeman.com), used with permission.


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Life zones also exist in lakes and ponds. The littoral zone is closest to shore.

The limnetic zone is the sunlit body of the lake. Below the level of sunlight

penetration is the dark profundal zone. At the soil-water interface we find the

benthic zone. The term benthos is applied to animals and other organisms that

live on or in the benthic zone.

Rapidly flowing, bubbling streams have insects and fish adapted to oxygen-richwater. Slow moving streams have aquatic life more similar to lake and pond

life.

Community Density and Stability | Back to Top

Communities are made up of species adapted to the conditions of that

community. Diversity and stability help define a community and are important

in environmental studies. Species diversity decreases as we move away from

the tropics. Species diversity is a measure of the different types of organisms in

a community (also referred to as species richness).Latitudinal diversitygradient refers to species richness decreasing steadily going away from the

equator. A hectare of tropical rain forest contains 40-100 tree species, while a

hectare of temperate zone forest contains 10-30 tree species. In marked

contrast, a hectare of taiga contains only a paltry 1-5 species! Habitat

destruction in tropical countries will cause many more extinctions per hectare

than it would in higher latitudes.
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Environmental stability is greater in tropical areas, where a relatively

stable/constant environment allows more different kinds of species to thrive.

Equatorial communities are older because they have been less disturbed by

glaciers and other climate changes, allowing time for new species to evolve.

Equatorial areas also have a longer growing season.

The depth diversity gradient is found in aquatic communities. Increasing

species richness with increasing water depth. This gradient is established by

environmental stability and the increasing availability of nutrients.

Community stability refers to the ability of communities to remain unchanged

over time.During the 1950s and 1960s, stability was equated to diversity:

diverse communities were also stable communities.Mathematical modeling

during the 1970s showed that increased diversity can actually increase

interdependence among species and lead to a cascade effect when a keystone

species is removed. Thus, the relation is more complex than previously thought.

Change in Communities Over Time | Back to Top

Biological communities, like the organisms that comprise them, can and do

change over time.Ecological time focuses on community events that occur

over decades or centuries.Geological time focuses on events lasting thousands

of years or more.

Community succession is the sequential replacement of species by immigration

of new species and local extinction of older ones following a disturbance thatcreates unoccupied habitats for colonization.The initial rapid colonizer species

are the pioneer community. Eventually a climax community of more or less

stable but slower growing species eventually develops.

During successionproductivity declinesand diversity increases. These trends

tend to increase the biomass (total weight of living tissue) in a

community.Succession occurs because each community stage prepares the

environment for the stage following it.

Primary succession begins with bare rock and takes a very long time to occur.Weathering by wind and rain plus the actions of pioneer species such as lichens

and mosses begin the buildup of soil. Herbaceous plants, including the grasses,

grow on deeper soil and shade out shorter pioneer species. Pine trees or

deciduous trees eventually take root and in most biomes will form a climax

community of plants that are stabile in the environment. The young produced
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by climax species can live in that environment, unlike the young produced by

successional species.

Secondary succession occurs when an environment has been disturbed, such as

by fire, geological activity, or human intervention (farming or deforestation in

most cases). This form of succession often begins in an abandoned field withsoil layers already in place. Compared to primary succession, which must take

long periods of time to build or accumulate soil, secondary succession occurs

rapidly. The herbaceous pioneering plants give way to pines, which in turn may

give way to a hardwood deciduous forest (in the classical old field succession

models developed in the eastern deciduous forest biome).

Early researchers assumed climax communities were determined for each

environment. Today we recognize the outcome of competition among whatever

species are present as establishing the climax community.

Climax communities tend to be more stable than successional communities.

Early stages of succession show the most growth and are most productive.

Pioneer communities lack diversity, make poor use of inputs, and lose heat and

nutrients. As succession proceeds, species variety increases and nutrients are

recycled more. Climax communities make fuller use of inputs and maintain

themselves, thus, they are more stable. Human activity (such as clearing a

climax forest community to establish a farm field consisting of a cultivated

pioneering species, say corn or wheat) replaces climax communities with

simpler communities.

Communities are composed of species that evolve, so the community must also

evolve.Comparing marine communities of 500 million years ago with modern

communities shows modern communities composed of quite different

organisms.Modern communities also tend to be more complex, although this

may be a reflection of the nature of the fossil record as well as differences

between biological and fossil species.

Disturbance of a Community

The basic effect of human activity on communities is communitysimplification, an overall reduction of species diversity. Agriculture is a

purposeful human intervention in which we create a monoculture of a single

favored (crop) species such as corn. Most of the agricultural species are derived

from pioneering communities.
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Inadvertent human intervention can simplify communities and produce stressed

communities that have fewer species as well as a superabundance of some

species.Disturbances favor early successional (pioneer) species that can grow

and reproduce rapidly.

Ecosystems and Communities | Back to Top

Ecosystems include both living and nonliving components. These living, or

biotic, components include habitats and niches occupied by organisms.

Nonliving, or abiotic, components include soil, water, light, inorganic nutrients,

and weather. An organism's place of residence, where it can be found, is its

habitat. A niche is is often viewed as the role of that organism in the

community, factors limiting its life, and how it acquires food.

Producers, a major niche in all ecosystems, are autotrophic, usually

photosynthetic, organisms. In terrestrial ecosystems, producers are usuallygreen plants. Freshwater and marine ecosystems frequently have algae as the

dominant producers.

Consumers are heterotrophic organisms that eat food produced by another

organism. Herbivores are a type of consumer that feeds directly on green plants

(or another type of autotroph). Since herbivores take their food directly from

the producer level, we refer to them as primary consumers. Carnivores feed on

other animals (or another type of consumer) and are secondary or tertiary

consumers. Omnivores, the feeding method used by humans, feed on both

plants and animals. Decomposers are organisms, mostly bacteria and fungi thatrecycle nutrients from decaying organic material. Decomposers break down

detritus, nonliving organic matter, into inorganic matter. Small soil organisms

are critical in helping bacteria and fungi shred leaf litter and form rich soil.

Even if communities do differ in structure, they have some common uniting

processes such as energy flow and matter cycling, shown in Figure 17.Energy

flows move through feeding relationships.The term ecological niche refers to

how an organism functions in an ecosystem.Food webs, food chains, and food

pyramidsare three ways of representing energy flow.

Producers absorb solar energy and convert it to chemical bonds from inorganic

nutrients taken from environment. Energy content of organic food passes up

food chain; eventually all energy is lost as heat, therefore requiring continual

input. Original inorganic elements are mostly returned to soil and producers;

can be used again by producers and no new input is required.
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Figure 17. The flow of energy through an ecosystem.Image from Purves et



permission.

Energy flow in ecosystems, as with all other energy, must follow the two laws

of thermodynamics. Recall that the first law states that energy is neither created

nor destroyed, but instead changes from one form to another (potential to

kinetic). The second law mandates that when energy is transformed from one

form to another, some usable energy is lost as heat. Thus, in any food chain,

some energy must be lost as we move up the chain.

The ultimate source of energy for nearly all life is the Sun. Recently, scientists

discovered an exception to this once unchallenged truism: communities of

organisms around ocean vents where food chain begins with chemosynthetic

bacteria that oxidize hydrogen sulfide generated by inorganic chemical

reactions inside the Earth's crust. In this special case, the source of energy is the

internal heat engine of the Earth.

Food chains indicate who eats whom in an ecosystem. Represent one path of

energy flow through an ecosystem. Natural ecosystems have numerous

interconnected food chains. Each level of producer and consumers is a trophic

level. Some primary consumers feed on plants and make grazing food chains;

others feed on detritus.


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The population size in an undisturbed ecosystem is limited by the food supply,

competition, predation, and parasitism. Food webs help determine

consequences of perturbations: if titmice and vireos fed on beetles and

earthworms, insecticides that killed beetleswould increase competition

between birds and probably increase predation of earthworms, etc.

The trophic structure of an ecosystem forms an ecological pyramid. The base of

this pyramid represents the producer trophic level. At the apex is the highest

level consumer, the top predator. Other pyramids can be recognized in an

ecosystem. A pyramid of numbers is based on how many organisms occupy

each trophic level. The pyramid of biomass is calculated by multiplying the

average weight for organisms times the number of organisms at each trophic

level. An energy pyramid illustrates the amounts of energy available at each

successive trophic level.The energy pyramid always shows a decrease moving

up trophic levels because:

Only a certain amount of food is captured and eaten by organisms on the

next trophic level.

Some of food that is eaten cannot be digested and exits digestive tract as

undigested waste.

Only a portion of digested food becomes part of the organism's body;

rest is used as source of energy.

Substantial portion of food energy goes to build up temporary ATP in

mitochondria that is then used to synthesizeproteins, lipids,

carbohydrates, fuel contraction of muscles, nerve conduction, and other

functions.

Only about 10% of the energy available at a particular trophic level is

incorporated into tissues at the next level. Thus, a larger population

canbe sustained by eating grain than by eating grain-fed animals since

100 kg of grain would result in 10 human kg but if fed to cattle, the

result, by the time that reaches the human is a paltry1 human kg!

A food chain is a series of organisms each feeding on the one preceding

it.There are two types of food chain: decomposer and grazer.Grazer food

chains begin with algae and plants and end in a carnivore.Decomposer chainsare composed of waste and decomposing organisms such as fungi and bacteria.

This is shown in Figure 18.

Figure 18.Energy flow and the relative porportions of various levels in the food

chain. Images from Purves et al., Life: The Science of Biology, 4th Edition, by

Sinauer Associates (www.sinauer.com) and WH Freeman
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(www.whfreeman.com), used with permission.

Food chains are simplifications of complex relationships.A food web is a more

realistic and accurate depiction of energy flow. Food webs are networks of

feeding interactions among species.

The food pyramid provides a detailed view of energy flow in an

ecosystem.The first level consists of the producers (usually plants).All higher

levels are consumers. The shorter the food chain the more energy is available to

organisms.

Most humans occupy a top carnivore role, about 2% of all calories available

from producers ever reach the tissues of top carnivores.Leakage of energy

occurs between each feeding level.Most natural ecosystems therefore do not

have more than five levels to their food pyramids.Large carnivores are rare

because there is so little energy available to them atop the pyramid.

Food generation by producers varies greatly between ecosystems.Net primary

productivity (NPP) is the rate at which producer biomass is formed. Tropical

forests and swamps are the most productive terrestrial ecosystems.Reefs and

estuaries are the most productive aquatic ecosystems.All of these productive

areas are in danger from human activity.Humans redirect nearly 40% of the net
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primary productivity and directly or indirectly use nearly 40% of all the land

food pyramid.This energy is not available to natural populations.

Learning Objectives| Back to Top

Be able to describe the major terrestrial biomes and the types of plants

and animals occuring there.

Relate the efect of increasing altitude as one goes up a mountain to

biome changes sen as one moves north of the equator toward the polar

regions.

Distinguish the different regions within the marine ecosystems.

Be able to describe a food chain in detail, with some indication of the

relative porportions of organisms at each trophic level.

Terms| Back to Top

benthic biomass biomes carnivoresclimax

community

closed

community

communitycommunity

simplification

community

stability

community

successionconsumers

depth

diversity

gradient

desertecological

timeecosystem ecotones energy flow epilimnion

euphotic

zone

freshwater

biome

Food

webs, foodchains,

and food

pyramids

grasslands herbivores hypolimnion

latitudinal

diversity

gradient

limnetic zone littoral zonemarine

biome

matter

cyclingnektonic

net primary

productivity

(NPP)

niches omnivoresopen

communitypelagic planktonic

primary

succession

profundal

zoneproducers

rain shadow

deserts

secondary

succession

shrubland

biome

species

diversitytaiga

temperate

forest biome

tropical rain

foreststundra

Links | Back to Top
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookcommecosys.html#Table%20of%20Contentshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookcommecosys.html#Table%20of%20Contentshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossB.html#benthic%20zonehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossB.html#biomasshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossB.html#biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#climax%20communityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#climax%20communityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#closed%20communityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#closed%20communityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#communityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#community%20simplificationhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#community%20simplificationhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#community%20successionhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#community%20successionhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossD.html#depth%20diversity%20gradienthttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossD.html#depth%20diversity%20gradienthttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossD.html#depth%20diversity%20gradienthttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossD.html#desert%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossE.html#ecological%20timehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossE.html#ecological%20timehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossE.html#ecosystemhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossE.html#ecotoneshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossE.html#energy%20flowhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossE.html#euphotic%20zonehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossE.html#euphotic%20zonehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#freshwater%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#freshwater%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#food%20webhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#food%20webhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#food%20chainhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#food%20chainhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#food%20pyramidhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#food%20pyramidhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossG.html#grasslands%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossL.html#latitudinal%20diversity%20gradienthttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossL.html#latitudinal%20diversity%20gradienthttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossL.html#latitudinal%20diversity%20gradienthttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossM.html#marine%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossM.html#marine%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossM.html#matter%20cyclinghttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossM.html#matter%20cyclinghttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossN.html#nektonic%20organismshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossN.html#net%20primary%20productivity%20(NPP)http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossN.html#net%20primary%20productivity%20(NPP)http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossN.html#net%20primary%20productivity%20(NPP)http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossN.html#nichehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossO.html#open%20communityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossO.html#open%20communityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossPQ.html#pelagic%20zonehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossPQ.html#planktonic%20organismshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossS.html#species%20diversityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossS.html#species%20diversityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossT.html#taiga%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossT.html#temperate%20forest%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossT.html#temperate%20forest%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossT.html#tropical%20rain%20forest%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossT.html#tundra%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossT.html#tropical%20rain%20forest%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossT.html#tundra%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookcommecosys.html#Table%20of%20Contentshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookcommecosys.html#Table%20of%20Contentshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookcommecosys.html#Table%20of%20Contentshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossB.html#benthic%20zonehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossB.html#biomasshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossB.html#biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#climax%20communityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#climax%20communityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#closed%20communityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#closed%20communityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#communityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#community%20simplificationhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#community%20simplificationhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#community%20successionhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossC.html#community%20successionhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossD.html#depth%20diversity%20gradienthttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossD.html#depth%20diversity%20gradienthttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossD.html#depth%20diversity%20gradienthttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossD.html#desert%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossE.html#ecological%20timehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossE.html#ecological%20timehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossE.html#ecosystemhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossE.html#ecotoneshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossE.html#energy%20flowhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossE.html#euphotic%20zonehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossE.html#euphotic%20zonehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#freshwater%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#freshwater%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#food%20webhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#food%20webhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#food%20chainhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#food%20chainhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#food%20pyramidhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossF.html#food%20pyramidhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossG.html#grasslands%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossL.html#latitudinal%20diversity%20gradienthttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossL.html#latitudinal%20diversity%20gradienthttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossL.html#latitudinal%20diversity%20gradienthttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossM.html#marine%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossM.html#marine%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossM.html#matter%20cyclinghttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossM.html#matter%20cyclinghttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossN.html#nektonic%20organismshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossN.html#net%20primary%20productivity%20(NPP)http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossN.html#net%20primary%20productivity%20(NPP)http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossN.html#net%20primary%20productivity%20(NPP)http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossN.html#nichehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossO.html#open%20communityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossO.html#open%20communityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossPQ.html#pelagic%20zonehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossPQ.html#planktonic%20organismshttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossS.html#species%20diversityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossS.html#species%20diversityhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossT.html#taiga%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossT.html#temperate%20forest%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossT.html#temperate%20forest%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossT.html#tropical%20rain%20forest%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossT.html#tropical%20rain%20forest%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossT.html#tundra%20biomehttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookcommecosys.html#Table%20of%20Contents


25/25

The Rain Forest Report Card Maps, images, morphed movies showing the effects

of deforestation, and more make this a site to see for further information about the

rain forests and their plight.

Manu: Peru's Hidden Rain Forest PBS documentary, part of the Living Edens

series. Links to animals, plants, and people of this area. Quite a nice resource, as

are many of the PBSwebsites! Population Ecology This site, maintained by Alexi Sharov of the Department of

Entomology at Virginia Tech provides a great start to the study of population

ecology. Links to people, organizations, online lectures, and other items ofinterest are provided.

Planet Earth - a suite of interactive learning activities on ecology Aimed at high

school students and teachers this site offers a series of great activities that will

allow application of the concepts learned to real world problems, such asthe Wolves of Yellowstone.

Types of Deserts This part of a larger U.S. Geological Surveypageprovides

additional details on the types of deserts and related features.

All text contents 1995, 2000, 2001, 2002, 2007, by M.J. Farabee. Use of the text for

educational purposes is encouraged.

Back to Table of Contents

Email: [email protected]

Last modified:

Tuesday May 18 2010

The URL of this page is:
http://www.bsrsi.msu.edu/rfrc/home.htmlhttp://www.pbs.org/edens/manu/http://www.pbs.org/http://www.pbs.org/http://www.ento.vt.edu/~sharov/popechome/http://powayusd.sdcoe.k12.ca.us/mtr/PlanetEarthMainPage.htmhttp://powayusd.sdcoe.k12.ca.us/mtr/ConflictYellowstoneWolf.htmhttp://pubs.usgs.gov/gip/deserts/types/http://pubs.usgs.gov/gip/deserts/contents/http://pubs.usgs.gov/gip/deserts/contents/http://pubs.usgs.gov/gip/deserts/contents/http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookTOC.htmlmailto:[email protected]://www.bsrsi.msu.edu/rfrc/home.htmlhttp://www.pbs.org/edens/manu/http://www.pbs.org/http://www.ento.vt.edu/~sharov/popechome/http://powayusd.sdcoe.k12.ca.us/mtr/PlanetEarthMainPage.htmhttp://powayusd.sdcoe.k12.ca.us/mtr/ConflictYellowstoneWolf.htmhttp://pubs.usgs.gov/gip/deserts/types/http://pubs.usgs.gov/gip/deserts/contents/http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookTOC.htmlmailto:[email protected]

Documents

Community and Ecosystem Dynamic1