Showing posts with label ecosystems. Show all posts
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11:07 PM
In adaptive radiation, numerous species evolve from a common ancestor introduced into an environment with diverse ecological niches. The progeny evolve genetically into customized variations of themselves, each adapting to survive in a particular niche.

In 1898 Henry F. Osborn identified and developed the evolutionary phenomenon known as adaptive radiation, whereby different forms of a species evolve, quickly in evolutionary terms, from a common ancestor.

According to the principles of natural selection, organisms that are the best adapted (most fit) to compete will live to reproduce and pass their successful traits on to their offspring. The process of adaptive radiation illustrates one way in which natural selection can operate when members of one population of a species are cut off or migrate to a different environment that is isolated from the first.

Such isolation can occur from one patch of plantings to another, from one mountain top or hillside to another, from pond to pond, or from island to island. Faced with different environments, the group will diverge from the original population and in time become different enough to form a new species.


Genetic Changes

In a divergent population, the relative numbers of one form of allele (characteristic) decrease, while the relative numbers of a different allele increase. New environmental pressures will select for favorable alleles that may not have been favored in the old environment.

Over successive generations, therefore, a new gene created by random mutation (change) may replace the original form of the gene if, for example, the trait encoded by that gene allows the divergent group to cope better with environmental factors, such as food sources, predators, or temperature.

The result in the long term is that deoxyribonucleic acid (DNA) changes sufficiently through the growth of divergent populations to allow new generations to become significantly different from the original population. In time, they are unable to reproduce with members of the original species and become a new species.

Galápagos Islands Case Study

Adaptive radiation occurs dramatically when a species migrates from one landmass to another. This may occur between islands or between continents and islands. A classic example of adaptive radiation is the evolution of finches noted by Charles Darwin during his trips to the Galápagos Islands off the west coast of South America.

Several species of plants and animals had migrated to these islands from the South American mainland by means of flight, wind, ocean debris, or other means of transport. Finches from the mainland—perhaps aided by winds—settled on fifteen of the islands in the Galápagos group and began to adapt to the various unoccupied ecological niches on those islands, which differed.

Over several generations, natural selection favored a variety of finch species with beaks adapted for the different types of foods available on the different islands. As a result, several species of different finches evolved, roughly simultaneously, on these islands.

Hawaiian Silversword Alliance

Although plants seem unable to "migrate" as birds and other animals do, adaptive radiation occurs in the plant world as well. In the Hawaiian Islands, for example, twenty-eight species of the Asteraceae family are known together as the Hawaiian silversword alliance. The entire group appears to be traceable to one ancestor, thought to have arrived on the island of Kauai from western North America.

The silverswords—which compose three genera, Argyroxiphium, Dubautia, and Wilkesia— have since evolved into twenty-eight species, and this speciation came about due to major ecological shifts. These plants are therefore prime examples of adaptive radiation.

Within the silversword alliance, different species have adapted to widely varying ecosystems found throughout the islands. Argyroxiphium sandwicense, for example, is endemic to the island of Maui and grows at high elevations from 6,890 to 9,843 feet (2,100 - 3,000 meters) on the dry, alpine slopes of the volcano Haleakala.

This species has succulent leaves covered with silver hairs. It is thought that the hairs lessen the pace of evaporative moisture loss and protect the leaves from the sun. In contrast, species of the genus Dubautia that grow in wet, shady forests have large leaves that lack hairs.

Despite their "customized" physiologies, the silverswords that have evolved in Hawaii are all closely related to one another, so much so that any two can hybridize. Studies of the silverswords have provided what geneticist Michael Purugganan called a "genetic snapshot of plant evolution". Adaptive radiation is one window into how new plant structures arise.
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7:44 PM
Animal-plant Interactions
Animal-plant Interactions
The ways in which certain animals and plants interact have evolved in some cases to make them interdependent for nutrition, respiration, reproduction, or other aspects of survival.

Ecology represents the organized body of knowledge that deals with the relationships between living organisms and their nonliving environments. Increasingly, the realm of ecology involves a systematic analysis of plant-animal interactions through the considerations of nutrient flow in food chains and food webs, exchange of such important gases as oxygen and carbon dioxide between plants and animals, and strategies of mutual survival between plant and animal species through the processes of pollination and seed dispersal.

A major example of animal-plant interactions involve the continual processes of photosynthesis and cellular respiration. Green plants are classified as ecological producers, having the unique ability, by photosynthesis, to take carbon dioxide and incorporate it into organic molecules.

Animals are classified as consumers, taking the products of photosynthesis and chemically breaking them down at the cellular level to produce energy for life activities. Carbon dioxide is a waste product of this process.


Mutualism

Mutualism is an ecological interaction in which two different species of organisms beneficially reside together in close association, usually revolving around nutritional needs.

One such example is a small aquatic flatworm that absorbs microscopic green algae into its tissues. The benefit to the animal is one of added food supply. The mutual adaptation is so complete that the flatworm does not actively feed as an adult.

The algae, in turn, receive adequate supplies of nitrogen and carbon dioxide and are literally transported throughout tidal flats in marine habitats as the flatworm migrates, thus exposing the algae to increased sunlight. This type of mutualism, which verges on parasitism, is called symbiosis.

Coevolution

Coevolution is an evolutionary process wherein two organisms interact so closely that they evolve together in response to shared or antagonistic selection pressure. A classic example of coevolution involves the yucca plant and a species of small, white moth (Tegitecula).

The female moth collects pollen grains from the stamen of one flower on the plant and transports these pollen loads to the pistil of another flower, thereby ensuring cross-pollination and fertilization. During this process, the moth will lay her own fertilized eggs in the flowers’ undeveloped seed pods.

The developing moth larvae have a secure residence for growth and a steady food supply. These larvae will rarely consume all the developing seeds; thus, both species (plant and animal) benefit.

Although this example represents a mutually positive relationship between plants and animals, other interactions are more antagonistic. Predator-prey relationships between plants and animals are common. Insects and larger herbivores consume large amounts of plant material. In response to this selection pressure, many plants have evolved secondary metabolites that make their tissues unpalatable, distasteful, or even poisonous. In response, herbivores have evolved ways to neutralize these plant defenses.

Mimicry and Nonsymbiotic Mutualism

Mimicry
Mimicry
Inmimicry, an animal or plant has evolved structures or behavior patterns that allow it tomimic either its surroundings or another organism as a defensive or offensive strategy.

Certain types of insects, such as the leaf hopper, walking stick, praying mantis, and katydid (a type of grasshopper), often duplicate plant structures in environments ranging from tropical rain forests to northern coniferous forests. Mimicry of their plant hosts affords these insects protection from their own predators as well as camouflage that enables them to capture their own prey readily.

Certain species of ambush bugs and crab spiders have evolved coloration patterns that allow them to hide within flower heads of such common plants as goldenrod, enabling them to ambush the insects that visit these flowers.

In nonsymbiotic mutualism, plants and animals coevolve morphological structures and behavior patterns bywhich they benefit each other but without living physically together.

This type of mutualism can be demonstrated in the often unusual shapes, patterns, and colorations that more advanced flowering plants have developed to attract various insects, birds, andmammals for pollination and seed dispersal purposes. Accessory structures, called fruits, form around seeds and are usually tasty and brightly marked to attract animals for seed dispersal.

Although the fruits themselves become biological bribes for animals to consume, often the seeds within these fruits are not easily digested and thus pass through the animals’ digestive tracts unharmed, sometimes great distances from the parent plant. Some seeds must pass through the digestive plant of an animal to stimulate germination.

Other types of seed dispersal mechanisms involve the evolution of hooks, barbs, and sticky substances on seeds that enable them to be easily transported by animal fur, feet, feathers, or beaks. Such strategies of dispersal reduce competition be- tween the parent plant and its offspring.

Pollinators

Pollinators
Pollinators
Because structural specialization increases the possibility that a flower’s pollenwill be transferred to a plant of the same species, many plants have evolved a vast array of scents, colors, and nutritional products to attract pollinators. Not only does pollen include the plant’s spermcells; it also represents a food reward.

Another source of animal nutrition is a substance called nectar, a sugar-rich fluid produced in specialized structures called nectaries within the flower or on adjacent stems and leaves.

Assorted waxes and oils are also produced by plants to ensure plant-animal interactions. As species of bees, flies, wasps, butterflies, and hawk-moths are attracted to flower heads for these nutritional rewards, they unwittingly become agents of pollination by transferring pollen from stamens to pistils.

Some flowers have evolved distinctive, unpleasant odors reminiscent of rotting flesh or feces, thereby attracting carrion beetles and flesh flies in search of places to reproduce and deposit their own fertilized eggs.

As these animals copulate, they often become agents of pollination for the plant itself. Some tropical plants, such as orchids, even mimic a female bee, wasp, or beetle, so that the insect’s male counterpart will attempt to mate with them, thereby encouraging precise pollination.

Among birds, hummingbirds are the best examples of plant pollinators. Various types of flowers with bright, red colors, tubular shapes, and strong, sweet odors have evolved in tropical and temperate regions to take advantage of hummingbirds’ long beaks and tongues as an aid to pollination.

Because most mammals, such as small rodents and bats, do not detect colors as well as bees and butterflies do, some flowers instead focus upon the production of strong, fermenting, or fruit-like odors and abundant pollen rich in protein. In certain environments, bats and mice that are primarily nocturnal have replaced day-flying insects and birds as pollinators.
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3:32 AM
Antarctica
Antarctica
The harsh climate of Antarctica makes it one of the most inhospitable places on the earth, allowing only a relatively small number of organisms to live there. Permanent terrestrial (land) animals and plants are few and small. There are no trees, shrubs, or vertebrate land animals. Native organisms are hardy, yet the ecosystem is fragile and easily disturbed by human activity, pollution, global warming, and ozone layer depletion.

The Antarctic continent has never had a native or permanent population of humans. In 1998 the United States, Russia, Belgium, Australia, and several other countries signed one of an ongoing series of treaties to preserve Antarctica. The continent is used for peaceful international endeavors such as scientific research and ecotourism.

Terrestrial Flora

There are only two types of flowering plants in Antarctica, a grass and a small pearlwort (Deschampsia antarctica). These are restricted to the more temperate Antarctic Peninsula. Antarctic hairgrass (Colobanthus quitensis) forms dense mats and grows fairly rapidly in the austral summer (December, January, and February). At the end of summer, the hairgrass’s nutrients move underground, and the leaves die. Pearlwort forms cushion-shaped clusters and grows only 0.08 to 0.25 inch (2 to 6 millimeters) per year.


Numerous species of primitive plants, such as lichens, mosses, fungi, algae, and diatoms, live in Antarctica. Lichens are made up of an alga and a fungus in a symbiotic (interdependent) relationship. They can use water in the form of vapor, liq uid, snow, or ice.

Lichens grow as little as 0.04 inch (1 millimeter) every one hundred years, and some patches may be more than five thousand years old. Mosses are not as hardy as lichens and also grow slowly; a boot print in a moss carpet may be visible for years. Fungi are found in the more temperate peninsula, and most are microscopic.


Algae grow in Antarctic lakes, runoff near bird colonies, moist soil, and snow fields. During the summer, algae form spectacular red, yellow, or green patches on the snow. Bacteria are found in lakes, melt water, and soils. As elsewhere on the earth, bacteria play a role in decomposition. Because of the extreme conditions, they are not always as efficient in Antarctica as they are in warmer climates, and carcasses may lie preserved for hundreds of years.
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6:56 AM
Biological Invasions
Biological Invasions

Biological invasions are the entry of a type of organism into an ecosystem outside its historic range. In a biological invasion, the "invading" organism may be an infectious virus, a bacterium, a plant, an insect, or an animal.

Species introduced to an area from somewhere else are referred to as alien or exotic species or as invaders. Because an exotic species is not native to the new area, it is often unsuccessful in establishing a viable population and disappears.

The fossil record, as well as historical documentation, indicates that this is the fate of many species in new environments as they move from their native habitats. Occasionally, however, an invading species finds the new environment to its liking.


In this case, the invader may become so successful in exploiting its new habitat that it can completely alter the ecological balance of an ecosystem, decreasing biodiversity and altering the local biological hierarchy. Because of this ability to alter ecosystems, exotic invaders are considered major agents in driving native species to extinction.

Biological invasions by notorious species constitute a significant component of earth’s history. In general, large-scale climatic changes and geological crises are at the origin of massive exchanges of flora and fauna. On a geologic time scale, migrations of invading species from one continent to another are true evolutionary processes, just as speciation and extinction are.

On a smaller scale, physical barriers such as oceans, mountains, and deserts can be overcome by many organisms as their populations expand. Organisms can be carried by water in rivers or ocean currents, transported by wind, or carried by other species as they migrate seasonally or to escape environmental pressures.

Humans have transplanted plants since the beginning of plant cultivation in pre-Columbian times. The geological and historical records of the earth suggest that biological invasions contribute substantially to an increase in the rate of extinction within ecosystems.

Invasive Plants

Invasive Plants
Invasive Plants

In modern times, most people are not aware of the distinction between native plants and exotic species growing in their region. Recent increases in intercontinental invasion rates by exotic species, brought about primarily by human activity, create important ecological problems for the recipient lands. Invasive plants in North America include eucalyptus trees, morning glory, and pampas grass.

It would seem logical to assume that invading species might add to the biodiversity of a region, but many invaders have the opposite effect. In all ecosystems the new species are often opportunistic, driving out native species by competing with them for resources.

For example, Pueraria lobata, or kudzu, is a vine native to Japan. Introduced in the United States at the 1876 Philadelphia Exposition, kudzu was planted to control erosion on hillsides and for livestock forage. By the end of the twentieth century, it could be found from Connecticut to Missouri, extending south to Texas and Florida.

Kudzu covers everything in its path and grows as much as 1 foot (0.3 meter) per day. Similarly, English ivy (Hedera helix), a native of Eurasia, is considered a serious problem in West Coast states. It forms "ivy deserts" in forests and crowds out native trees and shrubs that make up essential wildlife habitat.

The invasion of an ecosystem by an exotic species can effectively alter ecosystem processes. An invading species does not simply consume or compete with native species but can actually change the rules of existence within the ecosystem by altering processes such as primary productivity, decomposition, hydrology, geomorphology, nutrient cycling, and natural disturbance regimes.

Invasive Insects and Microorganisms

The invasion of native forests alone by nonnative insects and microorganisms has been devastating on many continents. The white pine blister rust and the balsam woolly adelgid have invaded both commercial and preserved forest lands in North America. Both exotics were brought to North America in the late 1800’s on nursery stock from Europe.

The balsam woolly adelgid attacks fir trees and causes their death within two to seven years from chemical damage and by feeding on the trees’ vascular tissue. The adelgid has killed nearly every adult conebearing fir tree in the southern Appalachian Mountains.

The white pine blister rust attacks five-needle pines; in the western United States fewer than 10 pine trees in 100,000 are resistant. Because white pine seeds are an essential food source for bears and other animals, the loss of the trees is having severe consequences across the food chain.

Since the 1800’s the deciduous trees of eastern North America have been attacked numerous times by waves of invading exotic species and diseases. One of the most notable invaders is the gypsy moth, which consumes a variety of tree species. Other invaders have virtually eliminated the once-dominant American chestnut and the American elm.

Tree species that continue to decline because of new invaders include the American beech, mountain ash, white birch, butternut, sugar maple, flowering dogwood, and eastern hemlock. It is widely accepted that the invasion of exotic species is the single greatest threat to the diversity of deciduous forests in North America.

Effects on Humans and Humans as Invaders

Some introduced exotic species are beneficial to humanity. It would be impossible to support the present world human population entirely on species native to their regions. Humans, the ultimate biological invaders, have been responsible for the extinction of many species and will continue to be in the future.

At the beginning of the twenty-first century, the United States was spending $4 billion annually to eradicate invasive plant species, a figure that does not take into account loss of biodiversity or wildlife habitat.
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7:01 PM
Biomass Related to Energy
Biomass Related to Energy
The relationship between the accumulation of living matter resulting from the primary production of plants or the secondary production of animals (biomass) and the energy potentially available to other organisms in an ecosystem forms the basis of the study of biomass related to energy.

Biomass is the amount of organic matter, such as animal and plant tissue, found at a particular time and place. The rate of accumulation of biomass is termed productivity. Primary production is the rate at which plants produce new organic matter through photosynthesis.

Secondary production is the rate at which animals produce their organic matter by feeding on other organisms. Biomass is an instantaneous measure of the amount of organic matter, while primary and secondary production give measures of the rates at which biomass increases.

Plant and animal biomass consists mostly of carbon-rich molecules, such as sugars, starches, proteins, and lipids, and other substances, such as minerals, bone, and shell. The carbon-rich organic molecules are not only the building blocks of life but also the energy-rich molecules used by organisms to fuel their activities.


Solar Energy and Photosynthesis

Ultimately, all energy used by organisms to produce the building blocks of life and to drive life processes originated as solar energy captured by plants. Only a small fraction, less than 2 percent, of the total solar light energy received by a plant is absorbed and transformed by photosynthesis into energy-containing organic molecules.

The rest of the sun’s energy passes out of the plant as heat. The rate at which plants capture light energy and transform it into chemical energy is called primary production.

Because plants do not rely on other organisms to provide their energy needs, they are referred to as primary producers, or autotrophs (meaning “self-feeding”). In addition to light energy, plants must absorb water, carbon dioxide gas, and simple nutrients, such as nitrate and phosphate, to produce various organic molecules during photosynthesis. Oxygen gas is also produced.

Sugars are the first energy-containing organic molecules produced in photosynthesis, and they can be changed to other, more complex, molecules, such as starches, proteins, and fats.

The energy in the sugar molecules can be used immediately by the plants to maintain their own respiration needs, stored as starches and fats, or can be converted to new plant tissue. It is the stored organic matter plus new tissue that contributes to the growth of plants and to biomass.

Because the energy-containing products of photosynthesis can be used either immediately in respiration or in the formation of new plant biomass, two types of primary production can be distinguished. Gross production refers to the total amount of energy produced by photosynthesis.

It includes both the energy used by the plant for respiration and the energy that goes into new biomass. Net production refers only to the amount of energy that accumulates as new biomass. It is only the energy in net production that is potentially available to animal consumers as food.

The rate of primary production varies directly with the rate of photosynthesis; therefore, factors in the environment that affect the rate of photosynthesis affect the rate of primary production. These factors most often include light intensity, temperature, nutrient concentrations, and moisture conditions.

Each species of plant has a specific combination of these factors that promotes maximum rates of primary production. If one or more of these factors is in excess or is in short supply, then the rate of primary production is slowed.

Primary Production

Primary Production
Primary Production
On land, the rate of primary production by plants is determined largely by light, temperature, and rainfall. The favorable combination of intense sunlight for twelve hours per day, warm temperatures throughout the year, and considerable rainfall make the tropical rain forests the most productive ecosystems on land.

In contrast, Arctic tundra vegetation is exposed to reduced light intensity, very cold winters, and cool summers. Primary production there is very low. In deserts, the lack of water severely limits primary production even though light and temperature are otherwise favorable.

In aquatic habitats, rates of primary production by algae, such as phytoplankton, are determined by nutrient concentration and light intensity. As sunlight penetrates water, it is quickly absorbed by the water molecules and by small suspended particles.

Thus, all primary production occurs near the surface, as long as nutrients are available. Although thewaters of the open ocean are very clear, and sunlight can penetrate to great depths, the scarcity of nutrients reduces the rate of primary production to less than one-tenth that of coastal bays.

Secondary Production

The energy and material needs of some organisms are met by consuming the organic materials produced by others. These consumer organisms are called heterotrophs; there are two types. Those that obtain their food from other living organisms are called consumers and include all animals. Those that obtain their energy from dead organisms are called decomposers and include mostly the fungi and bacteria.

The energy available to each type of consumer becomes progressively less at each level of the food chain. Each consumer level uses most of its food energy, about 90 percent, to fuel its respiratory activities. In this energy-releasing process, most of the food energy is actually converted to heat and is lost to the environment.

Only 10 percent or less of the original food energy is used to form new biomass. It is only this small amount of energy that is available for the next consumer level. The result is that food chains are limited in their number of links or levels by the reduced amount of energy available at each higher level.

Generally, the greater the amount of primary production, the larger the number of consumer organisms and the longer the food chain. Most food chains consist of three levels; rarely are there examples of up to five levels. It should be noted that the food chain concept is a simplified view of a more complex network of energy pathways, known as food webs, that occur in nature.

Another outcome of the reduction in energy flow up the food chain is a progressive decrease in production and biomass. The most productive level, and the one with the greatest biomass, is therefore the primary producers, or plants.

Human Threats to Primary Production

Human Threats to Primary Production
Human Threats to Primary Production
The total natural primary production of the earth is limited, and human efforts to increase total world primary production much beyond its present levels may be futile. One reason for this is that much of the earth’s surface lacks optimal conditions for plant growth. The open ocean, which covers about 71 percent of the earth’s surface, has very little plant growth.

On land, the Arctic, subarctic, and Antarctic regions are very unproductive most of the year. Human attempts to increase primary production in the form of food or fuel crops usually involve changing the characteristics of the land, converting forests into croplands, for example, and adding large quantities of nutrients and water.

It has been estimated that humans are currently utilizing most of the easily workable croplands and that the development of additional lands for agriculture would require major changes to currently unworkable habitats, changes that would be expensive and demand much fuel energy.

The study of production processes is vitally important in understanding the ecology of natural ecosystems. Such information is necessary to manage and conserve habitats and their organisms in the face of human pressures.

These processes provide insight into the general health of ecosystems. Pollutants, such as acid rain or industrial toxic wastes, are known to reduce the primary and secondary productivity of forests and lakes.

Throughout the world, humans are reducing the biomass of the world’s primary producers through deforestation. This is particularly true in the tropics, where high population pressures have necessitated that land be cleared for agriculture and development. There is a world wide demand for lumber. One obvious consequence is the dramatic reduction in the primary and secondary production of these areas.

The clear-cutting (removal of all the trees) of tropical forests allows unprotected soils to wash away quickly during the heavy tropical rains. It will take hundreds of years for new soils to develop and for the forest to return—if it can return at all.

Deforestation is also harmful in that tropical forests form a major part of the world’s life-support system. For millions of years these forests have buffered the earth’s atmosphere by producing the oxygen gas needed by animals and by removing carbon dioxide and other toxic gases.

The low level of carbon dioxide in the atmosphere is believed to have moderated the earth’s temperature, counteracting the so-called greenhouse effect. It is therefore of great importance to understand and preserve these forests and other primary producers of the world.
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5:24 AM
Types of Biomes
Types of Biomes

The major recognizable life zones of the continents are divided into biomes, characterized by their plant communities.

Temperature, precipitation, soil, and length of day affect the survival and distribution of biome species. Species diversity within a biome may increase its stability and capability to deliver natural services, including enhancing the quality of the atmosphere, forming and protecting the soil, controlling pests, and providing clean water, fuel, food, and drugs. Major biomes include the temperate, tropical, and boreal forests; tundra; deserts; grasslands; chaparral; and oceans.

Temperate Forests

The temperate forest biome occupies the so called temperate zones in the midlatitudes (from about 30 to 60 degrees north and south of the equator). Temperate forests are found mainly in Europe, eastern North America, and eastern China, and in narrow zones on the coasts of Australia, New Zealand, Tasmania, and the Pacific coasts of North and South America. Their climates are characterized by high rainfall and temperatures that vary from cold to mild.


Temperate forests contain primarily deciduous trees—including maple, oak, hickory, and beech-wood—and, secondarily, evergreen trees—including pine, spruce, fir, and hemlock. Evergreen forests in some parts of the Southern Hemisphere contain eucalyptus trees. The root systems of forest trees help keep the soil rich. The soil quality and color are due to the action of earthworms.

Where these forests are frequently logged, soil runoff pollutes streams, which reduces spawning habitat for fish. Raccoons, opossums, bats, and squirrels are found in the trees. Deer and black bears roam forest floors. During winter, small animals such as marmots and squirrels burrow in the ground.

Tropical Forests

Tropical Forests
Tropical Forests

Tropical forests exist in frost-free areas between the Tropic of Cancer and the Tropic of Capricorn. Temperatures range from warm to hot year-round. These forests are found in northern Australia, the East Indies, Southeast Asia, equatorial Africa, and parts of Central America and northern South America.

Tropical forests have high biological diversity and contain about 15 percent of the world’s plant species. Animal life thrives at each layer of tropical forests. Nuts and fruits on the trees provide food for birds, monkeys, squirrels, and bats. Monkeys and sloths feed on tree leaves.

Roots, seeds, leaves, and fruit on the forest floor feed larger animals. Tropical forest trees produce rubber and hardwood, such as mahogany and rosewood. Deforestation for agriculture and pastures has caused reduction in plant and animal diversity in these forests.

Boreal Forests

Boreal Forests
Boreal Forests

The boreal forest is a circumpolar Northern Hemisphere biome spread across Russia, Scandinavia, Canada, and Alaska. The region is very cold. Evergreen trees such as white spruce and black spruce dominate this zone, which also contains larch, balsam, pine, fir, and some deciduous hardwoods such as birch and aspen. The acidic needles from the evergreens make the leaf litter that is changed into soil humus. The acidic soil limits the plants that develop.

Animals in boreal forests include deer, bears, and wolves. Birds in this zone include red-tailed hawks, sapsuckers, grouse, and nuthatches. Relatively few animals emigrate from this habitat during winter. Conifer seeds are the basic winter food.

Tundra

Tundra
Tundra

About 5 percent of the earth’s surface is covered with Arctic tundra and 3 percent with alpine tundra. The Arctic tundra is the area of Europe, Asia, and North America north of the boreal coniferous forest zone, where the soils remain frozen most of the year.

Arctic tundra has a permanent frozen subsoil, called permafrost. Deep snow and low temperatures slow the soil-forming process. The area is bounded by a 50 degrees Fahrenheit (122 degrees Celsius) circumpolar isotherm, known as the summer isotherm. The cold temperature north of this line prevents normal tree growth.

The tundra landscape is covered by mosses, lichens, and low shrubs, which are eaten by caribou, reindeer, and musk oxen. Wolves eat these herbivores. Bears, foxes, and lemmings also live there. The most common Arctic bird is the old squaw duck. Ptarmigans and eider ducks are also very common. Geese, falcons, and loons are some of the nesting birds of the area.

The alpine tundra, which exists at high altitude in all latitudes, is acted upon by winds, cold temperatures, and snow. The plant growth is mostly cushion- and mat-forming plants.

Deserts

Deserts
Deserts

The desert biome covers about one-seventh of the earth’s surface. Deserts typically receive no more than 10 inches (25 centimeters) of rainfall per year, and evaporation generally exceeds rainfall. Deserts are found around the Tropic of Cancer and the Tropic of Capricorn.

As warm air rises over the equator, it cools and loses its water content. The dry air descends in the two subtropical zones on each side of the equator; as it warms, it picks up moisture, resulting in drying the land.

Rainfall is a key agent in shaping the desert. The lack of sufficient plant cover contributes to soil erosion during wind- and rainstorms. Some desert plants—for example, the mesquite tree, which has roots that grow 40 feet (13 meters) deep—obtain water from deep below the earth’s surface.

Other plants, such as the barrel cactus, store large amounts of water in their leaves, roots, or stems. Some plants slow the loss of water by having tiny leaves or shedding their leaves. Desert plants have very short growth periods, because they cannot grow during the long drought periods.

Grasslands

Grasslands
Grasslands

Grasslands cover about one-quarter of the earth’s surface and can be found between forests and deserts. Treeless grasslands exist in parts of central North America, Central America, and eastern South America that have between 10 and 40 inches (250-1,000millimeters) of erratic rainfall per year. The climate has a high rate of evaporation and periodic major droughts. Grasslands are subject to fire.

Some grassland plants survive droughts by growing deep roots, while others survive by being dormant. Grass seeds feed the lizards and rodents that become the food for hawks and eagles. Large animals in this biome include bison, coyotes, mule deer, and wolves.

The grasslands produce more food than any other biome. Overgrazing, inefficient agricultural practices, and mining destroy the natural stability and fertility of these lands, resulting in reduced carrying capacity,water pollution, and soil erosion.

Diverse natural grasslands appear to be more capable of surviving drought than are simplified manipulated grass systems. This may be due to slower soil mineralization and nitrogen turnover of plant residues in the simplified system.

Savannas are open grasslands containing deciduous trees and shrubs. They are near the equator and are associated with deserts. Grasses there grow in clumps and do not form a continuous layer.

Chaparral

Chaparral
Chaparral

The chaparral, or mediterranean, biome is found in the Mediterranean Basin, California, parts of Australia, middle Chile, and the Cape Province of South America. This region has a climate of wet winters and summer drought. The plants have tough, leathery leaves and may have thorns. Regional fires clear the area of dense and dead vegetation.

The seeds from some plants, such as the California manzanita and South African fire lily, are protected by the soil during a fire and later germinate and rapidly grow to form new plants. Vegetation dwarfing occurs as a result of the severe summer drought and extreme climate changes.

Oceans

The ocean biome covers more than 70 percent of the earth’s surface and includes 90 percent of its volume.Oceans have four zones. The intertidal zone is shallow and lies at the land’s edge. The continental shelf, which begins where the intertidal zone ends, is a plain that slopes gently seaward.

The neritic zone (continental slope) begins at a depth of about 600 feet (180 meters), where the gradual slant of the continental shelf becomes a sharp tilt toward the ocean floor, plunging about 12,000 feet (3,660 meters) to the ocean bottom, which is known as the abyss. The abyssal zone is so deep that it does not have light.

Plankton are animals that float in the ocean. They include algae and copepods, which are microscopic crustaceans. Jellyfish and animal larva are also considered plankton. The nekton are animals that move freely through the water by means of their muscles.

These include fish, whales, and squid. The benthos are animals that are attached to or crawl along the ocean’s floor. Clams are examples of benthos. Bacteria decompose the dead organic materials on the ocean floor.

The circulation of materials from the ocean’s floor to the surface is caused by winds and water temperature. Runoff from the land contains pollutants such as pesticides, nitrogen fertilizers, and animal wastes. Rivers carry loose soil to the ocean, where it builds up the bottom areas. Overfishing has caused fisheries to collapse in every world sector.

Human Impact on Biomes

Human interaction with biomes has increased biological invasions, reduced species biodiversity, changed the quality of land and water resources, and caused the proliferation of toxic compounds. Managed care of biomes may not be capable of undoing these problems.
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Biosphere Concept
Biosphere Concept

The term "biosphere" was coined in the nineteenth century by Austrian geologist Eduard Suess in reference to the 20-kilometer-thick zone extending from the floor of the oceans to the top of mountains, within which all life on earth exists.

Thought to be more than 3.5 billion years old, the biosphere supports nearly one dozen biomes, regions of climatic conditions within which distinct biotic communities reside.

Compounds of hydrogen, oxygen, carbon, nitrogen, potassium, and sulfur are cycled among the four major spheres, one of which is the biosphere, to make the materials that are essential to the existence of life.


The other spheres are the lithosphere, the outer part of the earth; the atmosphere, the whole mass of air surrounding the earth; and the hydrosphere, the aqueous vapor of the atmosphere, sometimes defined as including the earth’s bodies of water.

The Water Cycle

The most critical of these compounds is water, and its movement among the spheres is called the hydrologic cycle. Dissolved water in the atmosphere condenses to form clouds, rain, and snow. The annual precipitation for any region is one of the major factors in determining the terrestrial biome that can exist.

The precipitation takes various paths leading to the formation of lakes and rivers. These flowing waters interact with the lithosphere (the outer part of the earth’s crust) to dissolve chemicals as they flow to the oceans. Evaporation of water from the oceans then supplies most of the moisture in the atmosphere. This cycle continually moves water among the various terrestrial and oceanic biomes.

The Water Cycle
The Water Cycle

Solar Energy

The biosphere is also dependent upon the energy that is transferred from the various spheres. Solar energy is the basis for almost all life. Light enters the biosphere as the essential energy source for photosynthesis.

Plants take in carbon dioxide, water, and light energy, which is converted via photosynthesis into chemical energy in the form of sugars and other organic molecules.

Oxygen is generated as a by-product.Most animal life reverses this process during respiration, as chemical energy is released to do work by the oxidation of organic molecules to produce carbon dioxide and water.

Incoming solar energy also interacts dramatically with the water cycle and the world wide distribution of biomes. Because of the earth’s curvature, the equatorial regions receive a greater amount of solar heat than the polar regions.

Convective movements in the atmosphere—such as winds, high- and low-pressure systems, and weather fronts—and the hydrosphere—such as water currents—are generated during the redistribution of this heat. The weather patterns and climates of earth are a response to these energy shifts. Earth’s various climates are defined by the mean annual temperature and the mean annual precipitation.
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