Lecture 02

Environmental Science

The Environment & Ecosystems

 

 

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 Ecosystems      Ecosystem Roles        Human Risk and the Environment          Depletion Degradation of Species

      An ecosystem is composed of all the living organisms in an area plus the surrounding physical environment with which they interact. Each species has a role to play in their ecosystem.  This niche is where they live (habitat), what they eat, how they reproduce, how it interacts with other species and the role they play in the flow of energy and the cycling of matter.  One needs to know about the niche of a species to be able to see that conditions of an area meet the needs of the species so that they can reproduce and survive.  Determining the niche of a species is difficult, time-consuming and expensive.  Basic research needs to be done to manage ecosystems and maintain their biodiversity.

      Species can be broadly classified as specialists or generalists.  Specialists have narrow niches: usually only one type of habitat, a narrow tolerance range of environmental factors and one or a few types of food.  They are prone to becoming endangered when environmental changes occur.  Generalists have a broad niche: they can live in many habitats, tolerate wide ranges of environmental conditions and a wide variety of foods.  When environmental factors are stable, specialists are favored because there is little competition, however when environments change, the generalists can adapt better than the specialists. Species can be assigned the title native, nonnative, indicator or keystone species, depending on their ecological role.  Native species live and thrive in a particular ecosystem. 

      Deliberately or accidentally introduced species are called non-native, exotic or alien species.  Many of these non-natives thrive and crowd out many native species.  Indicator species give early warnings that an ecosystem is being damaged.  Birds are excellent biological indicators because they are found everywhere and respond quickly to environmental change. Fish and amphibians are indicators of aquatic and land ecosystems.  Keystone or Indicator species are more important in an ecosystem than their numbers or biomass suggest.  They are critically linked to large numbers of other species.  Some keystone species are: bees, bats, ants, hummingbirds, sea otters, gophers, tortoises, American alligators, elephants, rhinoceroses, beavers, wolves and the great white shark.  The loss of a keystone species can lead to population crashes and extinctions of other species that depend on the keystone species.

      Different species in an ecosystem interact with each other.  Interactions among species are: interspecific competition, predation and symbiosis.  Symbiotic relationships are parasitism, mutualism and commensalism. Mutualism occurs when both species benefit from their interaction.  Many mutualistic relationships provide nutrition and protection.  Research indicates mutualism increases when resources are scarce. (When both species in a close relationship that is beneficial, it is called mutualism.  These relationships often provide nutrition and protection.)  The acacia trees of  Central America, South America and East Africa are protected from herbivores by ants and the ants get nutrients and housing from the Acacia trees.

      Commensalism occurs when one species is neither harmed or benefits and the other species is helped.  In the tropics, trees and air plants or epiphytes occur.  The tree is unaffected and the orchid or bromeliad gains water, nutrients and sunlight. Some species interact so one benefits and the other is unaffected.  Coral reef interactions and tropical forest epiphytes are examples of commensalism.

      Symbiotic relationships are parasitism, mutualism and commensalism.  Parasites are specialized predators that live closely with and draw nourishment from its prey, the host.  The relationship is a long term chronic one.  Some parasites kill their host by feeding on the host tissue.  Some disease causing bacteria, viruses and worms live inside their hosts.  They are called endoparasites.  Parasites living outside the host are called ectoparasites.  Lice, ticks, fleas, mites, fungi are examples.

      The purpose of thinking in terms of an ecosystem is to link relationships that form the whole much as individual ingredients go together to make a cake. Shifts in the constituents of either change the end product. Biosphere is the term used to denote the collective ecosystems of the world. Biosphere studies are difficult to undertake because of the magnitude of interactions involved, yet examination of carbon dioxide levels, radiation changes, and energy flow must be applied to the biosphere as a whole.

      Ecologists refer to all living organisms in a given ecosystem as a community. Life within a forest is called a forest community. Sometimes we study subdivisions of a community, such as the plant, vertebrate, mammal, or rodent communities. Life above ground and in the soil may be called terrestrial communities, while plants and animals in bodies of water may be called aquatic communities.

      Within the community, organisms are grouped into populations. A population is a group of one species of organisms occupying a particular space at a particular time. Each population has characteristics, such as birth rate, death rate, age distribution, and genetic composition, which no individual in the population has by itself. Population characteristics are frequently expressed statistically. The ecologist uses populations and communities to tell us more about the ecosystem.

  

Reasons to Save the Diversity of Life

Why does biodiversity—the diversity of life—matter? Why should we even worry about extinction, when fossil records show that species have always gone extinct naturally over periods of millions of years? We should be concerned because the pace of extinction, and its cause, is dramatically different. Now, according to estimates by world-renowned conservation biologists, man's impact on the environment is causing species to vanish at a rate faster than the natural rate.

1. Feeding the World

A mere 20 species provide about 90 percent of the world's food. All major food crops, including corn, wheat, and soybeans, depend on the introduction of new strains from the wild to cope with evolving disease and pests. If those strains are lost, the security of our food supply will be threatened. For example, a wild relative of corn is exceptionally disease-resistant and is the only perennial in the corn family. If successfully interbred with domestic corn, its genes could boost corn production.

2. Cures

Forty percent of all prescriptions dispensed in the United States are for substances derived from plants, animals, or microorganisms. The list of wonder drugs originated from wild species includes aspirin for pain relief (from meadowsweet), penicillin for antibiotics (from the pencillium fungi), digitoxin for cardiac treatment (from common foxglove), L-dopa for Parkinson's disease (from velvet bean), taxol for ovarian cancer (from the Pacific yew), and quinine for malaria (from yellow cinchona). Like unread books in the library of the universe, who knows what treasures await us in as yet undiscovered species?

3. A Wealth of Natural Resources

Society derives most of life's necessities—food, clothing, medicine—from just a small number of plants and animals. Thousands of natural products are used routinely by industry to produce everyday goods. Consider just one wild source, the humble seaweeds. Compounds derived from seaweeds are used in plastics, polishes, paints, deodorants, detergents, dyes, fire-extinguishing foams, lubricants, meat preservatives, and chicken feed, to name a few among hundreds of products. By preserving the diversity of life, we act as trustee for the planet, preserving genetic capital for use by future generations.

4. Healthy Ecosystems

Ecologists and economists are beginning to understand the value of the services that healthy ecosystems provide to our planet. Bacteria break down organic material, building and fertilizing the soil. Wetlands filter pollutants from drinking water. Trees and plants return oxygen to the air through photosynthesis. Vast South American forests create rainfall on a continental scale, and store carbon as a buffer against global climate change.

5. Losing Vital Species

Some species appear to be "keystones in the arch," supporting entire ecosystems, such as the sea otter in the Pacific coastal ecosystem. When these keystone species disappear, it indicates that the web of life is beginning to unravel. It would be a shame to lose what we might one day realize was vitally important.

6. Value of Diversity

The diversity of life is indescribably rich: each species on Earth is the result of millions of years of adaptation, an incredible and unique wonder of the universe with its own lessons to teach. More than any machine, species awe us with their complexity, particularly as science begins to unravel the mysteries of the genetic code and the interaction of all species.

 

How Energy Moves Through The Ecosystem    The Limits of Life

      All living organisms require energy to live. The sun supplies the basic source of this energy. As animals eat food in the form of plants or other animals, energy passes from one organism to another in the ecosystem.

      Sunlight is stored by green plants as chemi­cally bound energy during the process of pho­tosynthesis. In the overall process, carbon dioxide and water are used as raw materials to produce sugar and oxygen.

      Photosynthesis is a complex process involv­ing many chemical reactions. The reactions take place in small green organelles (chloro­plasts) containing chlorophyll, which gives plants their characteristic green color. Photosynthesis occurs in two stages, a light dependent stage (light reaction) and a light-independent stage (dark reaction).

      In the first stage, chloroplasts absorb light and transform it into chemical energy. The chemical energy is coupled with the bonds of chemical compounds in the chloroplasts. Chemical bonds are the forces holding chemi­cal elements and compounds together.

During the dark reaction, compounds formed by bonds of chemical energy (originally solar energy) are used to break water and carbon dioxide into free elements. In a series of steps the carbon is combined with the hydro­gen and oxygen of water to form sugar. Free oxygen is released.

      The use of the sun’s energy to form new bio­mass (weight of living organisms), called productivity, varies in different types of ecosystems. Typically, it is expressed as the amount of usable energy produced per unit of area per unit of time. Examples are kilocalories per square meter per day (kcal/m2/day) or grams of food per square meter per year (g/m2/yr). The gross primary productivity is the rate at which green plants convert solar energy (by means of photosynthesis) to chemical energy usable by life. Plants use much of this con­verted energy to maintain respiration. The net primary productivity, or the energy available for consumption by the plant itself for growth and reproduction, equals the gross primary productivity minus the rate of plant respiration. Transfer of energy from one trophic level to the next is not 100 percent efficient because the animals in each trophic level require energy for survival and reproduction. Energy is also lost as organisms consume one another. Not all of the animals in each trophic level are eaten by others; some die and decay, transferring this energy to the detritus food chain. Producers use energy for respiration and lose en­ergy as heat in the photosynthetic reaction. Energy uptake by herbivores represents the to­tal amount of energy available not only to herbivores but to all animals.

To summarize, three things can happen to energy assimilated at each trophic level:

* It can be used for respiration of organisms on that trophic level and lost as heat.

* It can become part of the detritus food chain either when the organisms of that level die and decay or as it passes through the bodies of other animals without being assimilated.

* It can be passed on to the next trophic level when animals are consumed and assimilated.

During transfer of energy from one trophic level to the next, 80 to 90 percent of the energy is lost through respiration or decay, leaving only 10 to 20 percent. Thus energy available at each trophic level is shown as decreasing pyramids. Energy, however, is basic in our examination of trophic levels, for it provides the power necessary to sustain life.

      Respiration is the process whereby sugar produced by green plants is broken down into energy that living organisms can use for growth, reproduction, and tissue repair. Respiration, as discussed here, is cellular and should not be confused with the breathing process of inhaling oxygen and exhaling carbon dioxide. Cellular respiration combines oxygen with sugars to form carbon dioxide, water, and energy.

      Most organisms use the free oxygen gas (O2) released in photosynthesis for body respiration. This is called aerobic (requires oxygen) respiration. People, fish, earthworms, and most bacteria are aerobic organisms.

Anaerobic organisms, which include some species of bacteria, can grow in the absence of free oxygen. Anaerobic respiration uses oxygen from the breakdown of compounds such as nitrate or sulfate. Those organisms that must grow in the complete absence of free oxygen are obligate anaerobes; those that can grow in the presence or absence of free oxygen are facultative anaerobes.

      Anaerobic organisms are essential to life. They help to break down food in digestive tracts of many organisms, including humans, and decompose organic matter in lake sediments, landfills, and sewage treatment plants. Swamp gas, tetanus, and gangrene are also caused by anaerobes.

      Fermentation, a specialized form of anaerobic respiration in which oxygen is supplied by an organic compound, satisfies many human needs. Yeast fermentation produces carbon dioxide that makes bread rise. Alcohol production for beers, wines, and other liquors involves fermentation. Fermentation can be a preserva­tive process when its products inhibit the growth of microorganisms that cause decay or spoilage of food. Silage is made by allowing green hay, grass, and/or cereal crops to undergo fermentation that preserves the food value of crops and provides feed for animals during the winter season.

 

Food Chains

      Energy flow from green plants to consumer or­ganisms, as each population is eating and being eaten, is called a food chain. Food chains are relatively uncomplicated, involving energy movement from one population to another. Complex or interlinked food chains, where one population feeds on a number of other populations, are called food webs. Food is the means by which energy moves from one organism to another.

      Autotrophs, including green plants and a few species of bacteria, convert the sun’s energy to chemically bound energy-food used for life-through photosynthesis. All other forms of life, called heterotrophs, depend on autotrophs either directly or indirectly for their life’s energy. Green plants, then, are the primary producers. Consumers feed on produc­ers, but not all consumers feed directly on plants. Animals that eat only plants are herbivores; others that feed only on animals are carnivores, or flesh eaters. Omnivores feed on both plants and animals.

      To trace the sequence of energy flow in eco­systems, ecologists superimpose trophic levels on food chains or food webs. All green plants (producers) are members of the first trophic level; all herbivores constitute the second trophic level; animals that feed primarily on herbivores make up the third trophic level. The fourth and fifth levels are composed of animals that feed on the consumers of the trophic level just below them. Some animals, including humans, can occupy more than one trophic level. Very few populations occupy the fifth level or higher.

      Natural systems have two types of food webs-grazing and detritus. The terrestrial grazing food web involves moving energy and minerals from green plants to herbivores to carnivores. The decomposition or detritus food web becomes operative when organisms die. Millions of decomposer organisms break down dead bodies, using energy and releasing nutrients from plant and animal matter back into mineral cycles. Organisms such as earthworms and beetles, called macro-decomposers, begin the process by removing large pieces of the dead organism. Microde­composers such as bacteria and fungi then finish the process.

      Phytoplankton, minute floating plants, form the base of the grazing food web in aquatic systems. They are eaten by small floating animals called zooplankton, which in turn are food for small fish and filter feeders. Filter feeders obtain their food by straining plankton from the water. They then are eaten by other animals. Decomposers, including crabs, worms, and bacteria, tend to operate rapidly in the aquatic system by beginning to break down organic matter immediately after death or sometimes even before death.

The interaction of food chains produce food webs.      Food Webs

 

Natural Cycles

Mineral Cycles

      Each mineral or element within a mineral has a natural cycle that involves changing the min­eral from one chemical state to another. Some are necessary constituents of living tissue, while others accumulate in tissue and disrupt physiological processes. Most studies of natu­ral mineral cycles are conducted by ecologists, who observe relationships between the living and nonliving components of elements essential to life. Cycles of elements essential to life are called biogeochemical cycles. Mineral cycles are a part of the dynamics of an ecosystem.

Carbon Cycle      Carbon Cycle    

    Carbon is the major mineral component in coal, oil, and natural gas. All living organisms contain carbon; it is the basis of life on earth. Carbon is found in the atmosphere as carbon dioxide. From this reservoir it is used by green plants in the process of photosynthesis and some of it is returned to the atmosphere as carbon dioxide from plant respiration. Other carbon is passed along the food chain or forms carbon dioxide via the decay process. The carbon cycle is not only part of the terrestrial web of life, but also a part of the aquatic web as carbon dioxide diffuses in and out of both fresh and salt water. In the ocean, photosynthesis is confined to the light zone, where a large proportion of carbon becomes bound in the shells and exoskeletons of ocean invertebrates as calcium carbonate (CaCO2). When these organisms die, some of their body coverings are buried in the mud and sand and are thereby isolated from biological activity. Through geological time, some of these carbon deposits become part of coral reefs or limestone rock and some are slowly transformed into fossil fuels.

      Carbon dioxide is currently building up in the atmosphere. Some scientists believe the buildup is a result of burning fossil fuels; others feel the loss of forests which use carbon dioxide is the cause.

 

Oxygen Cycle     

      Oxygen is necessary for life, yet as a free gas it is toxic to organisms like anaerobic bacteria. It is freed from water in photosynthesis and reconstituted into water during plant and animal respiration. The oxygen cycle is quite complex because oxygen is highly reactive. We know how oxygen is necessary for the combustion of organic matter and that it is a component of water. It also circulates freely as carbon dioxide and combines with some metals, such as iron, to form oxides.

 

Nitrogen Cycle      Nitrogen Cycle

      Even though nitrogen makes up approximately 78 percent of the atmosphere, it is not usable by most living organisms in its atmospheric form (N2). Life processes generally require nitrate or some other nitrogen compound. A few microorganisms, including some bacteria, fungi, and the blue-green algae, can use at­mospheric nitrogen to form substances usable by other organisms. This process, called nitrogen fixation, also occurs to a small degree as a result of lightning or other electrical discharge, but it is a biological nitrogen fixation by microorganisms that provides most of the usable nitrogen.

      Some species of plants, primarily legumes (members of the pea family such as alfalfa, clover, and beans, live in a symbiotic relation­ship with nitrogen-fixing bacteria. The bacteria enter plant roots via the root hairs, where they are stimulated by secretions from the roots to grow and multiply and eventually form a swollen nodule. There the bacteria become immo­bile and carry on the process of nitrogen fixation. Soils low in nitrogen can have the nitrogen supply replenished by legume crops.

 

 

Succession        Succession            Fire and Succession

      Examining an ecosystem over a period of years shows that it is in a constant state of change. Population interactions, species composition, organic structure, and energy flow do not remain constant. Plants and animals are gradually eliminated and replaced by other species.

      Ecologists find that orderly, predictable changes occur in the populations of undisturbed ecosystems. This process of predictable change occurs over a period of time and is

called succession. The time sequence of succession is different for each ecosystem. As one community of animals and plants grows and utilizes a space, it gradually makes the area unsuitable for itself.

      Two forms of succession occur. Primary succession occurs in terrestrial areas where no soil exists. As plants become established, soil formation or deposition occurs, allowing other plants to grow. Rocks and cliffs are one example. Rock can be colonized by plants as soil accumulates in crevices. As plants spread out, more soil accumulates, allowing additional plants to become established. When pulverized rock is deposited by winds and water in the form of sand dunes, plants can become established. Communities change with time as the soil becomes established and continues to accept other plants.

      Secondary succession occurs when a disturbance to a community has caused it to revert to an earlier stage in succession. An old abandoned field provides a good place to study secondary succession. First, annual grasses colonize the area. After two or three years, perennial grasses appear; shrubs and a few trees follow ten to fifty years later. Finally, a forest develops.

      In western Oregon, the Oregon white oak develop dense forests at the eastern base of the Coast Range. Eventually, the canopy becomes so dense that the oak seedlings underneath cannot grow for lack of sunlight. As a result, Douglas fir seeds blown or carried into the woods begin to germinate without competition from the oak seedlings. Thus, a Douglas fir forest slowly replaces the oak forest. Eventually the Douglas fir chemically alter the soil so that their own seedlings cannot germinate. Then, depending on slope, moisture, and altitude, other trees become established. Thus, succession occurs as species within the system change their own environment. Such changes generally take hundreds of years. In any one physical environment, succession ends in a relatively stable climax community.

      A climax community can live so that its habitat remains suitable for continued reproduction by constituent species. It maintains a type of equilibrium or steady state, is highly organized and stratified, and has a net productivity lower than the earlier developmental stages because the organisms use most of the energy it assimilates. In a climax community, most energy needs are supplied by the organisms within it and losses of energy and nu­trients are minimal.

      This tells us something about the manage­ment of natural systems. If we remove energy in the form of food or wood products, we create a disturbance. For example, a community logged early in the successional stage will reestablish itself faster than one logged in a late or climax stage. Some changes are sudden and put stresses on ecosystems.  These disturbances move succession from one stage to the next.  All ecosystems will have patches of disturbance.  Fire succession is common in grasslands and some forests.  Conifer forests often need fire to germinate their seeds.

      Ecosystems are always changing in response to their environments changing.  This change in plants and other species is a normal process.  Ecologists classify these changes as primary or secondary depending on the environment at the beginning of the process.  Primary succession begins from an area that has no soil or bottom sediment such as a newly dug pond, a parking lot, cooled lava, or severe soil erosion.  Living communities need soil.  Hardy pioneer species such as lichens form soil by trapping air borne particles, producing organic matter and chemically breaking down rock. These pioneer species accumulate small amounts of soil that support microorganisms, insects and small invertebrates such as worms.  Their life cycle adds to soil development.  Eventually small perennials, herbs or ferns colonize the area and add to soil development.  These early succession plants, which are adapted to harsh conditions, continue to add to soil development until the soil is deep enough to store nutrients and water that will support mid-successional  plants.  These grass and low shrubs will be replaced by sun tolerant trees.  The areas soil and climate will determine the tree species. Suppressing fire succession in these fire-maintained forest can create conditions that alter their structure and physiology or function.  Changes in nutrient cycling following deforestation have been documented by field studies.

 

Biomes     Biome Maps

      The large recognizable communities in differ­ent parts of the world are called biomes. Biomes are the biological expressions of the interactions of organisms with the physical fac­tors in different regions of the world. Similar environmental conditions create similar biomes in different parts of the world.

      People can live almost everywhere, except in a few areas with extreme environmental conditions. Locations with ideal conditions for human living and food gathering are modified and developed to satisfy people’s desires. These regions are often altered to such an extent that the natural biotic region no longer exists. Although each biome is normally named for its climax community, it is composed of life in all developmental stages. We shall examine the interactions of abiotic and biotic factors in the environmental regions of North America with some reference to the rest of the world biomes.

 

Tundra

      The northern parts of the North American continent, Europe, and Asia are known as tundra. The tundra has a short summer and growing season and a very long, cold winter. The ground below the surface is frozen most of the year. In the summer, the top few inches of soil can thaw; however, the rest of the ground remains frozen (permafrost). During the short summer months, grasses grow quickly and at­tract a number of species of breeding birds and mammals. Insects also thrive abundantly during this time. Very few woody plants can live here because of the short growing season, so most of the vegetation is in the form of grasses, lichens, sedges, and dwarf willow. All of the plants must complete their life processes very quickly before the cold winter returns. There are two classifications of animals existing in the tundra: those present only in the summer and those resident throughout the year. A number of migratory birds, including many waterfowl, are attracted to these areas. The insects generally pass the winter as eggs or larvae, grow quickly, then reproduce.

      Because of the short growing season, the tundra ecosystem is very delicate. Disturbances are likely to take many years to heal. For example, tire ruts in the ground might remain for fifty or more years. This delicate system has been of great concern during the con­struction of the Alaskan oil pipeline.

 

Boreal Forest

      Moving south from the tundra, we find a large number of coniferous forests—mostly black spruce, white spruce, balsam fir, and tamarack. This biotic region exists throughout most of Canada, Scandinavia, and the northern part of the Soviet Union. Here we find the climate slightly warmer than the tundra with much more precipitation—about 38 to 102 centimeters. The frozen soil melts in the summer, allowing tree roots to penetrate more deeply and the soil to be more fully developed. This region is referred to as the boreal forest, or taiga in the Soviet Union. Conifers deposit litter on the soil which decays slowly because of the cold climate. The acid products from this decay are carried into the soil by rain or melting snow, making the soil relatively infertile for most crops. The growing season is also very short. Many migratory birds, some of the larger animals (moose, caribou, and wolverine), and the snowshoe hare live here.

 

Deciduous Forest

      Continuing south into the United States, Eu­rope, or the central Soviet Union, we come to deciduous forests. In the United States this region extends throughout the East and down into parts of the South. The domi­nant trees in a deciduous forest are oak, ma­ple, hickory, beech, and other hardwood. Most shed their leaves in the late fall and over winter in a dormant state. Precipitation is relatively high and distributed throughout the year. Rain­fall averages 76 to 152 centimeters per year, so there is an abundance of plant growth. The warm humid summers and cool winters encourage vegetation to become very dense. Animal species characteristic of this area are white-tailed deer, ruffed grouse, cottontail rabbit, red fox, raccoon, fox squirrel, and wild turkey.

 

Grassland or Prairie

      Between the eastern deciduous forest and the western desert is the grassland of the central United States.. Most continents have a similar biotic region in their central area. Grassland cli­mate is intermediate between the forest and desert with a relatively low rainfall. Wet and dry cycles often alternate for periods of several years. The summers are warm and winters cool.

      Several forms of grasses exist in this region. A prairie community maintained by periodic fires is dominant adjacent to forests, while shorter, less dense grass is found in the drier areas further west. The prairie has an ideal climate for crop production. The soil is rich in organic matter because litter from the grass de­cays in the upper soil. Minerals are not leached out because rainfall is light, under 76 centi­meters (30 inches) per year, and the leaching processes are retarded during winter. This is what people refer to as prime agricultural land. Animal life includes many grazing animals such as bison and pronghorn antelope.

 

Deserts

      Deserts are usually found where mountain re­gions block the flow of water (both as rivers and rain) into an area and where potential evaporation and transpiration exceeds rainfall. Desert-type areas also exist in continental interiors. The desert region of the southwestern United States has less than 25 centimeters of rainfall per year. The two main deserts are the high desert or Great Basin, ex­tending between the Rocky Mountains and the Sierra Nevada; and the low desert, including the Mojave in California and the Sonoran in New Mexico, Arizona, and southern California. The Great Basin vegetation is characterized by sagebrush, low shrubs, and often small coni­fers and junipers. The low desert has desert shrubs, creosote bush, and a variety of cacti. While most plants in the world reproduce each spring, taking their clues from daylight, desert plant reproduction is triggered by rainfall.

 

Chaparral

      The biotic region along coastal southern Cali­fornia is part of the chaparral biome and has a relatively stable climate throughout the year. This is referred to as Mediterranean climate since the area around the Mediterranean Sea is very similar. People consider the climate ideal, and they move in great numbers to this area. The chaparral is interspersed with bushes, trees and shrubs. Thoroughly dry in the summer, it receives most of its rainfall in the short winter season; thus, fire is common in the summertime. The many people now living in this region often have to fight huge fires, which are spread quickly up the canyons by the strong, dry Santa Ana winds. Some plants have evolved seed structures which open only following fires.

 

Tropics

      Tropical biotic regions exist in southern Mexico, Central America, and the northern part of South America, as well as in Africa and Asia, where rainfall exceeds 229 centimeters  per year. Instead of the usual four seasons, most tropical forests have two.We once thought tropical forests were impenetrable, but now know that they are very susceptible to human impact. Soil in many tropical regions is very poor because the vegetation contains most of the minerals and nutrients. When trees die or leaves fall, the min­erals are recycled into other living material quickly. Removing vegetation robs these areas of the nutrients necessary for the survival of plants and animals. Extremely varied habitats exist in the Central and South American tropics primarily because of the heavy rainfall, which can vary from 229 to 762 centimeters per year.

 

Forest Influence on the Environment

      There are three types of forests, depending on their climate:  tropical, temperate and polar.  If used sustainably (cutting and degradation does not exceed regrowth) and protecting biodiversity - forests are renewable resources. Currently forests are being fragmented and degraded almost everywhere, especially in the tropical countries.  Old growth forests are uncut, virgin forests and regenerated forests that have not been seriously disturbed for hundred of years.  Old growth forests provide ecological niches for many wildlife species.  Dead trees provide snags and habitats for many species.  Dead vegetation renews the soil and prevents erosion.  Most of the forests of U.S. and temperate areas are second-growth forests.  Some remain undisturbed long enough to become old growth forests, but many are not diverse forests but are tree farms.   These monocultures are harvested by clear-cutting.

      Forested watersheds act like sponges, slowing down run-off, absorbing and holding water that recharges surface and ground waters.  They regulate water flow, reduce sedimentation of water bodies and reduce soil erosion.  Forests influence local, regional and global climates.  Forests also are vital to the global carbon cycle, provide habitats for wildlife species, buffer noise, absorb air pollution and inspire the human spirit.  These ecological benefits; oxygen production, air purification, soil fertility, erosion control, water recycling, humidity control and wildlife habitat are valued at $196,250 over a tree's lifetime.  Sold as lumber, the tree would be worth $600.  The ecological benefits of complex, diverse forests are undervalued in the marketplace.  Long term ecological services are sacrificed for short term economical gain.

      Satellite scans and ground level surveys indicate that large areas of tropical rain forests are being cut and degraded.  About 40% of tropical deforestation is taking place in South America.  The rate of deforestation is even higher in Central America and Southeast Asia.

Tropical deforestation is a very serious ecological problem because these forests are home to more than 50% of the earth's land species.  Tropical forest products touch everyone's daily lives.  Hardwoods, food products, materials, prescription drugs and original strains of staple foods (rice, corn) come from the tropical plants.

      Tribal peoples and their cultures are vanishing as forests disappear for economic development.  Eliminating indigenous peoples is wrong and causes a tragic loss of earth wisdom and cultural diversity. Tropical deforestation results from population growth, poverty and government policies.  The process of degrading forests begins with roads cut by logging companies.  Shifting cultivators follow with subsidized cattle ranching taking over when the croplands are exhausted.  Torrential rains and over grazing further degrade the area into wastelands.  Clearing large areas of tropical forests with nutrient poor soil for cash crops also severely degrades these reservoirs of biodiversity.  Plantation export crops, mining, dam building and commercial logging degrade the forests.

      Fuel wood extraction is not a major cause in tropical deforestation but is a leading cause of deforestation in non-tropical developing countries.  Fuel wood shortages are common as a result of rapid population growth and poverty.  City dwellers' use of charcoal has expanded rings of deforested land around developing country cities.

      Experts have suggested ways to protect tropical rain forests, to use them sustainably and to use and restore degraded areas.  A detailed survey of forests has to be done with identifying "hot spots" of unique species in immediate danger.  Landless poor must be discouraged in moving to undisturbed forests.  Small-scale sustainable agriculture and forestry needs to be taught to new settlers.  Pressure to clear old-growth tropical  forests in various stages of secondary ecological succession.  Degraded forests and watershed need to be reforested and rehabited.

            Developing countries can reduce the severity of firewood crisis by planting fast growing fuel wood  trees, burning wood more efficiently and switching to other fuels.  Planting projects are most successful when local people participate in the planning and implementation  Women are major players in community environmental improvements.  In Kenya, Wangari Maathai started the Green Belt Movement in which 50,000 women and children planted trees for fuel wood and soil conservation.

      Most of the remaining old-growth forests in fragmented sections are in the U.S. northwestern states  These ancient forests are rapidly being destroyed at a rate faster than Brazil's Amazonian forests.  Fragmenting these forests into small patches or replacing them with tree plantations and second-growth forests, disrupts the vital network of symbiotic relationship that sustain old-growth forest communities.  The decline of the salmon population is linked to stream sedimentation due to excessive logging in the region.  Scientists have suggestions on how to protect and sustain both old-growth forests and forested related jobs.

      People the world over are working to protect forests.  The Rainforest Action Network followed by the Rainforest Conservancy has developed worldwide on college campuses and with grassroots persons.  The Kenya Green Belt Movement has been copied in many developing countries.  World leaders need to now develop an international agreement to curb deforestation, protect biodiversity and ecological services of forests, and help reforest degraded areas.

      Conservation biology deals with maintaining genes. Species communities and ecosystems that make the earth's biological diversity.  Natural processes and conditions such as energy flow and matter recycling need to be preserved (ecological integrity).  Practical ways of protecting ecosystems and preventing premature extinction of species differ from wildlife management which manipulates animal population sizes.

      There are two basic forest management systems:  even-aged and uneven-aged.  Even-aged, industrial forestry maintain trees at the same age and size.  Biologically diverse natural forests are replaced with simplified tree farms of one or two fast-growing species.  The trees are clear-cut and replanted with one or more species.  Uneven-aged management uses a variety of tree species maintained at many ages and sizes to foster natural regeneration.  The goals are biological diversity, long-term production of high-quality timber, reasonable economic return, and multiple use.  The rate of return owners expect on forest asset is the major factor in determining the type of management.  If no monetary value is assigned to ecological and recreational services of forests, short-term management appears more economically profitable.

      In healthy diverse forests, tree diseases and insect pest populations are controlled by multiple species interactions.  Some of the most destructive tree diseases such as Dutch-elm disease, Chestnut blight and White-pine blister were accidentally introduced into forests.  Some insect species have harmed tree farms or simplified forests.  Gypsy moth larvae eating leaves over several years weakens trees.

      Intermittent natural forest fires set by lightning are part of many forest ecological cycles.  There are different types of forests.  Surface fires burn undergrowth and leaf litter.  Some surface fires go underground and burn decayed vegetation.  U.S. Park Service allows most lightning-caused forests to burn themselves out as long as it doesn't threaten human lives, properties or endangered wildlife.  

      Timber harvesting has become the dominant use in most national forests because the Forest Service is allowed to keep most of the money from times sales.  Beauty strips along public access hides the extensive clear cutting of public lands.  National forest timber is often sold at prices lower than paid to private timber owners. According to the Worldwatch Institute, up to 60% of the wood used in the U.S. is wasted.  If waste of wood and paper products were reduced there could be less demand for national forest lumber.  Treeless paper is currently used in many countries that have depleted their forests.  U.S. consumers should put pressure on paper companies to make treeless paper products available.

      Parks everywhere are threatened.  In developing countries, local people invade to get resources, poachers kill animals and dams are being built.  In developed countries, popularity is the major problem.  Too many visitors to the parks and surrounding areas threaten the park systems.  Integrated management plans that combine conservation with sustainable development of park area resources look good but usually are inadequately  funded and enforced. 

      Most parks in the U.S. are too small to sustain themselves and be isolated from the harmful effects of nearby human activities.  The Park Services have conflicting goals:  to preserve park nature and to make nature more available to the public. National Park restoration is being tested with the partial restoration of the Florida Everglades.  This world's largest ecological restoration project is an attempt to undo and redo an engineering project that has been destroying the Everglades.  Projects like these need the support of citizens and elected officials over an extended period of time.  Clearly, prevention of environmental harm is cheaper and better.

      Conservation biologists say to keep biodiversity and ecological integrity from being depleted by human activities, a minimum of 10% of the world's land area should be protected.  Currently about 6% of the world's land area is protected in nature reserves, parks and wildlife refuges.  Biosphere reserves need to be set up in each of the earth's bio-geographic zones.  In reality, few countries are physically or politically able to set aside and protect areas as large as 10,000 square kilometers.  Wilderness areas need to be protected and managed.

 

The Changing of Life or Evolution     Miller & Urey - Life from Chemicals      Revolutionary Tree

       Evolution by natural selection is the most widely accepted explanation (high confidence theory)  for the origins of the plants, animals, and microbes populating Earth. The evidence for it is abundant and strong.  Nevertheless, evolution is also on of the most controversial scientific theories ever presented.  Part of the controversy over evolution stems from uncertainties about its mechanisms.  As we have seen, although Darwin postulated speciation by gradual changes accumulating over long periods of time, the fossil record supports the more recent view of punctuated equilibrium.  The debate over gradualism versus more abrupt processes continues to this day.  the debate centers on macro-evolutionary changes--those changes involved in the appearance of major new features and new classes of organisms.  Trying to deduce what happened over millions of years of evolution is like trying to solve a crime. We can see that it happened, but we have only a limited amount of events that brought it about.  

     Some of the evidence for evolution involves present-day processes and is less controversial.  This is microevolution--evolution within a species, or speciation itself.  Speciation can be observed at various stages in many species.  Natural selection by diverse components of environmental resistance can be evaluated for many organisms.  Also, the DNA of living organisms is a powerful argument for evolution;  closely related species show very minor differences in their DNA code sequences, while more distantly related species show greater differences.  Indeed, the DNA of a species can be thought of as a capsule view of its evolutionary past.   The base sequences of the DNA code of many groups of species are now yielding new information about the relationships between closely and more distantly related species. 

     In spite of the strong scientific support for the evolution, the theory continues to generate opposition from people who object to it on grounds that it conflicts with their religious beliefs.  There are, however, many people in the scientific and religious communities who have found that the controversy can be resolved without sacrificing either scientific integrity or religious faith.  This view holds that the scientific and religious explanations of origins are complementary.  Although this is an interesting topic, a thorough treatment is beyond the scope of the text.

     Whereas controversy over evolution is focused on what has happened in the past, it could be argued that a more important question facing us is what is happening now and what will happen in the future as life continues to develop over time.  As we continue to alter the surface of the Earth, move species from continent to continent, degrade and eliminate entire ecosystems, and drive a rising number of species to extinction, our choice is not whether we will change the future course of evolution.  We are already doing so.  The real alternative is whether we will simply chronicle the loss of more and more species as we pursue "business as usual" or whether we will choose to adopt a stewardship perspective, understanding that we have a responsibility to care for life in all of its diversity.

 

Conclusion

      The nonliving component and living organisms in a particular area function together as an ecosystem. Components of the ecosystem are unified through mineral cycles and energy flow. Human interference with the dynamic interrelations in ecosystems can lead to disrupted mineral cycles and broken links in the energy chain. People often change habitats, enabling organisms of different competitive abilities to become established. Maintenance of natural communities must be based on the principles of ecosystem dy­namics, community formation, and population interaction. Our knowledge of energy flow and ecosystem dynamics will help us control many environmental pollution problems and determine the most effective means for people to feed and improve their living conditions.