The Environment & Ecosystems
This lecture may be listened to by clicking
It will take several seconds to begin.
Degradation of Species
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.
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
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.
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
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.
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
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 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 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.
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.
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.
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.
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
The northern parts of the North American continent, Europe, and
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.