THE BIOSPHERE AND ECOSYSTEMS

A. BIOSPHERE

- is that part of the earth (upwards at least to a height of 10,000m, and downwards to the depths of the ocean, and a few hundred metres below the land surface) and the atmosphere surrounding it, which is able to support life.

- is the term used to denote the collective ecosystems of the world.

B. ECOSYSTEM

An ecosystem is composed of all the living organisms in an area plus the surrounding physical environment with which they interact.

Any area of nature which includes living organisms and nonliving substances I interacting to produce an exchange of materials between the living and nonliving parts, e.g. a pond, lake, or forest. It comprises four constituents, abiotic substances, producers, consumers, and decomposers, and is the basic functional unit in ecology.

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 be itself. Population characteristics are frequently expressed statistically. The ecologist uses populations and communities to tell us more about the ecosystem.

Movement of Energy 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.

Photosynthesis

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. This is a summary of the equation for photosynthesis.

Carbon dioxide water

6CO 2 + 6H 2 0 sunlight

sugar oxygen

C 6 H 12 O 6 + 60 2

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 step, 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.

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. As discussed here respiration 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:

Sugar oxygen

C o H 12 0 6 + 60 2

Carbon dioxide water

6CO 2 + 6H 2 O + energy

Respiration takes place largely in another specialized organelle of the cell, the mitochondrion . When the glucose is broken down, energy-packaged in high energy bonds – is released. Plants use some of the bound energy for survival and pass the rest on to animals that eat the plants.

Most organisms use the free oxygen gas (02)) 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 (Figure). 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

Basic kinds of Ecosystems

Since energy is an important common denominator in all ecosystems, whether designed by nature or by man, it provides a basis for what might be called a “first- order” classification.

Ecosystems rely on two major sources of energy, the sun and chemical (or nuclear) fuels. Thus, we can conveniently distinguish between solar-powered and fuel-powered systems on the basis of the major input, while recognizing that in any given situation both sources may be utilized. It is important to note that although the total solar energy impinging upon the earth is enormous, solar radiation on an area basis is a dilute energy source, because only a small portion of that which falls on a square meter is directly usable by organism. In contrast, fuel may provide a highly concentrated source in terms of conversion to useful work within a small area.

The systems of nature that depend largely or entirely on the direct rays of the sun can be designated as unsubsidized solar-powered ecosystems ( category 1 in Table 1). They are unsubsidized in the sense there is little, if any, available auxiliary source of energy to enhance or supplement solar radiation. The open oceans, large tracts of upland forests and grasslands, and large deep lakes are examples of relatively unsubsidized solar-powered ecosystems. Frequently, they are subjected to other limitations as well, as, for example, a shortage of nutrients or water. Consequently, ecosystems in this broad category vary widely, but are generally low powered and have a low productivity, or capacity to do work. Organisms that populate such systems have evolved remarkable adaptations for living on, and efficiently using, scarce energy and other resources.

Although the “power density” of natural ecosystems in this first category is not very impressive, nor could such ecosystems by themselves support a high density of people, they are none the less extremely important because of their huge extent (the oceans alone cover almost 70 percent of the globe). From the human interest standpoint the aggregate of solar-powered, natural ecosystems can be thought of, and they certainly should be highly valued, as the basic life-support module which provide desirable stability and homeostatic control for spaceship earth. It is here that large volumes of air are purified daily, water recycled, climates controlled, weather moderated, and much other useful work accomplished. A portion of man's food and fiber needs are also produced as a by-product without economic cost or management effort by man. This evaluation of course, does not include the priceless aesthetic values inherent in a sweeping view of the ocean, or the grandeur of an unmanaged forest, or the cultural desirability of green open space.

Where auxiliary sources of energy can be utilized to augment solar radiation the power density can be raised considerably, perhaps an order of magnitude (that is, ten times), as indicated in Table 1. In this frame of reference an energy subsidy is an auxiliary energy source that reduces the unit cost of self-maintenance of the ecosystem, and thereby increases the amount of solar energy that can be converted to organic production. In other words, solar energy is augmented by nonsolar energy freeing it for organic production. Such subsidies can be either natural or man-made (or, of course, both). For the purpose of our simplified classification we have listed naturally subsidized and man-subsidized solar- powered ecosystems as categories 2 and 3, respectively, in Table 1).

A coastal estuary is a good example of a natural ecosystem subsidized by the energy of tides, waves and currents. Since the back and forth flow of water does part of the necessary work of recycling mineral nutrients and transporting food and wastes, the organisms in an estuary are able to concentrate their efforts, so to speak, on more efficient conversion of sun energy to organic matter. In a very real sense, organisms in the estuary are adapted to utilize tidal power. Consequently, estuaries tend to be more fertile than, say, an adjacent land area or pond which receives the same solar input, but does not have the benefit of the tidal and other water flow energy subsidy. Subsidies that enhance productivity can take many other forms, as for example, wind and rain in a tropical rain forest, the flowing water of a stream, or imported organic matter and nutrients received by a small lake from its watershed.

Man, of course, learned early how to modify and subsidize nature for his direct benefit, and he has become increasingly skillful in not only raising productivity, but more especially in channeling that productivity into food and fiber materials that are easily harvested, processed, and used. Agriculture (land culture) and aquaculture (water culture) are the prime examples of category 3 (Table 1), the man- subsidized solar-power ecosystems. High yields of food are maintained by large inputs of fuel (and in more primitive agriculture, human and animal labor) involved in cultivation, irrigation, fertilization, genetic selection, and pest control. Thus, tractor fuel, as well as animal or human labor, is just as much an energy input in agro-ecosystems as sunlight, and it can be measured as calories or horsepower expended, not only in the field, but also in processing and transporting food to the supermarket. As H. T. Odum (1971) has so aptly expressed it, the bread, rice, corn, and potatoes which feed the masses of people are “partly made of oil.” This is why fuel, or some comparable auxilliary energy, is vital to food production for man.

It is very important to note that recent increases in crop yield, the so-called “green revolution,” has resulted from genetic selection of plants, not so much for their ability to utilize solar energy as for their ability to benefit from fuel subsidies. Thus, what has been called in the popular press “miracle” rice and wheat are dwarf plants with small root systems and just enough leaves and stem to capture a maximum of useable solar radiation. Since man's fuel and chemicals do most of the work of protection and maintenance that a wild plant would have to do with an expenditure of its own energies, the crop plant is able to covert more of the sun energy into grain. It can do this because it is highly selected (that is, genetically programmed) to produce grain at the expenses of nonedible tissue. Pouring the fertilizer, or other subsidies, on a wild rice plant would not have such a great effect on grain yield since the wild plant would be programmed to use the additional resources for stalks and leaves as well as grain. Man's skill in augmenting the natural conversion of sun energy into food in this fashion parallels nature's own design and has, at least temporarily, staved off starvation in some parts of the world. However, the fuel-subsidized agro-ecosystem is not without its economic and pollution costs resulting from the heavy energy consumption; also, the high degree of genetic specialization produces an inherent vulnerability to disease. Whether fuel-subsidized food production and rising per capita expectations can keep up with world population growth is now the question.

`In Table 1 the productivity, or power level, of natural and man-subsidized solar- powered ecosystems are listed as the same. This evaluation is based on the observation that the most productive natural ecosystems and the most productive agriculture are at about the same level; about 50,000 kcal m -2 yr -1 seems to be the upper limit for any plant-photosynthetic systems in terms of continuous, long- term function. The real difference in these two classes of systems is in the distribution of the energy flow, as indicated in the previous paragraph; man works to channel as much energy as possible into food he can immediately use, while nature tends to distribute the products of photosynthesis among many species and products and to store energy as a “hedge” against bad times.

We now come to man's crowning achievement, the fuel-powered ecosystem (category 4, Table 1), otherwise known as the urban-industrial system, Here, highly concentrated potential energy of fuel replaces, rather than merely supplements, sun energy. As cities are now managed, solar energy is not only unused within the city itself, it becomes a costly nuisance by heating up the concrete, contributing to the generation of smog, and so on. Food a product of solar-powered systems is here considered to be an externality since it is largely imported from outside the city. As fuel becomes more expensive for man it is likely that interest in utilizing solar energy in cities will increase, so we can anticipate a new class of ecosystems, the “sun-subsidized, fuel-powered city” Also, man may find it prudent to develop a whole new technology designed to concentrate solar energy to a level where it might partially replace fuel, rather than merely supplement it. Only time will tell what should be man's best strategy for survival, but one thing seems certain; it will have to be based on a better partnership between man and nature than now exists.

The fuel-powered system, in contrast to natural sun-powered ones, is an incomplete or dependent ecosystem in terms of life support since it produces no food, assimilates very few wastes, and recycles only a small portion of its water and other material needs; and most of the energy that runs it comes from outside, often from great distances. Thus, an acre of a city requires not only many acres of agro-ecosystems to feed it, but even more acres of general life support, natural or seminatural environment to take care of the carbon dioxide and other large volume wastes and to supply it with high volumes of water and other materials. The per capita use of water, including irrigation, is something like 2000 gallons per day, of which 730 gallons are consumed (that is, not returned to streams or other sources). A city person also consumes a ton of woods products (paper, lumber, and so on) per year which requires from 0.3 to 1 acre producing) (depending on the intensity of forest management). These are just two examples of an affluent individual's impact on his environment.

To summarize, the stress that a high powered fuel system places on the adjacent lower-powered sun system is enormous. The power differential between them increases with the power level of the city since there is a sharp upper limit to the work capacity of any system powered only by dilute sun energy. The richer the city in terms of energy use the greater the area of life support that is required, a reality city planners and developers are often strangely unaware. It is no accident that all of the world's great industrial cities are located on coasts, large estuaries, large rivers, or fertile deltas where life-support capacity of the natural environment is high, or extensive, or both. As we become more concerned with land-use planning it is important to recognize that natural, self-sustaining solar- powered ecosystems have a direct value to man for their life support and waste assimilation capacities as well as for their food, fiber, or recreational potential. Any city that overtaxes its life-support module, or fails to preserve enough of it, can find itself caught in a vicious downward spiral of declining cost-benefits as costs of paying for what was once the “free work of nature” overrides the benefits of life in the city.