While energy decreases and becomes more dispersed at each step in the food chain, some substances become more concentrated with each link in a process that has become known as biological magnification. The dramatic increase in concentration of four dangerous substances in four different ecosystems is shown in table 4 -1. In each case the amount in the water or soil (that is, the general environment ) at the time of measurement was extremely small and certainly ‘’harmless,’’ but by the time the poisonous substances reached the end of the food chain, concentrations were high enough to cause sickness and death. Physical, chemical, and biological processes may all contribute to the concentration process. DDT may be absorbed through the skin of a fish as well as be ingested with food. It becomes concentrated in vertebrates because it tends to become stored in fat deposits. Radio active phosphorus is concentrated simply because of the natural tendency of biota to concentrate and store this scarce but vital element, as noted earlier in this chapter. Strontium – 90 concentrates in bone, where its tadiation may damage sensitive blood – making tissue because of its chemical resemblance to calcium. In other cases reason for concentrations are unknown. We must observe, experiment with and model the ecosystem level because neither laboratory experiment nor general theory are adequate for predicting the behavior of each of the thousands of new chemicals that man is constantly introducing . concentration factors tend to be higher in aquatic than in terrestrial system since water is a more ‘’dilute’’ medium than soil.

Biological magnification has come as a great surprise to physical scientists and technologists who were enthusiastic about the idea that ‘’the solution to pollution is dilution’’ or, in other words, the belief that poisons would be quickly lost in the vast confines of nature. Because predatory birds are being wiped out by DDT and man himself threatened (since he cannot escape being part of food chains) society has been forced to consider reducing or banning outright, this pesticide that was once heralded as the solution to all insect pest problems. Likewise, the difficulty of dispersing or containing large amounts of radioactive wastes without contaminating man’s food chain is one major reason why you cannot as yet enjoy the peaceful uses of atomic energy.

Since energy is an important common denominator in all ecosystems, whether designed by man, it provides a basis for that might be called a ‘’first – order’’ classification. Energy is always a major forcing function. The source a quantity of available energy determines to greater or lesser degree the kinds and numbers of organisms, and the pattern of functional and developmental processes – not to mention the life – style of man. Therefore, knowledge about the energetic of an ecosystem is always of key importance in understanding its properties.

An energy – based classification of ecosystems is outlined in table 2 -1 together with an order of magnitude estimate of the range of energy utilized in terms of kilo calories that flow through a square meter on an annual basis. Since different units (joules, calories, BTU’s, kilowatts and so on) are used by specialists in dealing with different forms of energy, and since the figures in table 2 -1 may not mean much to you in the absolute sense, this might be a good time to refer to appendix table 1 for explanations of units and for a list of convenient conversion factors. It is important to note that some energy units, such as the watt, have time built into the definition and are thus are thus energy time or power units. Other units, such as the calorie, represent potential energy (not time – specific): a unit of time must be added to convert these units to power rates. In this book we shall mostly use kilo calories (abbreviated: kcal) per day or per year to quantify energy flow. Thus, when we speak of ‘’power level’’ or use the term ‘’powered’’ we are referring to energy flow per unit of time. In order to compare various kinds of ecosystems we add a unit of area such as the square meter, acre, hectare, and so on.

        Pond ecosystem after a number of years of experimentation in the teaching of a beginner’s course in ecology we have found that a series of field trips to a small pond provide a good beginning for the ‘’lab’’ part of the course. A pond has a distinct boundary and is thus a recognizable unit in terms of both structure and function, even though it is not a closed system. Just as the pond frog is a classical type for the introductory study of the animal organism, so the pond itself proves to be an excellent type for the beginning study of ecosystems. A small pond ‘’managed’’ for sport fishing is the best type to start with because the number of species of organisms present is small, but almost any pond, even a marine employment ill do. The four basic components can be sampled and studied without be beginner becoming lost in too much detail. Furthermore, measurement of oxygen changes over a diurnal cycle provides a ready means of measuring the rate of metabolism and of demonstrating the interaction of autotrophic and heterotrophic components in the ecosystem as a whole.

samples ponds ecosystem        
The dissecting tool’s that the ecologist uses in his study of ponds are shown in figure 2 – 3A, and some of the laboratory apparatus needed for quantitative measurements are shown in figure 2 -3 B. in a class study, students may be grouped into teams, each of which is assigned to the job of sampling a major component or making a key measurement. One team, for example, dissects out the producers by taking a series of water samples with a special sampler that traps a column of water at any desired depth. Back in the laboratory, part of the water samples are filtered to concentrate the tiny phytoplankton organisms for microscopic study and counting. Another part of the samples is then passed through a very tight filter that removes all of the organisms the dried filter with the organisms is then placed in acetone to extract the chlorophyll and the field and laboratory tools uses an ecology class in the study of pond ecosystem. (A) field equipment including sampling devices for water, chlorophyll, plankton, bottom, fauna, and fish together with apparatus for measuring oxygen metabolism of the pond. (B) laboratory apparatus for further study of samples collected in the field.

       Other pigments. The resulting clear green solution can be laced in a photoelectric spectrometer to determine quantitatively the actual amount of chlorophyll and other pigments. The total quantity of chlorophyll in a water column, or in a community in general on an area basis (that is, per square meter), tends to increase or decrease according to the amount of photosynthesis. Therefore, chlorophyll per square meter (m2) is an index of the food – making potential at a given time, since it adjusts to light, temperature, and available nutrients. A general model for chlorophyll in ecosystems will be presented later in this chapter. Chlorophyll data can be used to estimate the living weight or biomass of producers, while the amounts of other pigments tell other stories should we wish to go more deeply into the study.

      Similarly, other teams obtain numbers, kinds, and weights for the consumer groups. Zooplankton, which are the small consumers associated with the water column, can be sampled by dragging a plankton net made of very fine – mesh silk through the water, fish can be sampled by seining, and small animals living on and in the bottom sediments can be quantitatively collected with a ‘’grab’’ built on the principle of a steam shovel. From these data a picture of the structure of heterotrophic populations is obtained.
ponds ecosystem model

As already emphasized, simply inventorying the components of an ecosystem does not tell us much about what goes on the system; for full understanding it is also necessary to make measurements of the rate of energy flow, the rate of nutrient exchange, and other functional properties. For example, oxygen changes in the water column can be measured as a means of assaying the metabolism. One way to do this is to suspend light and dark bottles in the pond to measure oxygen changes resulting from autotrophic and heterotrophic metabolism, respectively. A portion of a sample of water from each of several levels is placed in glass bottles. One or more bottles are covered with aluminum foil or black tape so that no light can reach the sample;  these are called the dark bottles, in contrast with the light bottles that have no such cover. Other bottles are ‘’fixed’’ with reagents immediately so that the amount of oxygen in the samples at the beginning of the experiment can be known. Then pairs of light and dark bottles are suspended in the pond at the levels from which the water samples were drawn. At the end of the 24 hour period the string of bottles is removed from the pond and the oxygen in each is ‘’fixed’’ by addition of a succession of the three reagents: manganous sulfate, alkaline iodide, and sulfuric acid. This treatment releases elemental iodine in proportion to the oxygen content. The water in the bottles is thus now brown in color; the darker the color the more oxygen. The brown water is then titated in the laboratory measuring the metabolism of a pond by the light and dark bottle method. Pairs of black and transparent bottles are lowered into position, while the plastic jug acts as a float to hold the pairs at the desired levels for measurement of oxygen changes   See text for explanation.

       By adding sodium thiosulfate (the ‘’hypo’’ used to fix photographs) until the color disappears. The volume of sodium thiosulfate needed can be calibrated to indicate the concentration of oxygen in milligrams of milliliters per liter ; milligrams per liter is also parts per million, anotherway in which oxygen content of water is expressed. This chemical method of measuring oxygen in water is known as the winkler method; it has been and continues to be the standard method, although newer electronic methods involving the use of oxygen electrodes offer advantages, especially where we wish to have a continuous record of change over time. 

       The decrease of oxygen in the dark bottles indicates the amount of respiration (that is, heterotrophic metabolism) in the water column whereas the oxygen change in the light bottles indicates the net photosynthesis (that is, net of photosynthesis and respiration); the two quantities added give an estimate of total photosynthesis or total food production for the 24-hour period, since oxygen production by green plants is directly proportional to fixation of light energy. One method of calculating the photosynthetic rate of the water column on a square meter basis is to average for each meter level and convert to oxygen per meter ( a simple shift of the decimal since milligrams per liter equal grams per cubic meter); the values for each meter level when added give an estimate of total oxygen production per square meter of pond surface. In the simplest case, if bottles had been placed at 0.5, 1.5, and 2.5 m deep, then each pair could be considered as sampling the first, second, and third cubic meter; the sum of these would give an estimate for a column 3m deep. Alternatively, a graph of bottle values plotted against depth can be constructed and the area under the curve used to estimate the column.

        A factor of 3.5 can be used as an approximate conversion of grams of oxygen produced to kilocalories of organic matter fixed by the plants in photosynthesis. This, if the accumulated change in oxygen in a square meter of water column was +3 g in the light and -2 g in the dark, the total, or gross, production would be 5 g, or 16.5 kcal/m2/day; of this amount 6 were used (that is, respired) by the plankton community, leaving 10.5 to be used or stored in the bottom of the pond. A cloudy day or organic pollution could result in more energy used than produced (that is, more oxygen consumed in the dark bottle than produced in the light bottle). Thus, measuring the oxygen metabolism provides not only an index of the energy flow, or power level, as discussed at the beginning of this chapter, but also an indication of balance between autotrophic and heterotrophic activity.

        Where phytoplankton density is very low, as in large deep lakes or the open ocean, the sensitivity of the light and dark bottle method can be greatly increased by adding a radioactive carbon tracer to the water in the bottles. After an interval of time the phytoplankton is removed by a filter is placed in a detector to determine the amount of radioactive carbon fixed (that is, transferred from water to phytoplankton). This method, which indicates the net photosynthesis, is widely used in oceanographic work. At sea it is not necessary to resuspend bottles in the water and stand by for 24 hours; the samples can be subjected to the light and temperature conditions of the sea on the deck of the ship as it moves to new sampling location. in another approach the whole pond can be considered as a dark and light bottle. If oxygen measurements are made at 2- or 3- hour intervals throughout a 24-hour cycle, a diurnal curve may be plotted that shows the rise of oxygen during the day when photosynthesis is occurring and the decline during the night when only respiration is occurring. The daytime period is equivalent to the light bottle and the night to the dark bottle. The advantage of this diurnal curve method is that photosynthesis of the whole pond including plants growing on the bottom (which would not be included in bottles) would be estimated. The difficulty is that physical exchange of oxygen between air and water and between water and sediments must be estimated to obtain the correct estimate for oxygen production of plants in the pond. Usually, the bottle methods give a sort of minimum and the diurnal curve a sort of maximum estimate.

      In addition to community structure and community metabolism, water chemistry is a third ecosystem property that should receive some attention in a class study. Recent advances in colorimetry and spectrophotometer have made water analysis relatively easy, assuming one has funds to purchase a colorimeter or spectrophotometer and ready – made reagents, or if one has access to a water chemistry laboratory. Even without such resources some idea of physical conditions of existence can be obtained with inexpensive water test kits of the type homeowners use to check out the water quality of swimming pools . in any event, the principle is the same; for a given substance to be inventoried, reagents are added to water samples to produce a color, the intensity of which is proportional to the concentration of substance in question. Nitrate nitrogen, ammonia nitrogen, and phosphate phosphorus are macronutrients well worth measuring. Temperature, PH, transparency (turbibity), and total alkalinity (hardness) are easy to measure and provide key information on the chemical state of nutrient elements and their availability to organisms.

         Concentrations of some of the metallic ions, such as iron, copper, zinc, lead, and chromium in the water, in bottom sediments and/ or in bottom organisms and fish are of especial interest as indicators of industrial pollution. Measurements of these elements in an unpolluted pond where concentrations should be very provides a good yardstick for assaying the condition of streams or ponds suspected of being polluted by the waste products of industrial and commercial operations. Comparison of number and variety of organisms in the two situations provides a means of assessing the impact on the pollutants in the biotic community; more about this later. Thus, a visit to polluted waters (which should not be hard to find these days ) is a natural follow – up to the pond study. The contrast can be educational, to say the least.

     
        solar energy is a very important factor in the world. Organisms at or near the surface of the earth are immersed in a radiation environment consisting of direct downward flowing solar radiation and long – wave heat radiation from nearby surfaces. Both contribute to the climatic regime that determines ‘’conditions of existence,’’ but only a small fraction of the direct solar component can be converted by photosynthesis to provide food energy for the biotic components of the ecosystem. Extra – terrestrial sunlight reaches the biosphere at a rate of 2 g- cal /cm2/ min. this quantity is known as the solar constant. Since the sun shines only for part of the day at any location, the amount coming in on a day or year basis is about half, more or less. On a square – meter basis this comes to about 14,400 kcal/ day or 5.25 million kcal/ year. This large flow is reduced exponentially as it passes through clouds, water vapor, and other gases of the atmosphere, so the amount actually reaching the autotrophic layer of ecosystems is on the order of 1.0 to 2.0 million kcal/ m^-2 year^-1       , depending on latitude, cloud cover, and 1 -5 percent of this converted to organic matter that structures and operates the solar – powered ecosystem.
solar energy radiation

       The sequence of energy of energy flow that we have just described, including further transfers to animals and man, is shown in the diagrammatic model of figure 3 -1. Quantities shown are much rounded off averages that are appropriate for a north temperate latitude such as midcontinent north America. On an annual basis we see how rapidly solar energy is lost into the heat ( I, ii, and so on in figure 3 –IB) as it passes through the atmosphere and the green belt. The organic food that plants are able to produce from sunlight is partly used by the plants themselves for their own maintenance and growth (with appropriate heat loss) and is partly passed on to the heterotrophy. In the diagram c1 represents the primary consumer or herbivore level and c2 the secondary or carnivore level. In the plant – animal portion of the energy flow chain about 80 to 90 percent of the energy is lost with each step. Or to put it another way, only 10 – 20 percent can be transferred to the next level. Thus, out of the millions of calories of solar energy coming into a column with a square – meter base, only a few hundreds are left to nourish a meat – eating, animal, or man. Two sets of figures are shown in the right – hand portion of figure3 – 1: (1) along the top of the line averages for biosphere as a whole, and (2) in parentheses below the line 10 times these figures for the favored for the favored solar powered ecosystem that receive supplemental energy.

       It is important to note that useful work is accomplished at each transfer, not just in the biological part but all along the chain. Thus, although we cannot eat much of it, or use it directly to run our machines, all of the incoming solar radiation is vital to the operation of the biosphere. For example, the dissipation of solar radiation (a and b, figure3 -1) as it passes into the atmosphere, the seas, and the green belts warms the biosphere of life – tolerable levels, drives the hydrological cycle and power weather systems. So delicate are the heat and other energy balances of the earth that meteorological models now show that only very small changes in the solar constant, or in the turbidity of the atmosphere (which would let more or less energy reach the surface of the earth) are needed to change the world’s climates. Just a little bit of decrease in heat brings on ice age, while a small increase brings on a tropical era, with a melting of all the polar ice raising the sea level to flood large areas of present continents. (Good – by new York and most of the world’s large coastal cities)

          In figure 3 -1 we introduce a symbolic ‘’energy language’’ which has been developed by Howard T. odum to facilitate communication between physical scientists and engineers on the one hand, and biologists and social scientists on the other (H. T. Odum, 1971,). In this and subsequent diagrams of this type circles signify energy sources, the sun in this instance. The heat sink symbol (I through v in figure 3 – 1B), shows where energy is lost in transformation from one form to another as required by the second law of thermodynamics. The heat sink symbol resembles an electrical ground symbol, but is one – way (as indicated by downward directed arrow). The bullet – shaped symbol represents an autotrophic system (or more broadly a unit capable of receiving Pire wave energy, such as light, and producing an energy – activated state, such as food, which can be deactivated to pass energy on to another step in a chain of energy flow). The hexagonal symbol represents a heterotrophic unit, or more broadly a self – maintaining component that is capable of receiving, storing, and feeding back energy received from an autotrophic, another heterotrophic, or another concentrated potential energy source. Additional symbols will be introduced in subsequent diagrams.
solar energy ultraviolet radiation

        The spectral, that is, the wavelength, distribution of sunlight is also altered as it passes through atmosphere, clouds, tater, and vegetation. The ozone belt of the upper atmosphere selectively absorbs the lethal short – wave ultraviolet radiation so that only about 10 percent reaches the earth’s surface on a clear day. Visible radiation (medium wavelength) on which photosynthesis depends is least attenuated as it passes through clouds and water, which means that photosynthesis can continue on cloud days, and at some depth in lakes and the sea (if they are not too turbid). Green plants sufficiently absorb the blue and visible red light that is most useful in photosynthesis and reject, as it were, the near infrared heat waves and thus avoid overheating. The long – wave infrared radiation, in general, which makes up the bulk of solar energy. Is absorbed and reradiated as heat in a complex manner by atmosphere, clouds, and various natural and man – made objects and surfaces. For more on these aspects, see gates (1963). Just because the world’s green belts convert only a small percentage of incoming solar energy to food energy, does not mean that they are inefficient actually. Photosynthesis is a very efficient process for tapping that small portion of sunlight that can readily be converted to high utility potential energy of organic matter.