Energy Enters The Biosphere

The Light That Gives Life

All living things, without exception, must have energy if they are to survive, to grow, and to multiply. And all living things, with a few exceptions, obtain this energy, directly or indirectly, from sunlight. The few exceptions will be described in chapter 12. The guarded phrase "directly or indirectly" allows for the fact that only some life forms—green plants—are able to convert solar energy directly into chemical energy. Other organisms obtain their solar energy at second hand (or third or fourth, or ...) by eating the green plants that first captured it, or by eating the animals that ate the green plants, and so on back through the whole series of organisms forming a food chain. Food chains themselves are usually connected to other food chains by lateral and diagonal links, to make food webs; to keep the discussion clear of needless complications, in what follows we concentrate on single food chains.

Green plants, as everybody learns at school, are the factories that capture energy from sunlight in a series of chemical reac-

tions collectively known as photosynthesis; the green pigment chlorophyll always takes part in the reactions. Before going, rather sketchily, into the chemical details, it is worth considering the energy that drives them—sunlight.

Sunlight—more formally, solar energy—consists of electromagnetic waves; the way they convey energy is the topic of chapter 18. The point to notice here is that electromagnetic waves are not all alike; on the contrary, they have a tremendous range of wavelengths. The shortest waves reaching the top of the atmosphere from the sun in appreciable amounts are about 0.1 micrometers long, the longest close to 4.0 micrometers. A micrometer (symbol ||m; || is the Greek letter mu) is one millionth of a meter.1

About 90 percent of the solar spectrum lies between these limits. As we shall see, much of the radiation reaching the top of the atmosphere never penetrates to ground level. Nearly all that does is visible to humans; it constitutes the visible spectrum, the wave band of electromagnetic wavelengths to which human eyes are sensitive; it ranges from 0.40 |im to 0.71 |im.

The visible spectrum for humans, seen in its entirety, appears as white light. For human observers, different wavelengths produce light of different colors; the shortest ones appear violet, the longest, red. All this is well illustrated by a rainbow, which splits white light into its component colors or wave bands (how it all looks to other species is outside the scope of this book).

What is not so obvious when you look at a rainbow is the pronounced difference in energy content of the wave bands that constitute the different colors. The solar spectrum between 0.1 |im and 2.0 |im is shown in figure 11.1. The total area of the spindle-shaped strip represents the total energy, between these limiting wavelengths, received at the outer limits of the earth's atmosphere. The "tails" of the spectrum—at wavelengths less than 0.1 |m and greater than 2.0 |m—have been cut off to keep the figure compact. Consider the width of the strip, disregarding the shading; the width at each level represents the energy at the relevant wavelength, as shown on the scale at the left. It is obvious that a large proportion of the total energy (about 41 or 42 percent) is in the visible spectrum. Most of the incoming radiation with wavelengths outside the visible spectrum is absorbed in the atmosphere and never reaches ground level. The wave bands absorbed, or partly absorbed, are shown black (the few narrow wave bands in which absorption is only partial are not distinguished in the figure). The short wave, ultraviolet radiation, with wavelengths less than 0.34 |m, is absorbed high in the atmosphere, by oxygen, O2, and by the ozone, O3, of the famous ozone layer, which saves us all from being seriously sunburned. The long wave, infrared radiation, with wavelengths

Wavelength (|m)

"O

Red, orange Yellow, green Blue, violet

Figure 11.1. The solar spectrum at the top of the atmosphere (the energy tapers off gradually beyond the limits shown here). The width of the spindle-shaped figure at any level is proportional to the energy at that level; wavelengths in micrometers, |m, are shown on the vertical scale at the left. Wave bands absorbed or partly absorbed in the atmosphere are black. The photosynthetically active wave bands are stippled.

Figure 11.1. The solar spectrum at the top of the atmosphere (the energy tapers off gradually beyond the limits shown here). The width of the spindle-shaped figure at any level is proportional to the energy at that level; wavelengths in micrometers, |m, are shown on the vertical scale at the left. Wave bands absorbed or partly absorbed in the atmosphere are black. The photosynthetically active wave bands are stippled.

greater than 0.71 ||m, is absorbed lower in the atmosphere, mostly within 10 km of the ground, by carbon dioxide and water vapor, the two chief "greenhouse" gases.

This leaves only radiation in the wave band 0.34 to 0.71 |im unabsorbed and able to reach the earth's surface; most of the energy in this region of the spectrum is visible: only that between 0.34 and 0.40 |im is ultraviolet, invisible to humans but visible to bees.

Now observe the two stippled segments in the unblackened part of the spectrum. They represent the wave bands absorbed by the chlorophyll in green plants or, equivalently, the energy used in photosynthesis.2 Photosynthesis consists of a long, complicated series of chemical reactions, so it isn't surprising that the necessary radiation is not confined to a single narrow wave band. The two wave bands bearing the energy essential to nearly all life on earth are 0.40 to 0.50 |im (violet to blue) and 0.65 to 0.70 |im (orange to red).

The atmospheric absorption of sunlight we have so far considered goes on steadily, changing only gradually as the composition of the atmosphere changes; global climate change, whatever its cause, gives persuasive evidence to both scientists and nonscientists that the atmosphere is changing.

In addition, rapid fluctuations in the amount and kind of sunlight reaching any given spot of ground go on all the time. Think of the contrast between day and night, between winter and summer, between high latitudes and low. Think of the effect of clouds and haze. Natural aerosols—the water droplets in clouds and fog, and the myriad fine particles creating haze—dust, sea salt, volcanic ash, and bacterial spores—scatter sunlight as well as absorbing it. Changes in the brightness of the light, and in its spectral composition, affect the rate of photosynthesis. Surprisingly, the diffuse white light of a hazy day contains a higher proportion of photosynthetically active energy than does unobstructed sunlight, even though its total energy is less.3

The amount of usable energy reaching vegetation from the sun is difficult to measure directly because the rate at which it arrives is forever changing: it is most easily estimated by measuring the rate at which plants grow, known as their productivity.

Ecological Productivity

The chemical reactions of plant photosynthesis can be summarized thus: carbon dioxide + water vapor + solar energy ^ glucose + oxygen. This statement shows only the raw materials (to the left of the arrow) and the final products (to the right); it skips all the intermediate steps. Glucose is a simple carbohydrate; glucose molecules are the building blocks for a host of more elaborate carbohydrate molecules, among them cellulose, the most abundant organic chemical in nature and the one that, apart from water, constitutes the bulk of all plant material.

The individual reactions omitted from the statement are enormously complicated; fortunately they need not detain us. We are concerned with the capture of solar energy by the earth's vegetation, a process known as primary production because it is the stage at which the sun's energy is captured by living things for the very first time. The production of flesh by plant-eating animals—of beef by grazing cattle, for instance—is secondary production.

The energy required to produce one gram of glucose by photosynthesis is 15,650 J. A few words on units are necessary here. In the past (and to some extent still in the present), research workers studying ecological energetics, as it is called, used calories or kilocalories (1 kcal = 1,000 cal) as the unit for energy. As we saw in chapter 3, the relation between these units is 1 calorie = 4.186 joules, often rounded to 4.19 J. In traditional units, the energy required to produce one gram of glucose could therefore be given as 3.74 kcal.

Dietitians have confused matters by using the word Calorie (with a capital C) to mean kilocalorie. If you are told that your daily food intake ought to be about 2,000 Cal, the amount meant is 2,000 kilocalories, or approximately 8,400 kilojoules. If you ask someone who doesn't know the difference between Calories and calories, you might be told 2,000 calories (and you would starve). And if you ask by phone, the reply may be uninterpretable.

Because of this astonishingly ill conceived and carelessly used measuring system, it is much safer to follow the physicists and use joules (J) or kilojoules (kJ) as energy units.

The energy captured in one growing season by photosynthesizing plants is usually measured by harvesting the plants, drying them, and weighing the "dry matter." It is generally assumed that a gram of dry matter contains 0.45g of carbon, and that 10 kcal, or 42 kJ, of solar energy are captured per gram of carbon incorporated in the dry matter.4 This is equivalent to saying that each gram of dry matter represents 4.5 kcal of energy, or 18.9 kJ. The conversion factors are only estimates and should not be expected to give exact results every time; but if the factors are used repeatedly, overestimates and underestimates tend to cancel out.

The result obtained by harvesting, drying, and weighing the plants on a chosen area and then converting this measured weight to the energy equivalent gives an estimate of what is known as the net primary production (or NP1) for the vegetation in the environment concerned. In the symbol, NP stands for net production, and the 1 indicates that it is primary.

It is net in the sense that it doesn't represent all the energy the plants have absorbed. Plants need additional energy, first, because they must photosyn-thesize for immediate nourishment as well as for growth and reproduction and, second, because they need some of the sun's heat energy in addition to the light energy used to drive photosynthesis. We consider these two extra energy inputs in turn.

First the "extra" photosynthetic energy: while plants are putting on weight by growing new tissue, they have to keep themselves alive, and to do this they must consume a fraction of the weight they have put on instead of storing it as new growth. This they do in the process of respiration, which "burns" carbohydrate "fuel" to produce the energy needed to maintain life. The total energy absorbed, both that used for new growth and reproduction and that used for maintenance, is gross primary production (GP1). The whole business of growing and maintaining life while doing so can be likened to running up a "down" escalator. Gross production is represented by the sum total of the heights of all the steps climbed in a given time; respiration is represented by the distance the escalator descends during this time; net production is represented by the actual height the climber gains—equivalently, by the difference between the gain and the loss.

Consider now the second kind of "additional" energy plants require: heat from the sun. Although photosynthesis creates all the organic matter in a plant, the plant also requires water and some essential inorganic minerals. The plant absorbs soil water, with the minerals dissolved in it, through its roots. To keep the flow moving, the water entering via the roots is finally "exhaled" through the leaves, after giving up its dissolved minerals on the way. The "exhalation" is called transpiration: water vapor evaporates from the interior cells of the leaves, first through their thin cell walls into spaces within the leaf and then to the outside air via stomata, tiny perforations in the comparatively tough outer "skin" of the leaf. Note that the process entails evaporation, which requires heat energy from the sun.

The sun's electromagnetic radiation thus provides two kinds of energy for plant growth: light energy for photosynthesis and heat energy to keep the plants' water circulation going. The heat is as important as the light.5 The difference in the vegetation of different latitude zones is determined at least as

Tropical forest

Temperate forest

Boreal forest

Grassland

Grassland

Tundra

Tundra

108 kJ/ha/year

Figure 11.2. Typical productivities for five ecosystems. For each, the length of the whole bar represents the gross primary production, GP1; the stippled section represents the net primary production, NP1. Note the scale bar at the bottom representing 1.0 x 108 kilojoules per hectare per year (1 hectare = 100 x 100 m).

much by the sun's heat, which controls the water circulation rate, as by the sun's light, which powers photosynthesis.

Now back to the products of photosynthesis. The magnitude of the GP1, and the proportion of it stored as NP1, varies widely among ecosystems. Figure 11.2 shows some typical examples.6 The bars show the magnitudes of both GP1 and NP1, per square kilometer, for five ecosystems. The proportions of the GP1 stored as NP1 are almost the same (0.3) in the three types of forest, in spite of the markedly different climates they grow in. The proportions stored by the low vegetation of middle and high latitudes are twice as great: 0.6 in grasslands and nearly 0.7 in tundra. Life is slow in the cold.

The total quantity of solar energy captured in a year and temporarily stored by all the world's terrestrial vegetation has been estimated at about 1.9 x 1018 kJ.7 This is the terrestrial NP1 of the whole world. This energy is captured by 1,840 billion metric tons (dry weight) of plants, the world's entire "standing crop" of land vegetation.8 Note that the NP1 is easier to estimate than the GP1. It is only necessary to cut, dry, and weigh a sample of vegetation at the end of the growing season to determine the NP1 of that particular sample, whereas to find the GP1 it would be necessary, in addition, to estimate the amount of weight loss owing to respiration during the same season, a much more difficult task.

The final statistic to be examined here is the efficiency of photosynthesis. The question is what proportion of the solar energy absorbed by green plants is converted, by photosynthesis, into chemical energy. The answer depends on whether you measure the captured energy (the GP1) as a proportion of the available photosynthetically active energy, in the violet-blue and the orangered wave bands, or as a proportion of the available energy of all colors, regardless of usefulness, which is about twice as great. Measuring the energy in the GP1 as a proportion of the total energy received, photosynthetic efficiency is typically in the neighborhood of 1 percent and rarely greater than 3 percent.

Primary Productivity of the Oceans

The "vegetation" of the sea, as well as that of the land, takes part in converting solar energy to chemical energy by photosynthesis. As with land plants, photosynthesis requires light, water, and a source of carbon, but the carbon need not all come as carbon dioxide. Carbon dioxide does dissolve in water to some extent, but a source more useful to most marine plants is bicarbonate compounds dissolved in the water.

Two entirely different groups of plants grow in salt water; one group consists of plankton species, the other of seaweeds. Plankton is the collective name for the swarms of tiny organisms that live all or part of their lives floating in the ocean and drifting with the currents. The individual organisms of the plankton range from insubstantial jellyfish down to microscopic and submi-croscopic one-celled organisms including bacteria. A portion of them, collectively known as phytoplankton, contain chlorophyll and carry on photosynthesis, thus feeding themselves; they also serve as fodder for the zooplankton, the nongreen members of the plankton, which cannot feed themselves.

In the open ocean, practically all green plants are phytoplankton; the only notable exception is sargasso weed, a seaweed that floats at the surface far from land. Seaweeds in general, forming the second component of the marine vegetation, are confined to very shallow waters and, though not rooted, live attached to shoreline rocks, anywhere from the high tide line down to as far below the low tide line as sufficient sunlight penetrates. Seaweeds feed themselves by photosynthesis; many species don't look green (for example, kelps and rockweeds are brown and so-called Irish moss and dulse are purplish red) because the green of their chlorophyll is masked by other pigments.

The phytoplankton of the open ocean, most particularly the abundant phy-toplankton of the ocean's upwelling zones, is more than four times as productive as the seaweeds worldwide.9 The total NP1 of all the green plants in the world's oceans (phytoplankton and seaweeds combined) is about 1.1 x 1018 kJ

per annum, or slightly more than half that of all the terrestrial vegetation, even though the oceans cover 70 percent of the earth's surface.10

This is astonishing at first when you compare the total biomass—the dry weights of the "standing crops"—of the land plants and the marine plants doing the producing. Estimates of these quantities are 1,840 billion metric tons for land plants (as already mentioned), and 4 billion metric tons for marine plants.11 Consider the land:ocean ratios of these quantities. They are land plant biomass:marine plant biomass = 1,840:4 = 460:1

and land plant NP1:marine plant NP1 = 1.9:1.1.

Why the spectacular difference?

The reason is that land plants grow exceedingly slowly compared with plankton organisms, or plankters as they are conveniently called. Land plants have life spans ranging from one year (for annuals) to hundreds of years (for forest trees). Plankters have life spans of days or weeks at most. Therefore, when you look at an expanse of land plants in the fall, you see all the growth they have made in the growing season just finished in addition to what was already there when growth started in spring; in other words, the year's NP1 is all present before you—or nearly all. Some twigs may have broken, some leaves may have fallen, and some grass blades may have been grazed. But a sample of living plankters will contain only what has been produced in the past few days or weeks; the rest of the year's NP1 is missing—it is either already dead or not yet born.

This also explains why the biomass of terrestrial vegetation is so much greater—460 times greater—than the biomass of all living marine plants. On land, plant material accumulates and persists; at sea it is transitory and quickly disappears.

The volume of water inhabitable by phytoplankton is not so great as the huge volume of the oceans at first suggests. Sunlight penetrates only the topmost layer of the ocean, as we saw in chapter 6. The brightness of the light drops off at increasing depths below the surface, quickly at first and then at an ever decreasing rate.

Excessively strong sunlight inhibits photosynthesis. Therefore, going down from the surface on a sunny day, to begin with the photosynthetic rate increases as the light loses intensity. The rate reaches a maximum at the level where the energy of the sunlight has decreased to about half of what it is at the surface; the depth, in clear water, is between 2 and 3 m.12 At progressively greater depths, with the light growing dimmer and dimmer, the photosyn-thetic rate decreases rapidly.13 Below about 10 m, the light is entirely blue, all other colors having been absorbed (see chapter 6); the unabsorbed wavelengths are in the photosynthetically active wave band, however.

At the level where the energy in the sunlight has dwindled to about 1 percent of its full intensity, the compensation level is reached. This is the level where the rate at which energy is captured by photosynthesis balances the rate at which it is lost by respiration. It is the level where, in terms of the escalator analogy, the climber is running up at the same rate as the escalator is moving down. Consequently the NP1 is zero. The level may be deeper than 100 m in exceptionally clear water.

Now compare the volume of water occupied by photosynthesizing phyto-plankton in very clear water with the volume of air occupied by evergreen coniferous forest on fertile soil, given equal areas of the two contrasted ecosystems. Rather surprisingly, the volumes occupied turn out to be about the same. This is because their vertical extents are similar: photosynthesis takes place only in the topmost 100 m of clear ocean water, and the average height of full-grown coniferous trees is usually in the neighborhood of 100 m.

Before leaving the topic of primary production, it's worth noting that not all photosynthesis follows the formula for plant photosynthesis given at the start of the preceding section. The word plant used there is not redundant; it is used to distinguish the process from bacterial photosynthesis.14 This happens in deep, clear lakes, clear enough for some sunlight to reach the bottom. So-called sulfur bacteria must be present; their habitat is deep fresh water. Oxygen must be wholly absent—it poisons them. And hydrogen sulfide, supplied by decaying material in the mud of the lake bed, must be present. Given these conditions, the bacteria create carbohydrate (CH2O), producing sulfur (S) in place of oxygen (O2) as a by-product; the sulfur forms granules inside the bacteria before being used in further reactions.

Except that it depends on a slightly different form of chlorophyll, the pho-tosynthetic reaction appears similar to that in ordinary green plants, with sulfur taking the place of oxygen. The "crude" formulas for the two processes, showing only the initial inputs and final outputs, and concealing pronounced differences in the intermediate steps, are

CO2 + H2O + light energy ^ (CH2O) + O2 in ordinary plants and

CO2 + 2H2S + light energy ^ (CH2O) + H2O + 2S in sulfur bacteria.

Secondary Production: Energy Climbs the Food Chains

Solar energy captured and converted into chemical energy by photosynthesis has merely begun its journey through the biosphere. It is in the "bottom link" of innumerable food chains, along which are transferred the total energy requirements of all the world's animals.

The several links along a food chain are known as trophic (feeding) levels, and all the species in an ecosystem belong to at least one trophic level. All the world's "vegetation"—green plants including seaweeds plus phytoplankton— belongs to the first trophic level. The second trophic level comprises all the herbivores, the third level all the carnivores that eat herbivores, and the fourth level (when there is one) all the carnivores that eat the carnivores that eat the herbivores. Carrion feeders, or scavengers, belong to the level they would occupy if they killed their prey for themselves.

Here are two representative examples of four-link chains: on the Atlantic coast, seaweeds, forming level 1, are eaten by sea urchins (level 2), which are eaten by lobsters (level 3), which are eaten by humans (level 4). On the subarctic tundra, grass and seeds (level 1) are eaten by ground squirrels (level 2), which are eaten by weasels (level 3), which are eaten by golden eagles (level 4).

All this is elementary, and far too simplified to be useful in working out the energy budget of an ecosystem. In the first place, organisms at a high trophic level often eat food from several lower levels: humans eat lobsters and sea urchins and seaweed; golden eagles eat weasels and ground squirrels. It follows that humans and golden eagles—indeed, most "top" carnivores—belong to two or three trophic levels: they are often called (with some exaggeration) omnivores. It is even possible for a plant to occupy two levels. Some species of plants (mistletoe is an example) have insufficient chlorophyll to photosyn-thesize all the carbohydrate they need, and they obtain the rest of it by parasitizing other plants. Totally parasitic plants—for instance, coralroot orchids— belong squarely in trophic level 2. They are herbivores.

Rarely, a food chain loops back on itself. For example, a pitcher plant performs photosynthesis and so belongs to level 1. It also ingests the insects it captures, making it a member of level 3 when it consumes plant-eating insects and of level 4 when it consumes bloodsuckers like mosquitoes.

In working out the energy flow through ecosystems, all these fascinating minutiae have to be disregarded. In devising an energy budget for an ecosystem, trophic levels are the units considered, not specific groups of plants and animals. A particular trophic level, level 2 for example, does not consist of a specifiable group of animal species that are confined to that level. Rather, it consists of all the animals (and plants) whose food (energy) comes from level 1 in a single step; that is, herbivores together with omnivores (such as bears) that eat both plants and herbivores. Likewise, animals in level 3 derive their energy from level 1 in two steps; and so on. Recall that level 1 is where solar energy is first converted to the chemical energy that provides the energy in food.

The biomass belonging to level 1 in an ecosystem is the mass of all the pho-tosynthesizing plants in the ecosystem. The biomass belonging to level 2 is the sum of the biomasses of all the strict herbivores plus the proportion of the biomass in carnivores that they obtained by eating plants; and analogously for successively higher levels.

Once the data have been gathered, a flowchart patterned like that in figure 11.3 can be drawn, showing how energy is transferred in an ecosystem. Note that the flow is upward. The chart applies equally well to a terrestrial or an aquatic (marine or freshwater) ecosystem. Although in describing the chart we speak of material objects such as plants, animals, food, and leftovers, in every case these materials contain captured solar energy; whenever quantities of biomass are measured, they can always be converted to joules or kilojoules.

The bottom panel of the figure shows a year's events in trophic level 1. All the living organisms at this level (they are all plants) are represented by the grass in the drawing. The gross primary production yielded by the plants is shown by the heavily outlined box labeled GP1. Note that the plants producing the GP1, or most of them, are perennial and go on living to produce again in future years; the perennial parts are not part of the GP1. As we saw earlier, the GP1 has three components, shown by the small boxes enclosed in the large box: E1, the plant tissues eaten by animals at level 2 and higher levels; A1, the additions—new growth and offspring—gained by level 1; and R1, the loss due to respiration by level 1. Note that E1 and A1 combined amount to the net production of level 1, otherwise the net primary production NP1; also, that R1 + NP1= GP1.

Now consider the "crumpled" box, D1. It is the detritus (or "waste," or "litter") pertaining to level 1. It consists of old plant fragments such as fallen leaves, broken twigs and branches, and any other bits of plants that grew in previous years and became withered or detached this year; plus that fraction of the year's growth (A1)—leaves, twigs, bark, fruits, and seeds—left uneaten; plus what has been eaten but not assimilated by animals at higher trophic levels. In a word, it is level 1's leftovers and higher levels' feces, which contribute nothing to any level in this food chain.

Figure 11.3. Diagram of a three-level food chain (read It from the bottom upward). The code letters are followed by the level's number. The standing crops at levels 1, 2, and 3 are symbolized by the grass, the rabbit, and the fox. Arrows lead from each symbol to an outer, partitioned box, GP (the gross production) and a "crumpled" box, D. The energy of GP is used in three ways (the small, inner boxes): E, eaten by animals at a higher trophic level; A, added, by growth and reproduction, to the biomass of the level that produced it; and R, lost through respiration. D is detritus ("litter" or "waste"), destined to decay or burn in the future. Note that A + E = NP, the net productivity of the level, and that A + E + R = NP + R = GP.

Figure 11.3. Diagram of a three-level food chain (read It from the bottom upward). The code letters are followed by the level's number. The standing crops at levels 1, 2, and 3 are symbolized by the grass, the rabbit, and the fox. Arrows lead from each symbol to an outer, partitioned box, GP (the gross production) and a "crumpled" box, D. The energy of GP is used in three ways (the small, inner boxes): E, eaten by animals at a higher trophic level; A, added, by growth and reproduction, to the biomass of the level that produced it; and R, lost through respiration. D is detritus ("litter" or "waste"), destined to decay or burn in the future. Note that A + E = NP, the net productivity of the level, and that A + E + R = NP + R = GP.

Leftovers don't accumulate, however. They are the basis for other food chains, called detritus food chains, to be considered further in chapter 12. It's impossible to emphasize too strongly the importance of detritus food chains in cycling energy and materials through the biosphere. In any ecosystem, the bulk of all production winds up as detritus. In this chapter we will do no more than list some of the ingredients in the detritus of different levels.

Back to level 2 of the flowchart, the herbivores: the energy of the standing crop of herbivores (represented here by the rabbit) comes from NP1; in other words, all the energy at level 2 is one step removed from solar energy. This energy goes to the destinations shown: E2 is eaten by carnivores; A2 is the herbivores' new growth, in the form of both increased size and offspring; R2 is their loss from respiration. The detritus at this level, D2, consists of dead, not yet decayed herbivore bodies, shed herbivore parts such as deer antlers, molted hair and feathers, and the shed outer skins of growing invertebrates (crustaceans such as crabs and lobsters, metamorphosing insects, and many more); also in D2 is undigested herbivore flesh and bones in the feces of carnivores.

Level 3 (represented by the fox) is the top level in this ecosystem, which accounts for the absence of E3. In other respects, level 3 matches levels 2 and 1. All the energy at level 3 is two steps removed from solar energy.

An interesting exercise for any naturalist is to visualize the flowchart for an aquatic ecosystem, in either fresh or salt water. The diagram would look the same except for the pictured organisms representing each level. The ingredients of the detritus would be very different. It would also be apparent that marine ecosystems tend to have a larger number of trophic levels than terrestrial ones and (if quantities were being measured) that the amount of energy dissipated by respiration is greater in warm-blooded than in cold-blooded animals because the former have to generate heat to stabilize their temperatures.

Another contrast between marine and terrestrial ecosystems that would emerge is a striking difference between the relative proportions of "producers" (level 1) and "consumers" (all levels above 1) in the two kinds of ecosystems.15 In terrestrial ecosystems, the biomass of the consumers is only about one thousandth of the biomass of the producers—land vegetation. In marine ecosystems, the biomass of the consumers is about twenty times as great as the biomass of the producers—seaweed and phytoplankton.

The striking difference between land and sea in their ratios of producers to consumers is easy to visualize. About one-third of the world's land surface is forested, and another third is grassland; the biomass of all this vegetation is, understandably, orders of magnitude greater than the biomass of all terrestrial an imals. The seeming abundance of consumers relative to producers in the ocean is equally obvious to the mind's eye. Except in a narrow zone near shore, the producers consist of swarms of tiny phytoplankters floating in the illuminated surface waters and seldom affecting its transparency to any noticeable degree; most of the consumers are carnivorous fishes belonging to several trophic levels, with the individual members of each level generally outweighing those at lower trophic levels, which they prey on. The biggest animals are whales.

The reason for the difference between land and sea in the ratio of producers to consumers was explained above in a different context: recall that plant material on land, especially trees, persists for decades or centuries, growing bigger all the time; compared with trees, land animals—even bears and moose—are comparatively tiny and have life spans that rarely exceed twenty years. At sea this order is reversed: plankters have life spans of days or weeks, whereas fishes at the top of their food chains, and also whales, live and keep on growing for many years.

Last, we come to the efficiency of energy transfer from each trophic level to the one above it in an ecosystem. The subject has been studied in tremendous detail, and not surprisingly the studies have supplied a torrent of numerical measures of efficiency for numerous ecosystems. The efficiency of the transfer from level 1 to level 2, say, is defined as the energy in GP2 measured as a fraction of the energy in GP1, and correspondingly for transfers to successively higher trophic levels.

To summarize the results for terrestrial ecosystems, it seems safe to say that the efficiency at every step is roughly 10 percent or a bit more. If figure 11.3 were drawn to scale (it isn't) each GP box would be one-tenth the size of the one below it and ten times the size of the one above it. In marine ecosystems, efficiencies may be considerably higher, in some transfers possibly as high as 70 percent.16

Consider the efficiency with which trained, well-fed humans can perform athletic feats; the efficiency is said to be as high as 30 percent.17 Admittedly, we are dealing here with a few high-quality specimens of one species rather than a whole trophic level; the athletes tested would belong to trophic levels 1 and 2 if they were meat-and-vegetable feeders, or to level 1 if they were strict vegans. In any case, humans at their best appear to take up energy more efficiently than the average for other terrestrial organisms, but much less efficiently than some marine organisms.

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    How energy enters the biosphere?
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    Do ground squirrels living in shoreline eat urchins?
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