http://www.mansfield.ohio-state.edu/~sabedon/campbl54.htm
FLOW OF ENERGY THROUGH ECOSYSTEMS
(1) Ecosystem
(a) “An ecosystem consists of all the organisms living in a community as well as all the abiotic factors with which they interact.”
(b) Note that the boundaries of ecosystems are typically not arbitrarily defined, but instead are defined in some meaningful way: A pond, a field, a forest, etc.
(c) Ecosystems are typically understood in terms of
(i) Energy flow through ecosystems
(ii) Chemical cycling within (and through) ecosystems
(d) Note that both involve the movement of “stuff” through both biotic and abiotic components of the ecosystem
(e) DIAGRAMME: Movement of ‘stuff’ through the Ecosystem
(f) Ecosystems ecologists view ecosystems as energy machines and matter processors. By grouping the species in a community into trophic levels of feeding relationships, we can follow the transformation of energy in the whole ecosystem and map the movements of chemical elements as they are used by the biotic community.
(2) Energy flow
(a) Energy does not cycle through ecosystems but instead enters ecosystems and is used up within ecosystems
(b) Ultimately energy is lost from ecosystems primarily as waste heat, the most thermodynamically unavailable form of energy
(c) “Energy enters most ecosystems in the form of sunlight. It is then converted to chemical energy by autotrophic organisms, passed to heterotrophs in the organic compounds of food, and dissipated in the form of heat . . . The movements of energy and matter through ecosystems are related because both occur by the transfer of substances through feeding relationships. However, because energy, unlike matter, cannot be recycled, an ecosystem must be powered by a continuous influx of new energy from an external source (the sun). Thus, energy flows through ecosystems, while matter cycles within them.”
(d) Note that energy flows through ecosystems mostly as bonds between carbon atoms and bonds between carbon and hydrogen atoms, e.g., as one finds in carbohydrates and lipids; consequently, within and between organisms the carbon cycle and the flow of energy are quite similar, at least until the two are decoupled in the course of cellular respiration (i.e., the separation of carbon atoms from their energy)
(e) DIAGRAMME: “Pyramid of Life”
PRODUCTIVITY
(3) Primary productivity
(a) Only a small fraction of the sunlight striking the earth is converted to chemical energy by primary producers
(b) That sunlight energy that is converted to chemical energy, over a given period, is termed primary productivity
(4) Gross primary productivity
(a) Gross primary productivity is all of the light energy that is converted to chemical energy by producers
(5) Net primary productivity
(a) Net primary productivity is all of the light energy that is converted to chemical energy and that is subsequently stored by the primary producer (i.e., the gross primary productivity minus that employed to run the primary producer’s metabolism)
(b) The ratio of net primary productivity to gross primary productivity gives an indication of the cost of keeping the organism going, with large ratios indicative of relatively few costs (e.g., algae, ~50%) and smaller ratios associated with many costs (e.g., complex plants such as trees, ~10%)
(6) Biomass
(a) Net primary productivity is stored as biomass (dry mass of organisms)
(7) Standing crop biomass
(a) Standing crop biomass is another way of saying accumulated net primary productivity
(8) Limiting nutrient
(a) The productivity of an ecosystem is dependent on the primary productivity of the primary producers within that ecosystem
(b) Other than sunlight, primary productivity is limited by nutrient availability
(c) A limiting nutrient is that nutrient which is found in the lowest, relative concentrations such that an increase in this nutrient will increase primary productivity while a decrease in this nutrient will decrease primary productivity (this is equivalent to the concept of limiting reagent in chemistry)
(d) Typically, either phosphorus or nitrogen serves as a limiting nutrient within a given ecosystem, though water availability can (and often does) also serve to limit the primary productivity of an ecosystem
(9) Secondary productivity
(a) “The rate at which an ecosystem’s consumers convert the chemical energy of the food they eat into their own new biomass is called the secondary productivity.”
(b) Note that secondary productivity is dependent, in part, on the efficiency of transfer of chemical energy between trophic levels.
(c) The transfer between trophic levels, however, is typically not highly efficient because of inefficiencies involved in energy transfers in general, and the fact that the consumer must use acquired energy to respire (i.e., keep their metabolism going, reproduce, repair themselves, etc.)
(d) The more energy required to keep the consumer going (e.g., endotherms = ?warm blooded? = more versus ectotherms = ?cold blooded? = less), the less efficiently primary productivity will be converted to secondary productivity
(e) “Of course, the energy contained in the feces is not lost from the ecosystem; it can still be consumed by decomposers. However, the energy used for respiration is lost from the ecosystem; thus, while solar radiation is the ultimate source of energy for most ecosystems, respiratory heat loss is the ultimate sink. This is why energy is said to flow through, not cycle within, ecosystems.”
(10) Trophic efficiency
(a) Trophic efficiency refers to the transfer of energy up trophic levels, e.g., the ratio of secondary productivity to primary productivity consumed
(b) Trophic efficiencies generally range from 5% to 20%; that is, only 5% to 20% of primary producer biomass consumed is converted into new consumer biomass
(c) Note that trophic inefficiencies arise note just due to the second law of thermodynamics but because of inefficiencies in digestion (i.e., not everything is assimilated but instead is pooped out); in addition, it is always important when looking at food pyramids to keep in mind that not everything at the lower trophic levels is eaten, i.e., there is a reason that much of the terrestrial world is green, animals do not consume all of the plant material; on the other hand, there is a reason that many aquatic environments are not quite as green, animals do consume most of the planktonic photosynthesizers within aquatic systems.
(11) Pyramid of productivity
(a) A common way of illustrating ecological efficiency is via pyramids of productivity
(b) In these, productivity consumed is compared to productivity acquired, going up trophic levels, e.g., each level represents a drop of net productivity of approximately 90% (95% to 80%)
(c) Note that this is the reason that eating “lower on the food chain” is more consistent with being a good world citizen than eating higher on the food chain, i.e., vegetarians make a substantially smaller per capita impact on our planet than do meat eaters
(d) A generalization exists among ecologists that on average, about 10% of the energy available in one trophic level will be passed on to the next; this is primarily due to the 3 reasons given above. Therefore, it is also reasonable to assume that in terms of biomass, each trophic level will weigh only about 10% of the level below it, and 10x as much as the level above it. It also seems, however, that every time I go to measure, test, or model this assumption I run into an inconsistency, so take this generalization with a big grain of salt. Still, it comes in useful in terms of human diet and feeding the world’s population, consider this. If we all ate corn, there would be enough food for 10x as many of us as compared to a world where we all eat beef (or chicken, fish, pork, etc.). Another way of looking at it is this. Every time you eat meat, you are taking food out of the mouths of 9 other people, who could be fed with the plant material that was fed to the animal you are eating. Of course, it’s not quite that simple, but you get the general idea.
12) Biomass pyramid
(a) Similar to the pyramid of productivity, pyramids can be constructed using biomass
(b) Again, the variable associated with the primary producer is placed on the bottom with blocks associated with trophic levels stacked one upon the other
(c) Just as with pyramids of productivity, biomass pyramids can show dramatically decreasing biomass with increasing trophic levels
(d) However, this is not always the case and the reason for exceptions has to do with biomass pyramids being constructed from standing-crop biomass rather than from consumed-biomass data
(e) Consequently, aquatic biomass pyramids can seemingly be upside down if net primary productivity does not accumulate in the ecosystem within primary producers (i.e., primary producers are eaten as fast as they grow/reproduce)
(13) Pyramid of numbers
(a) Just as with productivity, total numbers of individual organisms tend to decline as one goes up trophic levels
(b) All else held constant, this decline is a consequence of ecological efficiencies being less than 100%
(c) A consequence of the pyramid of numbers is that top predator numbers tend to be small, thus making top predators both slow to evolve (also because they tend to be long lived and have long generation times) and relatively easy to drive to extinction
CHEMICAL CYCLING
(14) Chemical cycling
(a) “Chemical elements such as carbon and nitrogen are cycled between abiotic and biotic components of the ecosystem. Photosynthetic organisms acquire these elements in inorganic form from the air, soil, and water and assimilate them into organic molecules, some of which are consumed by animals. The elements are returned in inorganic form to the air, soil, and water by the metabolism of plants and animals and by other organisms, such as bacteria and fungi, that break down organic wastes and dead organisms.”
(b) Chemical cycles may be divided into two broad categories
(i) Those elements that have a gaseous form
(ii) Those elements that do not have a gaseous form
(c) We will consider particularly
(i) The carbon cycle
(ii) The nitrogen cycle
(iii) The phosphorus cycle
(15) Biogeochemical cycles
(a) “Because nutrient cycles involve both biotic and abiotic components of ecosystems, they are also called biogeochemical cycles.”
(b) The inorganic nutrients cycle through more than the organisms? they also enter into the atmosphere, the oceans, and even rocks. Since these chemicals cycle through both the biological and the geological world, we call the overall cycles biogeochemical cycles. Each chemical has its own unique cycle, but all of the cycles do have some things in common. Reservoirs are those parts of the cycle where the chemical is held in large quantities for long periods of time. In exchange pools, on the other hand, the chemical is held for only a short time. The length of time a chemical is held in an exchange pool or a reservoir is termed its residence time. The oceans are a reservoir for water, while a cloud is an exchange pool. Water may reside in an ocean for thousands of years, but in a cloud for a few days at best. The biotic community includes all living organisms. This community may serve as an exchange pool (although for some chemicals like carbon, bound in a sequoia for a thousand years, it may seem more like a reservoir), and also serve to move chemicals from one stage of the cycle to another. For instance, the trees of the tropical rain forest bring water up from the forest floor to be evaporated into the atmosphere? The energy for most of the transportation of chemicals from one place to another is provided either by the sun or by the heat released from the mantle and core of the Earth.
(16) The Carbon cycle
(a) The carbon cycle is an example of biogeochemical cycle in which the element (carbon) has a gaseous form, i.e., CO2, carbon dioxide
(b) Carbon dioxide is converted to organic forms of carbon in the Calvin cycle of primary producers
(c) Organic carbon is converted back to carbon dioxide during respiration
(d) Not all fixed carbon is converted back to CO2 over medium-term time scales since some ultimately is buried as oil, coal, or limestone (the latter is calcium carbonate)
(17) The Nitrogen cycle [ammonification, nitrogen assimilation, denitrification, nitrification, nitrogen fixing]
(a) The nitrogen cycle, like the carbon cycle, involves a gaseous form, i.e., N2 or nitrogen gas
(b) Nitrogen gas may be removed from the atmosphere, particularly by bacteria, in a process called nitrogen fixing [which is relatively expensive since nitrogen gas is quite stable]
(c) Nitrogen gas may be returned to the atmosphere, again particularly by bacteria, in a process called denitrification (a form of anaerobic respiration);
(d) More typically, bioavailable nitrogen is found as ammonium ion (NH4+), nitrate ion (NO32-), and various organic, nitrogen-containing compounds (e.g., amino acids and nucleic acids)
(e) Nitrate and ammonium ion are converted back and forth between each other (and nitrite, NO22-), also by various bacteria via processes termed nitrification and ammonification
(f) The nitrogen cycle thus involves
(i) Nitrogen fixing, the fixing of nitrogen from the atmosphere [typically by free-living or plant-associated nitrogen-fixing bacteria]
(ii) Assimilation, the uptake of ammonium ion and nitrate ion from soil by plants and the uptake of organic nitrogen by animals from plants (amino acids, nucleic acids)
(iii) Ammonification, the conversion of organic nitrogen back to ammonium ion by decomposers (nitrogenous waste) (4NH4+); ?The decomposition of organic nitrogen back to ammonium, a process called ammonification, is carried out mainly by bacterial and fungal decomposers.?
(iv) Nitrification, the various conversions of nitrogen within the soil from ammonium ion (NH4+ 4NO22- 4NO32- ; note that this represents an oxidation of nitrogen); ?Although plants can use ammonium directly, most of the ammonium in soil is used by certain aerobic bacteria as an energy source; their activity oxidizes ammonium to nitrite (NO22-) and then to nitrate (NO32-).?
(v) Denitrification, also by soil bacteria (4N2; note that this process involves the reduction of nitrogen); ?Some bacteria can obtain the oxygen they need for metabolism from nitrate (NO32-) rather than from O2 under anaerobic conditions.?
(18) The Phosphorus cycle
(a) Unlike the nitrogen and carbon cycles, the phosphorus cycle does not involve a gaseous phase
(b) As a consequence, phosphorus tends to cycle more locally rather than entering into the world-wide cycling seen with nitrogen and carbon
(c) The ultimate source of phosphorous is phosphate minerals that make up rocks
(d) The form in which phosphorus is available is as the phosphate ion (PO43-)
(e) Phosphorous is lost from ecosystems by erosion; e.g., The Grand Canyon, an impressive example of erosion in action
(f) Phosphorous can be gained by ecosystems, sometimes significantly, particularly via the movement of animals
(g) “After producers incorporate phosphorus into biological molecules, it is transferred back to the soil by the excretion of phosphate by animals and by the action of decomposers on detritus.”
(h) Note that phosphorus is transferred to (and between) terrestrial environments also by processes other than just bird pooping (i.e., guano) including the migration of various others animals such as salmon, which carry phosphorous from the sea back to their mother streams and on the way are eaten by such things as bears which, yes, do go on to poop in the woods:
(i) [Heterotrophs (animals) obtain their phosphorous from the plants they eat, although one type of heterotroph, the fungi, excel at taking up phosphorous and may form mutualistic symbiotic relationships with plant roots. These relationships are called mycorrhizae; the plant gets phosphate from the fungus and gives the fungus sugars in return.
