Ecology/Energy in ecosystems

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Chapter 14. Energy in Ecosystems

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An example of the concept of energy flow through trophic levels of a food chain.

There are several different factors that control the primary productivity of energy and biomass flow. Energy flow is the amount of energy that moves through a food chain. The energy input, or energy that enters the ecosystem, is measured in Joules or calories. Accordingly, the energy flow is also called calorific flow. In the study of energy flow, ecologists try to quantify the importance of different species and feeding relationships.

The largest source of energy for an ecosystem is the sun. Energy that is not used in an ecosystem is eventually lost as heat. Energy and nutrients are passed around through the food chain, when one organism eats another organism. Any energy remaining in a dead organism is consumed by decomposers. Nutrients can be cycled through an ecosystem but energy is simply lost over time.

An example of energy flow in an ecosystem would begin with the autotrophs that take energy from the sun. Herbivores then feed on the autotrophs and change the energy from the plant into energy that they can use. Carnivores subsequently feed on the herbivores and, finally, other carnivores prey on the carnivores.

In each case, energy is passed on from one trophic level to the next trophic level and each time some energy is lost as heat into the environment. This is due to the fact that each organism must use some energy that they received from other organisms in order to survive. The top consumer of a food chain will be the organism that receives the least amount of energy.

Hairston and Hairston (1993)^ , believe that there is a cause and effect relationship that results in any given trophic structure. Specifically, they state that it is trophic structure, rather than energetics that controls the amount of energy consumed at each trophic level and that "ecological efficiencies" are the product of a trophic structure, not a determining factor. Further, they state that trophic structure is instead the result of competition and predator-prey interactions. It is important to remember that many species may occupy each trophic level and are so subject to interspecific competition. This is especially true for producers, carnivores, and decomposers (Hairston, Smith, and Slobodkin, 1960)^ . Lastly it is not lost it is just consumed by its consumers.

Energy

The Sun is Most Common Source of Energy in Every Ecosystem

Energy is the ability to do work. Life manifests itself in energy changes, subject to the laws of thermodynamics. Ecosystems exist and operate by virtue of a flow of energy through the components of the system and thermodynamics (the movement of energy) forms the very basis of the biosphere organizing principles introduced in Chapter 2. Before proceeding into the relationship between ecology and thermodynamics, it is necessary to build a basic understanding of the physics of energetics, simply a further demonstration of the fact that ecology is multidisciplinary, requiring of its students a broad knowledge in all sciences.

• Read Energy - Follow links and cover details only as necessary to gain a basic understanding

• Read Laws of Thermodynamics - Read the expanded articles as needed to understand these basic principles of energy changes
• Read Principles of Energetics - You will find a constellation of related articles, including several with an ecological focus. You may wish to visit all of these to foster a deeper understanding

Although several sources of energy are available for exploitation on earth (e.g., geothermal, nuclear decay), the most significant is solar energy. Light and other radiation streaming out from the sun strikes the earth 93 million miles distant, providing energy to the atmosphere, the seas, and the land, warming objects that absorb this energy; that is, radiant energy is converted to heat energy (molecular motion). Differential heating causes winds and currents in the air and water, the heat energy becoming kinetic energy of motion. Warming results in evaporation of water into the atmosphere, setting up the hydrologic cycle (Chapter 4), the lifting of water into the atmosphere becoming potential energy that will convert to kinetic energy when the water begins to flow back downhill. However, the most significant solar energy driven process with respect to living systems is that of photosynthesis. Light energy is converted by photosynthesizing cells into a form of potential energy held in the chemical bonds of organic compounds. Organisms require both the substance and the stored energy of chemical compounds to function and grow, and eventually reproduce. Substance to provide the building blocks of cellular (and extracellular) components that comprise structure, and energy to move substances around, affect chemical reactions, and carry out all manner of intracellular and organismal processes.

Solar Constant

The Solar Constant is the average amount of radiant energy from the sun that reaches the Earth's atmosphere. This value is calculated at 2 calories per minute on each square centimeter of the Earth's upper atmosphere. This value can change because of Earth's elliptical path effecting seasonal changes and differences in the northern or southern slope which effect magnitude. The net radiation is what is left after some of the energy is reflected by the Earth's surface. Calculations for the solar constant are done by using the astronomical unit (AU) which is the mean distance between the Earth and Sun; One (AU) is equivalent to 92,960,000 miles (149,604,970 km).

Movement of the air and evaporation are important factors that regulate the temperature of the Earth from the sun's energy. The movements of air allow energy to be given off into space and without this reflection of energy the Earth would rapidly overheat and life would extinguish. This interaction is also a very important to the maintenance of the Earth's polar ice caps. A decrease in the solar constant of 2-5% would be enough to create a second ice age. Of course, as has been shown, the increasing use of industry by man, can increase earths temperature. If it were enough to cause the ice caps to melt, there would be an increase of approximately 7 degrees C. This is due to the fact that the polar ice caps help in reflecting back the Earth's radiation. This reflection of the solar radiation is an essential part of the maintenance of the current climate that the Earth maintains.

Organisms' Role in the Flow of Energy

A simplified overview of the process of photosynthesis.

When it comes to the flow of energy in ecosystems there are two types of organisms: producers and consumers. Plants are a common example of producers in all populations. They are able to convert carbon dioxide into oxygen and glucose, a common sugar consumed by most organisms. They do this through a process called photosynthesis which allows the plants to use sunlight as a source of energy. Producers convert energy from the environment into chemical energy in the form of carbon to carbon bonds. A classic example is the one previously mentioned where the plants convert CO₂ to O₂ and glucose.

The second type of organism is the consumer. Consumers are unable to make chemical energy the way plants do and have to use metabolic processes to get energy from carbon to carbon bonds, which is called respiration. Respiration breaks the carbon to carbon bonds and combines them with oxygen to make carbon dioxide. The energy released is used to help organisms move their muscles or as heat. Energy cannot be reused once it has been lost.

Read:

• Energy Flow Through the Ecosystem
• Ecosystems Background Information
• Ecology

Measuring Energy Flow

Flow diagrams are a visual way to understand how energy is transferred and/or transformed. A simple flow diagram usually starts with a primary producer[c]==> herbivore[c]==> predator[c]==> and so on. They can be as simple as that or as complex as the complete CO2 cycle. The symbol [c] is the consumption rate of that species. The consumption rate between forcing functions is denoted by r, (r=[c]/t). The forcing function, which is the flow of energy between species, is usually measured in kcalmol^-1year^-1. The primary producer efficiency is measured by (NPP/Total consumed)x 100. Secondary production efficiency is measured by (NSP/Total consumed)x 100.

For more detailed examples visit: Energy Flow of Organisms

Primary and Secondary Production

The Net Primary Production(NPP) is the amount of carbon fixed per unit time (usually per day). Measures of net primary production include:

1. change in dry biomass per unit time
2. calorimetry
3. carbon dioxide uptake over time
4. oxygen uptake over time (comparison of the plant's light cycle to its dark cycle)
5. change in chlorophyll-a concentration, which equals the concentration of carbon dioxide fixed per day.

Ecologists look at dry mass because the high variability of water content makes accurate measurements difficult. Essentially about 95% of the dry mass is carbon and about 99.9% of the global biomass is plants (which includes algae, cyanobacteria, and other organisms that contain chlorophyll).

Global oceanic and terrestrial photoautotroph abundance, from September 1997 to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it.

There are several different things that can limit NPP Terresterial limiting factors include:
(1) water/precipitation,
(2) sunligh/temperature, and
(3) nutrients.

Aquatic limiting factors include:
(1) sunlight/temperature, and
(2) nutrients.

The NPP of aquatic ecosystems has been vastly affected by global warming. Global warming is the primary issue due to human impacts and gas emissions. Projected increases in carbon dioxide and temperature for the next 50 years exceed the amounts in which great coral reefs have flourished over the past half-million years. Coral reefs are at a serious decline due to temperature increases and coral bleaching. ..

Two images showing the relationship of water temperature to w:coral bleaching along the Great Barrier Reef. Warm pink and yellow tones show where sea surface temperatures were warm in the top image. The warmest waters are the shallow waters over the reef near the coast, where coral bleaching was most severe in the summer. The lower image shows chlorophyll concentrations, where high concentrations (yellow) generally point to a high concentration of phytoplankton in surface waters of the ocean. In this image, the bright yellow dots actually represent the coral reefs, and not surface phytoplankton.

An estimated 30% of coral reefs are severely damaged and about 60% of coral reefs may be lost by 2030. Coral bleaching not only affects the coral reefs, but the diverse collection of species that interact with each other within the coral reef ecosystem. An example of this is the increased consuming of corallivorous fish on recently bleached coral (Pratchett et al. 2009). Corallivorous fish were found to increase stress on recently bleached coral due to increased feeding. The increased feeding on already stressed corals may have contributed to the mortality of bleached coral. The Corallivorous fish however, only increased consuming for the early stages of coral bleaching, and may be a good indication of coral mortality. Coral bleaching affects reefs at a regional and global scale, and has a huge impact on the NPP of coral reef ecosystems.

The net primary production is the main factor that limits the amount of net secondary production. Net secondary Production (NSP) determines the amount of biomass consumed and calculates the energy content of the consumed biomass. It also determines the amount of biomass and/or energy assimulated that is equivalent to an increase in the biomass of the consumer. In this case, a growth in biomass can be defined as a growth in individuals or growth in populations that includes reproduction. Most of the energy lost in secondary production comes from maintenance respiration. Secondary producers will consume oxygen and produce heat, which results in fairly inefficient production. Endotherms have a mean efficiency of about 2% and ectotherms have a mean efficiency of about 10%. There is a loss of energy due to respiration of 98% and 90%, respectively. All other organisms have a 30%-40% efficiency rate. All animals that live off plants(herbivores) are included in this group of secondary producers.

Limiting Nutrients

Excess Nitrogen can cause ill affects such as toxic algal bloom

As mentioned above nutrients can play a role in limiting the production of a community. Other than sunlight, primary productivity is limited by nutrient availability. A limiting nutrient is a 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. Typically, either phosphorus or nitrogen serves as a limiting nutrient within a given ecosystem, though water availability can also serve to limit the primary productivity of an ecosystem. Trace nutrients such as molybdenum and zinc are necessary for the growth of plants and can act as limiting agents even though they are in very small quantities. Aquatic communities can also have limiting nutrients that control how much production takes place. In freshwater ecosystems net primary production is limited by phosphorus, and in the ocean the limiting nutrient is iron.

Read: Limiting Nutrients

Too much of a nutrient can also have a limiting impact on a community. Recent studies have shown that excess nitrogen from human activities such as agriculture, energy production, and transport have begun to overwhelm the natural nitrogen cycle. The effects of the extra nutrients reach every environmental domain, threatening air and water quality and disrupting the health of terrestrial and aquatic ecosystems. In terrestrial ecosystems, nitrogen saturation can disrupt soil chemistry, leading to loss of other soil nutrients such as calcium, magnesium, and potassium. This means that while the nitrogen is not a limiting factor, it causes other nutrients in the soil to become limiting factors.

Read: Nutrient Overload

The Laws of Thermodynamics as they relate to Ecology

The laws of thermodynamics are fundamental concepts to chemical processes of the universe. They are extremely important in chemistry and physics, but are the basis for many biological concepts as well. The laws dictate how energy can be transported, which of course can be applied to ecology because energy transfer is what drives metabolism and, on a larger scale, food chains and food webs.

Zeroth Law of Thermodynamics

Since the temperature decreases going from block A to block C, assuming that all blocks are in thermal contact, heat energy will flow from the left to the right until all blocks are of equal temperature.

Zeroth Law of Thermodynamics is the most obvious of all three laws, simply stating that if the temperature of object A is equal to the temperature of object B, and the temperature of object C is equal to the temperature of object B then the temperature of object C equals the temperature of object A. Although this transitive property seems almost unnecessary to mention, it is of crucial importance to the subject of energy transfer in the form of heat energy, since temperature is a measure of heat. Its implications in ecology are obvious as well. Organisms require the means to survive in a climate of a certain range of temperatures, and evolution has created organisms with extremely different tolerances to temperature according to where they are located.

First Law of Thermodynamics

The First Law of Thermodynamics states that the total inflow of energy into a system must equal the total outflow of energy from the system, plus the change in the energy contained within the system. That is, energy is neither created nor destroyed, but may transform from one type to another. This is relevant to food webs in that the amount of energy being transferred through the food web cannot be larger than the amount of energy initially supplied by the primary producer (which was supplied by the sun's energy).

Second Law of Thermodynamics

The Second Law of Thermodynamics states that "energy of all kinds in our material world disperses or spreads out if it is not hindered from doing so." In other words, the randomness (entropy) of the universe is always increasing. This is quoted from a site devoted to demonstrating how the second law is involved in our open system of the earth and sun: http://www.entropysimple.com.

The second law of thermodynamics is definitely the most applicable of the four laws to ecology. It is consistent with Elton's Pyramid of foodwebs that states that although sometimes total size or number of organisms can either increase or decrease with increasing trophic levels, the total biomass ALWAYS decreases with increasing trophic levels, as energy is constantly being lost to the atmosphere (usually as CO₂. The energy from the sun allows living organisms on earth to temporarily decrease entropy, but our organized systems require an overall input of energy (provided by the sun). The second law of thermodynamics suggests a dire "heat death of the universe" will occur when all the energy of the universe is evenly distributed, and no life or concentrated matter (stars, galaxies) can exist, although this won't occur for at least 10¹⁰⁰ years.

Semipermeable membranes are prevalent in many organisms and are a way that tendency toward entropy is manipulated to create a gradient. Such gradients can allow thermodynamically favorable reactions to occur yet maintain an organized structure.

Ecosystems are far from thermodynamic equilibrium, which used to be an argument against the second law of thermodynamics. Galucci (1973) performed a literature review of the physical theory of thermodynamics in relation to mechanisms of energy transfer in the environment (passive and active) and ecosystem productivity. He also studied how community structure and diversity related to entropy. As a result of his research, he claimed that the earth is a "receiver, reflector, and degrader of energy." The movement of energy through a community structure is necessary for its maintenance. External energy from the sun provides primary producers with energy and with a range of temperature in which life in the community is possible. Internal energy that travels from primary producers to organisms higher in the food chain is in the form of metabolism (mass, bonds) which provides transferrable energy. Galucci claimed that diversity is a form of entropy and improves the stability of a community. His conclusion was that the hypothesis of the second law of thermodynamics being applicable to ecosystems is supported.

Hedin et al. (1998) studied biogeochemical processes involving nitrogen at soil to stream interfaces from a thermodynamic point of view. More than 1400 subsurface water samples from Michigan wetlands draining from a mixed forested-agricultural landscape were observed in this study. Thermodynamic principles could predict what form of nitrogen would be available to microbes if the number of electron donors and acceptors (water pH) is known. Microbes will transform nitrogen from NO₃^- to ammonia in acidic conditions present in shallow water, but covert NO₃ to N₂O gas in basic conditions present in deeper water. The findings agreed with this theory and were helpful because oxidizable carbon can be added to the shallow portions of the water that are not sufficient at denitrification. These findings would not have been possible (or understandable) if a thermodynamic approach to the metabolism of microorganisms had not been considered.

Third Law of Thermodynamics

The Third Law of Thermodynamics simply states that as temperature of a system reaches absolute zero (0 K), the entropy of the system decreases. Technically, it states that a system at absolute zero is at zero entropy, but this is theoretically not possible as it has been established that absolute zero is not able to be experimentally reached. This is why substances become gases (molecules spread out, entropy increased) at high temperatures, and freeze (become ordered crystalline structures, entropy decreased) at low temperatures. Decomposition also has a higher rate at higher temperatures for this reason.

Ecological Pyramids

An Ecological Pyramid (or Trophic pyramid) is a graphical representation designed to show the relationship between energy and trophic levels of a given ecosystem. Most commonly, this relationship is demonstrated through the number of individuals at a given trophic level, the amount of biomass at a given trophic level, or the amount of energy at a given trophic level. It is worth noting that all Ecological Pyramids begin with producers on the bottom and proceed through the various trophic levels, the highest of which is on top.

Pyramid of Biomass

An Ecological Pyramid of Biomass shows the relationship between energy and trophic level by quantifying the amount of biomass present at each trophic level (dry mass per trophic level). As such, is assumed that there is a direct relationship between biomass and energy. By doing this, the earlier discrepancy is avoided because even though there is only one tree, it is much more massive than the next trophic level.

The main problem with this type of Ecological Pyramid is that it can make a trophic level look like it contains more energy than it actually does. For example, all birds have a beak and skeleton, which despite taking up mass are not eaten by the next trophic level. In a Pyramid of Biomass, the skeleton and beak would still be quantified even though it does not contribute to the overall flow of energy into the next trophic level.

Pyramid of Energy (Ecology)

An Ecological Pyramid of Energy is the most useful of the three types, showing the direct relationship between energy and trophic level. It measures the number of calories per trophic level. As with the others, this graph begins with producers and ends with a higher trophic level.

When an ecosystem is healthy, this graph will always look like the standard Ecological Pyramid shown at the top of the page. This is because in order for the ecosystem to sustain itself, there must be more energy at lower trophic levels than there is at higher trophic levels. This allows for organisms on the lower levels to maintain a stable population, but to also feed the organisms on higher trophic levels, thus transferring energy up the pyramid.

When energy is transferred to the next trophic level, only 10% of it is used to build bodymass, becoming stored energy (the rest going to metabolic processes). As such, in a Pyramid of Energy, each step will be 10% the size of the previous step (100, 10, 1, 0.1, 0.01, 0.001 etc.).

The advantages of the Pyramid of Energy:

• It takes account of the rate of production over a period of time because each rectangle represents energy per unit area / volume per unit time. An example of units might be - kJ/m2/yr.
• Two species weight for weight may not have the same energy content therefore the biomass is misleading but energy is directly comparable.
• The relative energy flow within an ecosystem can be compared using pyramids of energy; also different ecosystems can be compared.
• There are no inverted pyramids.
• The input of solar energy can be added.

The disadvantages of the Pyramid of Energy:

• The energy value for a given mass of organism is required, which involves complete combustion of a sample.
• There is still the difficulty of assigning the organisms to a specific trophic level. As well as the organism in the food chains there is the problem of assigning the decomposers and detritivores to a particular trophic level.

The best way of showing what is happening in the feeding relationships of a community is to use Energy Pyramids.

References

• A.J. Colea, M.S. Pratchetta and G.P. Jonesa (2009) Effects of coral bleaching on the feeding response of two species of coral-feeding fish. Journal of Experimental Marine Biology and Ecology. 1: 11-15
• Galucci, V. F. (1973) On the principles of thermodynamics in ecology. Annual Review of Ecology and Systematics. 4,:329-357.
• Hairston, Nelson G., Jr. and Nelson G. Hairston, Sr. 1993. Cause and Effect Relationships in Energy Flow, Trophic Structure and Interspecific Interactions. The American Naturalist 142(3): 379-411.
• Hairston, Nelson G., Frederick E. Smith, and Lawrence B. Slobodkin. 1960. Community Structure, Population Control, and Competition. The American Naturalist XCIV(879): 421-425.
• Hedin, O. L., von Fischer, J. C., Ostrom, N. E., Kennedy, B. P., Brown, M. G., and Robertson, G. P. (1998). Thermodynamic constraints on nitrogen transformations and other biogeochemical processes at soil-stream interfaces. Ecology. 79,:684-703.
• Hughes, T. P. (2003) Climate Change, Human Impacts, and the Resilience of CoralReefs. Science (301): 929-933
• North, Gerald R. 1975. Anylitical Solution to a Simple Climate Model with Diffusive Heat Transport. Journal of the Atmospheric Sciences 32: 1301-1307
• Sellers, William D. 1969. A Global Climatic Model Based on the Energy Balance of the Earth-Atmosphere System Journal of Applied Metiorology 8: 392-400