An energy budget describes the ways in which energy is transformed from one state to another within some defined system, including an analysis of inputs, outputs, and changes in the quantities stored. Ecological energy budgets focus on the use and transformations of energy in the biosphere or its components.
Solar electromagnetic radiation is the major input of energy to Earth. This external source of energy helps to heat the planet, evaporate water, circulate the atmosphere and oceans, and sustain ecological processes. Ultimately, all of the solar energy absorbed by Earth is re-radiated back to space, as electromagnetic radiation of a longer wavelength than what was originally absorbed. Earth maintains a virtually perfect energetic balance between inputs and outputs of electromagnetic energy.
Earth’s ecosystems depend on solar radiation as an external source of diffuse energy that can be utilized by photosynthetic autotrophs, such as green plants, to synthesize simple organic molecules such as sugars from inorganic molecules such as carbon dioxide and water. Plants use the fixed energy of these simple organic compounds, plus inorganic nutrients, to synthesize an enormous diversity of biochemicals through various metabolic reactions. Plants utilize these biochemicals and the energy they contain to accomplish their growth and reproduction. Moreover, plant bio-mass is directly or indirectly utilized as food by the enormous numbers of heterotrophic organisms that are incapable of fixing their own energy. These organisms include herbivores that eat plants, carnivores that eat animals, and detritivores that feed on dead biomass.
Worldwide, the use of solar energy for this ecological purpose is relatively small, accounting for much less than 1% of the amount received at Earth’s surface. Although this is a quantitatively trivial part of Earth’s energy budget, it is clearly very important qualitatively, because this is the absorbed and biologically fixed energy that subsidizes all ecological processes.
Energy is defined as the ability, or potential ability, of a body or system to do work. Energy can be measured in various units, such as the calorie, defined as the amount of energy required to raise the temperature of one gram of pure water by one degree Celsius. (Note that the dietician’s calorie [the large calorie or kilogram calorie] is equivalent to one thousand of these calories [the small calorie or gram calorie], or one kilocalorie.) The Joule (J) is another unit of energy. One joule is generally defined as the amount of work required to lift a weight of 1 kg by 10 cm on the surface of Earth. It is equivalent to 0.24 calories. In addition, one calorie (small calorie) is equal to about 4.184 J.
Energy can exist in various states, all of which are interchangeable through various sorts of physical/chemical transformations. The basic categories of energy are: electromagnetic, kinetic, and potential, but each of these can also exist in various states.
Electromagnetic energy is the energy of photons, or quanta of energy that have properties of both particles and waves, and that travel through the vacuum of space at a constant speed of approximately 3× 108 meters per second (that is, at the speed of light). The components of electromagnetic energy are characterized on the basis of wavelength ranges, which ordered from the shortest to longest wavelengths are known as: gamma, x ray, ultraviolet, light or visible, infrared, and radio. All bodies with a temperature greater than absolute zero (that is, -459°F [-273°C], or zero degrees on the Kelvin scale [0K]) emit electromagnetic energy at a rate and spectral quality that is strictly determined by their surface temperature. Relatively hot bodies have much larger emission rates and their radiation is dominated by shorter wavelengths, compared with cooler bodies. The sun has a surface temperature of about 11, 000°F (6, 093°C). Most of its radiation is in the wavelength range of visible light (0.4 to 0.7 æm or micrometers) and shorter-wave infrared (0.7 to 2.0 æm), while Earth has a surface temperature of about 77°F (25°C) and its radiation peaks in the longer-wave infrared range at about 10 æm.
Kinetic energy is the energy of dynamic motion, of which there are two basic types, the energy of moving bodies, and that of vibrating atoms or molecules. The later is also known as thermal energy, and the more vigorous the vibration, the greater the heat content.
Potential energy has the capacity to do work, but it must be mobilized to do so. Potential energy occurs in various forms. Chemical potential energy is stored in the inter-atomic bonds of molecules. This energy can be liberated by so-called exothermic reactions, which have a net release of energy. For example, heat is released when the chemically reduced sulfur of sulfide minerals is oxidized to sulfate, and when crystalline sodium chloride is dissolved into water. All biochemicals also store potential energy, equivalent to 4.6 kilocalories per gram of carbohydrate, 4.8 Kcal/g of protein, and 6.0 to 9.0 Kcal/g of fat.
Gravitational potential energy is stored in mass that is elevated above some gravitationally attractive surface, as when water occurs above the surface of the oceans, or any object occurs above the ground surface. Unless obstructed, water spontaneously flows downhill, and objects fall downwards in response to gradients of gravitational potential energy. Other types of potential energy are somewhat less important in terms of ecological energy budgets, but they include potential energies of compressed gases, electrical potential gradients associated with voltage differentials, and the potential energy of matter, which can be released by nuclear reactions.
As noted previously, energy can be transformed among its various states. Electromagnetic energy, for example, can be absorbed by a dark object and converted to thermal kinetic energy. This action results in an increased temperature of the absorbing body. As another example, gravitational potential energy of water high on a plateau can be transformed into the kinetic energy of moving water and heat at a waterfall, or it can be mobilized by humans to drive a turbine and generate electrical energy. Thirdly, solar electromagnetic radiation can be absorbed by the chlorophyll of green plants, and some of the absorbed energy can be converted into the chemical potential energy of sugars, and the rest converted into heat.
All transformations of energy must occur according to certain physical principles, known as the laws of thermodynamics. These are universal laws; which means that they are always true, regardless of circumstances. The first law states that energy can undergo transformations among its various states, but it is never created nor destroyed—so the energy content of the universe remains constant. A consequence of this law for energy budgets is that there must always be a zero balance between the energy inputs to a system, the energy outputs, and any net storage within the system.
The second law of thermodynamics states that transformations of energy can only occur spontaneously under conditions in which there is an increase in the entropy of the universe. (Entropy is related to randomness of the distributions of matter and energy). For example, Earth is continuously irradiated by solar radiation, mostly of visible and near-infrared wavelengths. Some of this energy is absorbed, which heats the surface of Earth. The planet cools itself in various ways, but ultimately this is done by radiating its own electromagnetic radiation back to space, as longer-wave infrared radiation. The transformation of relatively short-wave solar radiation into the longer-wave radiation emitted by Earth represents a degradation of the quality of the energy, and an increase in the entropy of the universe.
A corollary, or secondary proposition of the second law of thermodynamics is that energy transformations can never be completely efficient, because some of the initial content of energy must be converted to heat so that entropy can be increased. Ultimately, this is the reason why no more than about 30% of the energy content of gasoline can be converted into the kinetic energy of a moving automobile. It is also the reason why no more than about 40% of the energy of coal can be transformed into electricity in a modern generating station. Similarly, there are upper limits to the efficiency by which green plants can photosynthetically convert visible radiation into biochemicals, even in ecosystems in which ecological constraints related to nutrients, water, and space are optimized.
Interestingly, plants absorb visible radiation emitted by the sun, and use this relatively dispersed energy to fix simple inorganic molecules such as carbon dioxide, water, and other nutrients into very complex and energy-dense biochemicals. Heterotrophic organisms then use the biochemicals of plant biomass to synthesize their own complex biochemicals. Locally, these various biological syntheses represent energy transformations that substantially decrease entropy, rather than increase it. This occurs because relatively dispersed solar energy and simple compounds are focused into the complex biochemicals of living organisms.
Are biological transformations not obeying the second law of thermodynamics? This seeming physical paradox of life can be successfully rationalized, using the following logic: The localized bio-concentrating of negative entropy can occur because there is a constant input of energy into the system, in the form of solar radiation. If this external source of energy was terminated, then all of the negative entropy of organisms and organic matter would rather quickly be spontaneously degraded, producing heat and simple inorganic molecules, and thereby increasing the entropy of the universe. This is why life and ecosystems cannot survive without continual inputs of solar energy. Therefore, the biosphere can be considered to represent a localized island, in space and time, of negative entropy, fueled by an external (solar) source of energy. There are physical analogues to these ecological circumstances—if external energy is put into the system, relatively dispersed molecules of gases can be concentrated into a container, as occurs when a person blows energetically to fill a balloon with air. Eventually, however, the balloon pops, the gases re-disperse, the original energy input is converted into heat, and the entropy of the universe is increased.
Physical energy budgets consider a particular, defined system, and then analyze the inputs of energy, its various transformations and storages, and the eventual outputs. This concept can be illustrated by reference to the energy budget of Earth.
The major input of energy to Earth occurs as solar electromagnetic energy. At the outer limits of Earth’s atmosphere, the average rate of input of solar radiation is 2.00 calories per cm2 per minute (this flux is known as the solar constant). About half of this energy input occurs as visible radiation, and half as near-infrared. As noted previously, Earth also emits its own electromagnetic radiation, again at a rate of 2.00 cal/cm2/min, but with a spectrum that peaks in the longer-wave infrared, at about 10 æm. Because the rate of energy input equals the rate of output, there is no net storage of energy, and no substantial, longer-term change in Earth’s surface temperature. Therefore, Earth represents a zero-sum, energy flow-through system. (Actually, over geological time there has been a small storage of energy, occurring as an accumulation of under-composed biomass that eventually transforms geologically into fossil fuels. There are also minor, longer-term variations of Earth’s temperature surface that represent climate change. However, these represent quantitatively trivial exceptions to the preceding statement about Earth as a zero-sum, flow-through system for energy.) Although the amount of energy emitted by Earth eventually equals the amount of solar radiation that is absorbed, there are some ecologically important transformations that occur between these two events.
The most important ways by which Earth deals with its incident solar radiations are:
- An average of about 30% of the incident solar energy is reflected back to outer space by Earth’s atmosphere or its surface. This process is related to Earth’s albedo, which is strongly influenced by the solar angle, the amounts of cloud cover and atmospheric particulates, and to a lesser degree by the character of Earth’s surface, especially the types and amount of water (including ice) and vegetation cover.
- About 25% of the incident energy is absorbed by atmospheric gases, vapors, and particulates, converted to heat or thermal kinetic energy, and then re-radiated as longer-wavelength infrared radiation.
- About 45% of the incident radiation is absorbed at Earth’s surface by living and non-living materials, and is converted to thermal energy, increasing the temperature of the absorbing surfaces. Over the longer term (that is, years) and even the medium term (that is, days) there is little or no net storage of heat. Virtually all of the absorbed energy is re-radiated by the surface as long-wave infrared energy, with a wavelength peak of about 10 æm.
- Some of the thermal energy of surfaces causes water to evaporate from plant and non-living surfaces (see entry on evapotranspiration), or it causes ice or snow to melt.
- Because of the uneven distribution of thermal energy on Earth’s surface, some of the absorbed radiation drives mass-transport, distributional processes, such as winds, water currents, and waves on the surface of waterbodies.
- A very small (averaging less than 0.1%) but ecologically critical portion of the incoming solar energy is absorbed by the chlorophyll of plants, and is used to drive photosynthesis. This photoautotrophic fixation allows some of the solar energy to be temporarily stored in the potential energy of biochemicals, and to serve as the energetic basis of life on Earth.
Certain gases in Earth’s atmosphere absorb long-wave infrared energy of the type that is radiated by heated matter in the second and third dissipation mechanisms (listed above). This absorption heats the gases, which then undergo another re-radiation, emitting even longer-wavelength infrared energy in all directions, including back to Earth’s surface. The most important of the so-called radiatively active gases in the atmosphere are water and carbon dioxide, but the trace gases methane, nitrous oxide, ozone, and chlorofluorocarbons are also significant. This phenomenon, known as the greenhouse effect, significantly interferes with the rate of radiative cooling of Earth’s surface.
If there were no greenhouse effect, and Earth’s atmosphere was fully transparent to long-wave infrared radiation, surface temperatures would average about 17.6°F (-8°C), much too cold for biological processes to occur. Because the naturally occurring greenhouse effect maintains Earth’s average surface temperature about 60 degrees Fahrenheit warmer than this, at about 77°F (25°C), it is an obviously important factor in the habitability of the planet. However, human activities have resulted in increasing atmospheric concentrations of some of the radiatively active gases, and there are concerns that this could cause an intensification of Earth’s greenhouse effect. This could lead to global warming, changes in the distributions of rainfall and other climatic effects, and severe ecological and socioeconomic damages.
Ecological energetics examines the transformations of fixed, biological energy within communities and ecosystems, in particular, the manner in which biologically fixed energy is passed through the food web.
For example, studies of a natural oak-pine forest in the state of New York found that the vegetation fixed solar energy equivalent to 11, 500 kilocalories per hectare per year (103 Kcal/ha/yr). However, plant respiration utilized 6.5× 103 Kcal/ha/yr, so that the actual net accumulation of energy in the ecosystem was 5.0× 103 Kcal/ha/yr. The various types of heterotrophic organisms in the forest utilized another 3.0× 103 Kcal/ha/yr to support their respiration, so the net
Electromagnetic energy— A type of energy, involving photons, which have physical properties of both particles and waves. Electromagnetic energy is divided into spectral components, which (ordered from long to short wavelength) include radio, infrared, visible light, ultraviolet, and cosmic.
Entropy— The measurement of a tendency towards increased randomness and disorder.
accumulation of biomass by all of the organisms of the ecosystem was equivalent to 2.0 x 103 Kcal/ha/yr.
The preceding is an example of a fixed-energy budget at the ecosystem level. Sometimes, ecologists develop budgets of energy at the levels of population, and even for individuals. For example, depending on environmental circumstances and opportunities, individual plants or animals can optimize their fitness by allocating their energy resources into various activities, most simply, into growth of the individual or into reproduction.
However, biological energy budgets are typically much more complicated. For example, a plant can variously allocate its energy into the production of longer stems and more leaves to improve its access to sunlight, or it could grow longer and more roots to increase its access to soil nutrients, or more flowers and seeds to increase the probability of successful reproduction. There are other possible allocation strategies, including some combination of the preceding.
Similarly, a bear must makes decisions about the allocation of its time and energy. It must decide on activities associated with resting, either during the day or longer-term hibernation, hunting for plant or animal foods, seeking a mate, taking care of the cubs, or just having fun, as wild bears are known to do.
Molles, Manuel C. Ecology: Concepts and Applications. Boston, MA: McGraw-Hill, 2005.
Smith, Thomas M. Elements of Ecology. San Francisco, CA: Benjamin Cummings, 2008.