We [the biological science community] recognize only a few “fundamental units” in Biology.
These are the Cell, the Individual, the Population (in Ecology & Genetics, or the Species in
Taxonomy), and the Ecosystem. When we narrow the discussion to the ecological sciences, only the
Population and Ecosystem are the fundamental units of interest.
The population is “all of the individuals of a
species which occupy the same site and interact with each other.”
The ecosystem is “all of the populations (Animal,
Plant, Fungi, Protista, & Bacteria) which occupy the same site, plus the physical & chemical
environment of the site.”
Initially, it is the goal of Ecology to determine all of the interactions between the populations, and
between the populations and the environmental factors; but, ultimately, the goal is to explain these
interactions under the umbrella of “Ecosystem function.”
There are two aspects of the structure & function of the Ecosystem which, although important to the understanding of the role of Plants in the Ecosystem, are primarily topics of interest to the Animal Ecologists. These include trophic dynamics (the subject of this chapter) and predator - prey relationships (the subject of the next chapter). In order to visualize the importance of these topics to Plant Ecology, it is necessary to recall that living systems are energy-intensive; all living systems require a considerable amount of energy simply to maintain the complex organization characteristic of living systems at every level [or for each of the fundamental units we currently recognize]. Since the lowest fundamental unit (the Cell) exhibits a very complex organization which requires a substantial amount of energy to maintain its organization, it seems reasonable to expect that the higher fundamental units the Population and the Ecosystem) must exhibit even greater complexity in organization and greater energy demands to maintain their organization. That said, we should (read “I intend to… ”) look at how ecosystems meet their energy requirements.
“Energy” is described as “the ability to do work” (energyquest.ca.gov, downloaded
28 Jan 2011). In biological systems, we measure energy in calories (1 cal = the energy required to raise
the temperature of 1 gram of water 1 Celsius degree starting at 20° C). In physical systems [and in
SI (Système International d'Unites) metric usage], we use the joule (1 j = 0.2390 cal). Power is
defined as energy per unit time and is measured in watts (1 W = 1 j/sec). The Laws of Thermodynamics
describe how energy behaves in the Universe, including within all living systems:
The 1st Law of Thermodynamics states that ‘Energy is neither created nor destroyed;’ it merely changes form…
The 2nd Law of Thermodynamics states that ‘whenever energy changes form, a portion is lost as heat.’ [Heat is a form of energy]. This law turns out to be extremely important to biological systems, including ecosystems.
ALL of the energy used by ecosystems comes ultimately from the Sun, as sunlight. Humans tend to think of sunlight only as visible light, but there are many other wavelengths which can not be seen by the ‘normal’ Human eye. Visible light can be separated with a prism [an example of indirect observation] into Red, orange, yellow, Green, Blue, indigo, violet (the mneumonic to remember this is a man's name: Roy G Biv). Most references list the longest wavelength of visible light, at the Red end of the visible sprectum, to be about 700 nanometers (nm) [1 nm is 1/1,000 micrometer (µm); 1 µm is 1/1,000 millimeter (mm); 1 mm is 1/1,000 meter (m), which is 3.37 inches longer than 1 yard], and the shortest wavelength, at the violet end of the visible spectrum, to be about 400 nm. Based on an experiment run with High School general science students, the exact limits of wavelengths visible to Human subjects vary between observers, as does the range which can be seen. The peak intensity of sunlight is at approximately 540 nm (yellow-green), but extends up to over 100 km [60 miles] and down to less than 0.001 nm (or 1 picometer, pm). The shorter wavelengths contain more energy (per photon) than do the longer wavelengths.
|kilometers||km = 10+3m|
|km = 10+3|
mm = 10-3m
|millimeters||mm = 10-3m|
|micrometers||µm = 10-6m|
|nanometers||nm = 10-9m|
|picometers||pm = 10-12m|
The amount of energy reaching the Earth above the atmosphere is 1,376 W/m². Because the
atmosphere ‘filters’ out some of the solar radiation by reflection and by absorption, only
688 W/m² reaches the Earth surface. However, some of the 688 W/m² [absorbed or reflected by the
atmosphere] is used to drive the winds and weather of our planet, and is therefore can not be ignored as
unimportant to the ecosystems of Earth. Of the 688 W/m² reaching the Earth's surface, most is
absorbed [not all, because some is reflected] by the Earth surface [land and water] and by objects on
the Earth surface. For that light energy which is absorbed, the second law of thermodynamics applies, so
the atmosphere and Earth surface are heated. All objects which are warmer than absolute zero, or
0° Kelvin (K) = -273.15° C = -459.67° F, emit or re-radiate light, and the peak wavelength
of this light is determined by the temperature of the object. At the average temperature of the Earth
(as seen from space) the light produced by the Earth should be in the range 100 - 4 µm, or thermal
infrared, IRt, which is felt by Human skin as heat (for example from a stove or fireplace).
Solar energy is the input to the Earth's energy budget, but strikes only the sun-lit side of the Earth. Re-radiation by the Earth is lost from the Earth's energy budget at a nearly steady rate 24 hours a day from the entire Earth surface. However, the re-radiated IRt can be reflected (and absorbed) by “greenhouse gases” in the atmosphere. That energy absorbed by the atmmosphere (whether on its way down, or back up) heats the atmosphere. Some of the heat of the atmosphere is transported by winds toward the layer of atmosphere near the Earth surface, some is re-radiated toward the Earth surface where it is absorbed and heats the Earth surface and the atmosphere near the surface. One of the most effective greenhouse “gases” is suspended liquid water, which you can verify for yourself by comparing the daytime high temperature to the following night's low temperature. On those nights when the skies are mostly cloud-free, the temperature drops by several degrees, while on mostly cloudy nights the temperature drops only a few degrees [unless a cold front, or a warm front, passes through the area during the afternoon, evening or night]. Note that you are observing the temperature drop (high temperature minus the low temperature) not the actual low temperature; and most newspapers, radio & TV stations in the morning news, report the previous days high and low temperatures, so you do not have to have a ‘minimum/maximum’ thermometer to verify this hypothesis. The other important greenhouse gases are Methane (CH4), and Carbon dioxide (CO2), … plus Carbon monoxide, Ozone, and a few others. Note: most articles concerning Human-caused climate effects emphasize CO2, but ignore that those proposed automobile engines which burn Hydrogen [claimed to be “carbon neutral”, or neutral with respect to Human-caused climate effects] produce water (H2 + 2O2 → 2H20) which is an effective greenhouse gas [a hypothesis which I told you how to verify for yourself]. Over very short time frames (days), there is a clear temperature cycle, called the diurnal cycle, with nighttime temperature several degrees cooler than the daytime temperatures. This has no effect on global temperatures because daytime covers only the sunlit side of the earth and it is night on the opposite side of the Earth. Similarly, in the temperate zones, the temperature exhibits a seasonal cycle with winters cooler by many degrees compared to summers, but this,too, has no effect on global temperatures because when it is winter in the Noorthern Hemisphere, it is summer in the Southern Hemisphere.
Over relatively short time frames (a few years or less), the global temperature (as annual average temperature) changes very little, so the Earth energy budget is relatively balanced. Over longer time frames (decades, millenia, and longer… ), the solar constant turns out not to be so constant. The temperature of the Sun changes as the number of sunspots changes (on an approximately 10 - 12 year cycle). As the number of sunspots increases the intensity of light in the visible spectrum (relatively low energy per photon) decreases slightly, but in the extreme UV (EUV) spectra (relatively high energy per photon) it increases more than enough to compensate for the decrease in visible light intensity. While the EUV is absorbed by the upper atmosphere, the absorbed energy is re-radiated (some downward) and mixed downward by winds. There is also an approximately 750 to 1,500 year cycle in the solar constant (Desprat, S., Goñi, M.F.S. and Loutre, M.-F. 2003. “Revealing climatic variability of the last three millennia in northwestern Iberia using pollen influx data.” Earth and Planetary Science Letters 213: 63-78. downloaded from www.co2science.org/articles/V6/N43/C2.php, 14 Jan 2011), at all wavelengths, as well as changes in the peak wavelength. These cycles are further complicated by the Milankovitch cycle describing changes in the Earth's orbit (www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm downloaded 14 Jan 2011), changing the distance from the Earth to the Sun, and therefore also the intensity of the sunlight at the top of the Earth's atmosphere. As a result of these cycles (and other natural cycles) the climate of the Earth undergoes cycles in climate affecting both Global temperatures and global precipitation patterns.
Early attempts to describe energy flow in ecosystems (before they were called “ecosystems”)
produced the “Food Chain” model. The species of the ecosystem were categorized as
“producers” (green plants), “herbivores” (animals which eat plants),
“carnivores” (animals which eat animals), and “top carnivores” (animals which eat
other animals, but are not eaten by other animals). The energy flow became:
producers → herbivores → carnivores → top carnivores
When we counted the number of individuals for all species at each level, we found that this model could be portrayed as a “Food Pyramid” since each category seemed to include progressively fewer individuals. This was repeated with biomass (estimated weight of all members of each level) and produced a comparable pyramid. Using biomass as the estimate of the size of each level of the food chain led to the concept of “standing crop biomass” as the sum of all organisms in each level of the food chain. As we moved from qualitative data to quantitative data, we determined that efficiency, or ecological efficiency, defined as “the biomass of the animal which is eating as a percentage of the biomass of the organism being eaten; and for producers, the biomass of producers as a percentage of the light energy striking the leaf surfaces,” is (for economically interesting organisms, because it is not only easier to do the research on domestic organisms, but because the breeders of economically interesting organisms are more willing to pay for research which could lead to more efficient organisms):
Hybrid corn (increase in plant biomass compared to light energy received in the field) = 10 - 12%, depending on which hybrid is involved
Hereford cows (increase in cow biomass compared to hay biomass fed to the cow) = 10%
German Shepard dogs (increase in dog biomass compared to the biomass of dogfood fed to the dog) = 10%
When we look at naturally occuring ecosystems, we find a surprisingly large number of species which do not fit neatly into the Ecological Pyramid model. There are, for example, omnivores (animals which “eat anything that doesn't eat them first”), and instances of the predator-prey relationship being reversed (an animal about to be eaten turns on the predator and eats it instead) or even cannibalism among herbivores. These complications led to attempts to identify all of the interactions between organisms where one organism is eaten by another, under the name “Food Webs,” but the work involved is quite extensive although the few successful attempts to describe food webs have not led to a productive theory to explain energy flow in ecosystems.
It is unclear where we should categorize those creatures which normally feed on rotting “road kill,” so we added the “detritus-based” food webs. Detritus is “the decomposing remains of dead plant and animal carcasses or parts” (including shed leaves and hair). In the usual description of a detritus-based system, the “producers” are the bacteria responsible for decomposition, the primary consumers are those creatures which eat bacteria, and so forth…
This model, the Ecologial Pyramid (either with number of individuals or biomass), turns out to be quite useful in explaining energy flow in ecosystems to students with the cognitive development of middle school or junior high school. Unfortunately, it served only briefly to explain ecosystem energy flow to Ecologists. What it actually accomplished was the design of many studies to elucidate the underlying processes involved in energy flow.
© 2010-2011 TwoOldGuys
revised: 26 Jan 2011