Ecosystem Structure

One way of describing an ecosystem is according to its trophic structure. The trophic structure constitutes the levels of feeding (trophic = food) and the feeding relationships of the components of the ecosystem. All ecosystems must be based upon autotrophs. Autotrophs (literally self feeders) produce organic food for themselves and all members of their community. A few types of bacteria are able to harness chemical energy to produce food, but mostly autotrophs are green plants which utilize photosynthesis to harness energy from sunlight to produce organic materials, such as carbohydrates, plus oxygen. The chemical reaction for this process is shown below:

Photosynthesis by Plants

The inorganic materials water and carbon dioxide are the raw materials for photosynthesis. Plants take them in from the environment and, using the energy from sunlight, assemble the large macromolecules virtually all organisms depend on for food energy. In addition the oxygen produced is necessary for the cell metabolism (respiration) carried on by animal cells (and plant cells too, in the dark). The macromolecules plants produce include not only carbohydrates (sugars, starches, etc.) but also fats, proteins and vitamins. In order to produce these substances other minerals such as nitrates and phosphorus must also be present. In fact a whole spectrum of trace minerals is necessary for healthy plant growth.

Metabolism by HeterotrophsAnimals which consume these plant products as they eat the plants and breathe in oxygen do the reverse reaction: They use the energy now contained in the organic molecules together with the oxygen, and produce carbon dioxide and water as waste products. This process is known as cell respiration or metabolism. The relationship between photosynthesis and metabolism is shown below. As you can see this clearly shows the complementarity between animals and plants. As I have heard someone say, we owe a debt of gratitude to green plants. We would not have the organic foods nor the oxygen we need without them.

Photosynthesis-MetabolismAll types of plants participate in this process, from the obvious trees, grass and shrubs to the often forgotten algae and phytoplankton of the oceans. In fact, because of the extent of the oceans on our planet such microscopic green plants produce at least 2/3 of all the oxygen on earth. This begs the question how are we treating the earth's plant life and especially how are we treating the oceans? Are our actions reducing the extent of phytoplankton and other plant life and its ability to take carbon dioxide from the atmosphere and put oxygen into it?

The organic food consumed (and its energy along with it) is not broken down all at once. It travels through many levels of consumers and decomposers, allowing each to reap some of the benefit of the food the green plants have produced.

The Trophic Cycle

Consumers live in levels according to whether they eat the green plants directly (herbivore = plant eater), or whether they get the plant food second hand by eating another animal (carnivore = meat eater, omnivore = eats all). Human beings are omnivores because sometimes we eat the plants directly and sometimes we eat animals who have eaten plants, or even animals who have eaten other animals. But in all cases the organic materials were originally produced by plants!

Organisms such as consumers which depend on others for food are called heterotrophs. (hetero = other, troph = food). Scavengers and decomposers are also heterotrophs, but instead of consuming living plants and animals they consume them in the form of detritus. Detritus is decaying organic material and can be anything from recently dead carrion to bacteria, to organic waste from plants and animals. When they think of scavengers people usually think of big ones like vultures, but the largest group of scavengers is the insects. Scavengers are the first step in the process which ultimately will completely break down the organic matter of detritus into carbon dioxide, water, and inorganic minerals. These substances are then available to plants to convert back to organic matter and the cycle occurs all over again.

The Biogeochemical Cycles

Another component of ecosystem structure is the pathway of each chemical element through the components of the biosphere. Every element that is used by living organisms passes between the biotic (living) and abiotic (non-living) components of the biosphere. The pathways taken by these chemical elements are called the biogeochemical cycles. These cycles fall into two categories:

the gaseous cycles are those of chemical elements which spend at least part of their time in a gaseous form in the atmosphere. The elements falling into this category are carbon, nitrogen, and oxygen. Carbon is found in the atmosphere in carbon dioxide (CO2), nitrogen as nitrogen gas (N2), and oxygen as oxygen gas (O2). The carbon and nitrogen cycles are shown below. The sedimentary cycles are those in which the element is not found in a gaseous form. All the other cycles are of this type, exemplified by phosphorus seen below.

To illustrate each cycle we superimpose on the trophic cycle the form taken by the element in each of the trophic levels and in the abiotic environment of the soil, water, air, etc.

Carbon CycleCarbon is found in the atmosphere as carbon dioxide which is taken into plants to become plant tissues (we ignore other products of photosynthesis such as oxygen because we are only interested in what is happening to the carbon). The plant tissues are consumed and either metabolized to CO2 (and water) or turned into animal tissues. The animal tissues become detritus eventually most of which will ultimately revert to the inorganic forms. But over time small amounts of detritus have built up in the form of coal, oil, gas, and peat, the fossil fuels. These fossil fuels have been built up very slowly over long periods of time, literally millions of years, as incompletely decomposed detritus was covered by other materials and subjected to heat and pressure. This process has meant a very gradual loss of carbon from the cycle.

But our industrial society has been using these fossil fuels very rapidly compared to their buildup,

adding carbon to the atmosphere in the form of carbon dioxide. This added CO2 is thought to contribute to the Greenhouse Effect and thus to global warming. [see The Coming Climate ]

The nitrogen cycle also uses the atmosphere as a component, but plants don't get their nitrogen from the atmosphere. Nitrogen gas is mostly inert, while plants obtain needed nitrogen from soluble nitrates in the soil. These nitrates are produced from the decomposition of detritus, i.e. composting.

Nitrogen CyclePlants absorb soluble nitrates, along with other minerals, and use them to make plant tissues. It's proteins that especially require nitrates, making the nitrogen cycle (the nitrogen budget) so critical for nutritious crops. The plant protein is passed on to animals to be made into animal protein. Animals metabolize some of the protein for energy, excreting urea in the process. Urea can be used by plants, but most of the nitrogen is converted to the soluble forms by the decomposers. Soil organisms such as worms, fungi, and bacteria are essential to the normal recycling of minerals and nutrients for continued plant growth. Another underappreciated component of our biota!

Among the important soil organisms are those indicated in the above chart by NFB, nitrogen fixing bacteria and other organisms which convert gaseous nitrogen to soluble forms useful to plants. These organisms take the nitrogen found in the small air pockets in the soil and fix it, turning it into a form plants can absorb. NFB live frequently in the company of legumes, a family of plants which includes peas, beans, clover, alfalfa, and many others. The relationship between the NFB and legumes constitutes a type of symbiosis. These plants evolved to live in poor soils. In the process of fixing nitrogen the soils are built up and that makes possible the growth of other plants. Organic gardeners take advantage of NFB by growing legumes in an area which will later be given over to corn or some other nitrogen consuming crop. They also utilize the abundant nitrogen available in compost and manure, including human manure.

[see Soil Amendment Web Site]

Minerals such as phosphorus are not found in the atmosphere as part of their cycle. Their cycles are sometimes called sedimentary because some of the mineral is always being transported to deep sediments. These sediments are only very slowly recycled as the sediments eventually uplift and are subjected to erosion. Phosphorus CycleThis works fine in nature, but when man depends on these minerals to fertilize crops they may become depleted. Often man replaces this with artificial fertilizer. But the reserves of phosphate used for fertilizer are also being used up. While manure is poor in phosphorus, composted plant materials are a rich source, together with bone meal. In nature bones and teeth, which don't break down rapidly, are put back into the cycle as they are gnawed on by rodents. (Another underappreciated group!)

The biogeochemical cycles are an example of the Law of Conservation of Matter, the principle that matter is not created or destroyed but simply changes from one form to another. The phosphorus atom which was at one time in the bones of a mastodon may have, through erosion, found its way into the shell of a nautilus-like creature which later settled into the ocean sediment. If this sediment were at the bottom of a shallow sea in an area which would later uplift to become part of New Mexico's Sandia Mountains (we can see such fossils in the rocks of the crest!) it may then be subject to weathering and erosion into the soil of the Rio Grande valley, there to be incorporated into a chile pepper which you add to your evening salad. We have right now all the phosphorus, nitrogen, iron, oxygen etc. which we will ever have. Chemical elements are neither appearing nor disappearing. They are changing form, and man will have to be able to utilize them in their new forms. Recycling and composting are the ways in which we can do that.

Unlike matter which cycles, energy does not cycle! It passes through the biosphere in a one-way flow following the Laws of Thermodynamics. The first law states that energy is not created or destroyed but changes from one form to another, just like matter. But, according to the second law, every time it changes form, some of it becomes heat, the lowest form of energy. This represents a constant loss, since heat cannot be captured and stored but is continually radiated from the earth. Heat is a waste product of every energy conversion. It is produced by the combustion in our automobile engines, along with the light from our light bulbs, even from our own bodies! Imagine the number of energy conversions inherent in producing light with electricity made by burning coal (fossil fuel!) We will enumerate these conversions in class. The vast majority of the coal's energy is used up as waste heat, only a minuscule amount making it to the form of light. And yet light energy is constantly available to us from the sun. Again, all we have to do is learn to use the form (s) of energy readily available to us in renewable form.

Biological PyramidThe laws of thermodynamics apply to ecosystems too. The greatest amount of energy is available as sunlight to the producers. But they aren't able to harness much of it, and pass along even less to the primary consumers. When all the loss is considered plants pass along, on average, only about 1% of the energy they receive as light as Net Primary Production(NPP). Some plants are more efficient, some less, but the average is 1%

Consumers are somewhat more efficient passing along to the next level, on average, 10% of the energy they receive. Referred to as the Rule of Tens, this means that each successive level of consumers has only 10% of the amount of energy available to the previous level.

These relationships are illustrated by the Biological Pyramid shown above. The pyramid shows how energy decreases at each successive trophic level. Actual pyramids are shown in your textbook and will vary according to the particular community involved. Note that the decomposers don't really fit into the pyramid since they receive energy from each of the other levels and are more efficient than the others as well. [See Figure 4.6 in Biosphere 2000 for a variety of Biological Pyramids]

The pyramid can also be used to illustrate the biomass and the numbers of organisms at each level. Imagine you have a piece of land which will produce 100 lb. of corn. You can eat the corn, or you can feed it to a cow. But if you feed it to a cow 90% will be lost to the cows metabolism, heat, etc. So you'll only realize 10 lb. of beef from your 100 lb. of corn!

A pyramid of numbers will only work if size relationships between trophic levels are not skewed. In other words the consumers can't be too much smaller than what they consume. So it won't work for a forest or for some other communities. But in general there are fewer herbivores than producers, fewer carnivores than herbivores, etc.

Biological Magnification is an application in which certain pollutants become concentrated as they pass through the pyramid. At each level they concentrate according to the inefficiency of the trophic level. Plants will concentrate certain pesticides etc. to 100 times their level in the water, and microscopic animals, larger fish, and birds of prey will each concentrate them about 10 times. By the time the contaminant reaches the ultimate consumers they are concentrated enough to be harmful. This was discovered with DDT and its effect on calcium metabolism in birds. Inability to make viable egg shells was the result, and it nearly wiped out the California and Texas brown pelican, the peregrine falcon, osprey and bald eagle. The discovery led to the banning of DDT in the US and the reevaluation of pesticides and their effects.