27 April 2005

Lecture 43

Reading, Chapter 29, 30, and 31

B. Ecosystem ecology

1. Biological energy flow

a. Food conversion efficiency

Previously, we noted that the conversion of food into growth is10 to 20% efficient, meaning that a consumer must consume 10 pounds of food to add 1-2 pounds of body mass. Reflecting this, most natural "food chains" are short, having just a few steps. This is also the reason that "top" consumers, such as sharks, swordfish, wolves, and great cats, are relatively few in number. Large numbers of producers and lower consumers are needed to support their energy needs.

b. Biological magnification

Toxins and pollutants that cannot be degraded or completely excreted from living things are found in much higher concentrations in the tissues of top consumers than in their environment. This is the result of "biological magnification". It happens like this. A consumer eats 10 pounds of food and burns 80 to 90% of it for metabolism. Stable toxins are not burned but are stored in the consumers tissues, accumulating by a factor of 10 or more over the concentration in the food. This magnification occurs at every step in the food chain and thus an environmental level of the toxin that is harmless can accumulate to health-threatening levels in top consumers, including humans.

A famous example of a toxin that was biologically magnified to the point of poisoning top consumers (birds of prey) was the insecticide DDT. This case is discussed on page 744 of your text. Another example is mercury. Mercury is released into the air from burning coal in power plants and from burning of refuse. It falls into the oceans from these human sources as well as from other natural sources. Ocean waters may have a concentration of mercury that is extremely low but it is magnified up to 100,000 times in the tissues of top consumers, such as swordfish and shark.

Mercury levels in swordfish and shark sold at fish markets can be above 1 part per million (ppm), which is the safe level of mercury in food established by the Food and Drug Administration. The FDA discourages consumption of shark and swordfish because of the mercury they contain. Canned tuna can have mercury levels up to 0.3 ppm. This is high enough that Consumers Union (publisher of Consumer Reports) recommends that pregnant women and young children, who are more susceptible to mercury poisoning than adults, eat no more than one tuna sandwich a week.

Low level mercury poisoning damages nerve cells and causes memory loss and loss of muscle coordination. These symptoms appear when blood levels of mercury exceed about 15 parts per billion.

2. Biogeochemical cycles

A "hot" topic in ecosystem ecology is "biogeochemistry", which is the study of how living things affect the atmosphere, climate, and geology of the Earth. A specific example of this is the global carbon cycle, which is the exchange of carbon between the atmosphere and all the living things on earth. It is related to biological energy flow because biological energy drives much of the cycle.

a. The global carbon cycle

Producers take up carbon dioxide from the air and build it into biological molecules by means of photosynthesis. Consumers and decomposers release some of this back to the atmosphere by respiration and decomposition. Some of the carbon dioxide taken up by photosynthesis is transferred to deep ocean sediments or deep layers of soil and rock where it can remain for great periods of time and be converted to coal, oil, and natural gas (fossil fuels).

Current estimates of "flux rates" and "pool sizes" of carbon in the global carbon cycle may be found at The Woods Hole Research Center . Pools are accumulations of carbon in different parts of the cycle. For example, all the carbon in the carbon dioxide of the atmosphere is a pool of about 750 billion metric tons of carbon. All the carbon in the organic matter of terrestrial soils and in living things on land is another pool of 2,000 billion metric tons. Carbon in the deep sediments of the oceans is the largest pool, with 40,000 billion metric tons. Carbon in the bodies of living things in the oceans and dissolved in ocean water as carbon dioxide is a pool of 800 billion metric tons. Finally, fossil fuels (coal, oil, and natural gas) represent a carbon pool of 10,000 billion metric tons

Flux rates are the annual movement of carbon between pools. For example, 100 billion metric tons of carbon move from the atmosphere to the bodies of plants, animals, and other land organisms every year by the process of photosynthesis. Roughly 99 billion metric tons of carbon move from those organisms back to the atmosphere every year as the carbon dioxide released in respiration.

A recent and important phenomenon is that in the last 200 years, humans have dug up coal, oil, and gas and burned it, releasing huge amounts of carbon dioxide into the atmosphere. Carbon dioxide from these fuels and from burning of forests to convert them to agricultural land represent an annual flux of carbon into the atmosphere of 8 billion metric tons. This input to the atmosphere is small but unbalanced by other fluxes and has raised the carbon dioxide concentration of the atmosphere by over 35% since 1750. This has caused concern about "global warming", which is predicted to result from rising carbon dioxide in the atmosphere.

b. Global climate change resulting from rising carbon dioxide

"Global warming" has been a topic of public debate because it is difficult to predict with confidence and because it will cost money to moderate it. For such issues, it is important to be aware of what is known with confidence and what is uncertain and still subject to debate.

Known facts about global warming:

1) It is known with confidence that the carbon dioxide levels in our atmosphere have increased by over 35% in the last 250 years. Unambiguous measurements of atmospheric composition in the last 40 years have shown this plainly. It is also known that this new carbon dioxide is largely the result of burning fossil fuels, since fossil fuels have a distinctive carbon isotope ratio that allows it to be detected in the atmosphere.

 

2) It is known with confidence that carbon dioxide is a greenhouse gas. This means that it allows sunlight to pass through the atmosphere but traps heat near the Earth's surface. In theory, increased carbon dioxide in the atmosphere should cause the Earth to warm. This was predicted over 100 years ago by the chemist Arrhenius, among others.

 

3) It is known that average global temperature has increased by over half a degree Centigrade in the last 100 years.