April 19, 1999

Readings - Starr text: Ch 41 cover page, 41.1-41.2, 41.4-41.5, 41.7, 41.10

"Three hundred trout are needed to support one man for a year. The trout, in turn, must consume 90,000 frogs, that must consume 27 million grasshoppers that live off of 1,000 tons of grass."

- G. Tyler Miller, Jr., American chemist (1971)


I. Who's who in ecosystems
II. Structure of ecosystems:
III. Biogeochemical Cycles
IV. Biological magnification

I. Who's who in ecosystems:

Ecosystem: An array of organisms and their physical environment, all of which interact through a one-way flow of energy and a cycling of materials. Most of the energy originally fixed by the autotrophs is lost to the environment as metabolic heat.


1. Producers: autotrophic ("self-feed") organisms; produce the carbon and energy they need. Examples: photoautotrophs (plants, plankton) and chemoautotrophs (sulfur bacteria)

2. Consumers: heterotrophic organisms; obtain energy and carbon by feeding on the tissues of other organisms. Herbivores, carnivores, and parasites are examples.


II. Structure of ecosystems: Who eats who in ecosystems

Trophic levels: All the organisms in an ecosystem that are the same number of transfer steps away from the energy input into the system.

Food chain: A straight-line sequence of who eats who in an ecosystem.


Food web: A network of cross-connecting, interlinked food chains with some number of producers and consumers, as well as decomposers, detritivores, or both.


Food chain - the game





Rule of 10's: Often the trophic structure of an ecosystem is represented as an ecological pyramid in which producers form a base for tiers of consumers above them. Some pyramids are based on biomass (the weight of all the members at each trophic level), others on energy flow. In general, there is roughly a 10-fold loss of energy for each level up in trophic level.



III. Biogeochemical Cycles - nutrient flow through ecosystems

Primary producers require carbon, oxygen, and hydrogen, which they obtain from water and air. They also require nitrogen, phosphorus, and other minerals. Chemical elements and nutrients move in biogeochemical cycles. In such cycles, ions or molecules of a nutrient are transferred from the environment to organisms, then back to the environment--part of which serves as a reservoir for them. They generally move through the reservoir slowly, compared to their rapid exchange between organisms and the environment. The relationship between geochemical cycles and most land ecosystems--a biogeochemical cycle--is shown below.

Hydrologic cycle:

Surf your watershed

Carbon cycle:


Nitrogen cycle

Enviro-gardening: create your own nitrogen cycle

Human alterations of the nitrogen cycle

Phosphorous cycle

A cute poem about the Phosphorous cycle...


IV. Biological magnification: From this website on DDT:

"Because of the roughly 90 percent loss of energy at each step in a food chain, any consumer has to eat far more food than it ultimately stores as body tissue. If the food contains some material which is not metabolized and cannot be excreted (usually fat soluble rather than water soluble) that material will accumulate in the consumer's body. A carnivore eating the first consumer will receive a high dose of the material, which it, in turn will retain and pass on to the next level if eaten."

Food Chain Concentration of DDT in a Long Island Marsh sprayed for Mosquito Control (1967)

Water  .00005
Plankton  .04
Silverside Minnow  .23
Sheephead Minnow  .94
Pickerel 1.23
Needlefish 2.07
Heron 3.57
Tern 3.91
Osprey 13.8
Merganser 22.6
Cormorant 26.4

"Starting in the 1940's, the chlorinated hydrocarbon DDT was used in vast quantities all over the world for killing insects. It was cheaper and much more effective than other insecticides against nearly all insects. It saved millions of lives by killing the mosquitoes that spread malaria and saved millions from starvation by killing crop pests. Paul Müller of Switzerland won the Nobel prize for discovering it.

But: in the 1950's and 1960's there was an alarming decline in the populations of several predatory birds, particularly fish-eaters such as bald eagles, cormorants, ospreys and brown pelicans but also including the Peregrine Falcon, which is a predator on other birds. The Brown Pelican, bald eagle and osprey almost went extinct.


In the case of DDT, it results from the following factors:

1. DDT is not metabolized, and does not break down in the body.

2. It is much more soluble in fat than in water. So it accumulates in body fat and is not excreted.

3. The transfer of energy from lower trophic levels to higher ones is inefficient -so herbivores eat large quantities of plant material, and carnivores eat many times their body weight of prey during their lifetime. Since DDT is not excreted, the carnivore accumulates most of the DDT that was present in all of the prey organisms.

The DDT episode led to the publication of a famous book, Silent Spring by Rachel Carson in 1963. It led to a great increase in environmental awareness. DDT was banned in this country in 1972 by the U.S. government, which found that it 'had an adverse impact on wildlife' and 'should be considered a potential carcinogen'. However, DDT released before that time is still present in the environment and in various organisms in the process of bioaccumulation."

DDT as an endocrine disruptor



Earth Day Indiana

NEXT Saturday, April 29, 2000



1. Explain the importance of autotrophs with respect to nutrient flow and energy cycling in ecosystems.
2. List and describe the types of consumers found in an ecosystems.
3. Desctive the trophic levels in an ecosystem and give an axample of an organism found in each level.
4. Describe a biogeological cycle, and how it results from the reciprocal processes of photosynthesis, cellular respiration, and the resevior effect of the earth.
5. Explain why toxic compounds like DDT have the greatest effect of the top-level carnivores.