4/21/11

1

Community and Ecosystem Ecology by Dr. W.

. . . After QUIZ 11!!!1!!1!! •  1. Draw a graph showing how a typical

population in nature might grow over time. Include and label the carrying capacity.

•  2. Define niche. •  3-4. Describe the two main possibilities that

result when two different species share a niche. •  5. Describe the difference between an r-selected

life history and a K-selected life history.

Energy Flow •  You can divide organisms into three types,

according to the roles they play in an ecosystem – Producers: carry out photosynthesis. These are the

ultimate sources of all living matter (biomass) and energy in an ecosystem. EXAMPLES: Most plants; photosynthetic protists; photosynthetic bacteria.

– Consumers: obtain matter and energy from feeding on other organisms. EXAMPLES: Most animals; many non-photosynthetic protists.

– Decomposers: obtain matter and energy from feeding on dead organisms. EXAMPLES: most fungi, many bacteria, some protists.

Energy Flow

•  You can then look at energy flow through an ecosystem. – The energy that organisms use is stored as chemical

energy in the form of chemical bonds. – As organisms feed on each other, etc., energy—in the

form of food—flows through the ecosystem. – Biomass—living matter—also "flows" through the

ecosystem, as some organisms grow larger by eating others.

– These pathways can be represented by a diagram called a food web.

4/21/11

2

A food web diagram for an open ocean

ecosystem.

Energy Flow

•  You can define several levels of energy flow, called trophic levels: –  Primary producters / Primary productivity –  Primary consumers—feed directly on the producers. A.k.a.

herbivores (plant eaters) –  Secondary consumers—feed on the primary consumers –  Tertiary consumers—feed on the secondary consumers –  Quaternary consumers—you guessed it, feed on the tertiary

consumers •  This is a bit of a simplification, because many organisms

occupy several levels at once. •  Also, some ecosystems may have fewer trophic levels than

this, and some may have more.

This diagram shows typical

organisms on the various trophic

levels in a prairie (left) and ocean

(right) ecosystem.

The highest-level consumers in an ecosystem—those that are not preyed on themselves—are sometimes called

apex predators.

4/21/11

3

Imagine taking the amount of biomass (living matter) of each trophic level and representing it as a bar. Stack the bars on top of each other, and you get

a diagram called a trophic pyramid. Pyramid Schemes

•  At each trophic level, energy and biomass are lost. –  Not all of the matter an animal eats goes into building

new biomass. In fact, most of it doesn't. •  Why? Metabolism isn't 100% efficient. Some food is lost as

waste. And a lot of energy is lost as heat or used for “maintenance activities” instead of new growth.

– The ecological efficiency of each trophic level averages about 10%, although in real life it varies quite a bit.

•  Typical beef cattle convert about 6% of their total food intake into beef; pigs are more efficient at about 14%, and broiler chickens can do about 21%.

Thus the rule of thumb is that each level of the trophic pyramid is only one-tenth the size of the level below.

This leads to the phenomenon of

biological magnification—toxins such as DDT, which

organisms can't easily break

down or eliminate,

become more concentrated at higher levels of

the pyramid.

4/21/11

4

Another toxin that builds up at higher trophic levels is mercury. The US government currently advises pregnant and nursing women and

young children not to eat shark or swordfish, and to limit their consumption of tuna— because these fish are high -level

consumers, and are likeliest to contain dangerous levels of mercury.