Ecosystems

Goals

1. To understand how ecosystem ecologists approach the study of the natural world

2. To understand how energy flows through ecosystems

3. To understand how matter (nutrients) cycle within ecosystems

Outline

1. What is an ecosystem?

ecosystem = abiotic environment. (= matter and energy) + biotic community

ecosystem ecologists consider the role organisms play rather than what organisms are out there.

2. How does energy flow through ecosystems?

Food chains describes how energy flows between trophic levels

producers = autotrophs = those who can get energy straight from the abiotic environment (the sun)

consumers = heterotrphs = those who must get energy from other organisms

• primary (1°) consumers = herbivores.... eat producers
• secondary (2°) consumers = carnivores = predators... eat 1° consumers
• tertiary (3°) consumers eat 2° consumers, and are eaten by quaternary (4°) consumers

decomposers = those who get energy from detritus (bacteria and fungi)

• detritus = nonliving organic matter

photosynthesis: how energy enters ecosystems:

light + CO₂ + H₂O -----> O₂ + organic carbon

respiration: how captured energy is put to use:

O₂ + organic carbon -----> CO₂ + H₂O + released energy

Once energy enters a trophic level, it can either

1. be used by that trophic level and is lost as heat
2. be passed on to the next trophic level
3. become detritus, going to the decomposers (lots of organic C ends up as detritus)

Food chains vs food webs: real life is much more complicated

• an organism may occupy more than one trophic level
• energy in detritus may reenter the food web as detritivores eat decomposers

There is a constant input of energy into ecosystems (as light), and energy constantly flows out of ecosystems (as heat). Energy flows through ecosystems.

3. How efficient is the flow of energy between trophic levels?

only about 1% of the light falling on an ecosystem is captured by autotrophs

this amount = gross primary production (GPP)

Of that energy that is captured by plants (GPP):

1. 10 - 50% will be used (respiration)
2. The remainder (NPP, Net PP) may be passed on to consumers or decomposers
3. Of the NPP, only 5-10% goes to herbivores in terrestrial ecosystems, up to 40% in aquatic.....the rest to decomposers

Thus, only about 10% of GPP goes to 1° consumers

DETRITUS!!!

• In general, only 10% of the energy entering a trophic level will enter the next.

This reflects ecological efficiency, which is equal to:

the amount of energy entering a trophic level divided by the amount of energy entering the previous trophic level

This loss of energy is illustrated in the Pyramid of Productivity

Consequences of the pyramid (i.e., of ecological inefficiency)

1. food chains are of finite length
2. "eating low on the food chain" is more efficient
3. Biological magnification of toxins can take place in higher trophic levels

4. How does matter cycle in ecosystems?

Matter cycles within ecosystems, energy flows through them

5. the Phosphorus cycle

P occurs only as PO₄^-3

(know the cycle -- its pools and fluxes)

PO₄^-3 is in rocks, dissolved, and as organic P (e.g., DNA and phospholipids): there is no gaseous form of PO₄^-3 ,the only free form is dissolved

P can be imported into a system -- humans do this a lot (fertilizer and sewage are the leading causes of the eutrophication of lakes)

• Eutrophication is the increase in a system's productivity.
• Lakes become eutrophic through the addition of P because P is the common limiting nutrient in lakes

3. the Nitrogen cycle

N has multiple forms:

• N₂ = gaseous N -- inert
• organic N -- DNA and proteins
• NO₃^- = nitrate ... NO₂^- = nitrite
• NH₃ = ammonia ... NH₄⁺ = ammonium

Bacteria are responsible for much of the transformation of N between forms

• Legumes are plants that have a mutualism with N-fixing bacteria
• N is most commonly the limiting nutrient in local terrestrial ecosystems

4. the Carbon cycle

(know the cycle -- its pools and fluxes)

the detrital carbon pool is 3 times the size of the biotic pool the CO₂ pool is much larger than either

Vocabulary

click here to go to Ecosystem ecology vocaulary