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Ecosystem Ecology

• Ecosystem = community and abiotic (chemical, physical) interactions

• Energy flow depends on abiotic (sunlight, water, nutrient) and community (who eats who)

Where does energy originate?

Who uses it first?

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Energy Flow through Ecosystems• Primary production: Energy absorbed (fixed) by producers

(autotrophs) in ecosystem.– Rate of primary production: Energy fixed over time.

• Gross primary production: Total energy fixed by autotrophs.• Net primary production: Energy leftover after autotrophs’

needs satisfied.

Which can consumers use?

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How could you measure primary production?• Ecologists usually use one of these…

– CO2 uptake

• CO2 uptake = sugar production = energy fixed

– O2 released– Biomass: mass of autotrophs

Remember this:6CO2 + 6H2O C6H12O6 + 6O2

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How does primary production affect consumers?

• Trophic Level: Position in food web; i.e. # of energy transfers from primary producers to current level:– Primary producers = 1st level.– Primary consumers = 2nd level.– Secondary consumers = 3rd level.– Tertiary consumers = 4th level.

There is a limit… at the very most ecosystems support 8

levels (usu. 2-5)

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Terrestrial Primary Production• Temperature and precipitation affect

primary production – duh…• Rosenzweig summarized temp. and

precip. using annual actual evapotranspiration (AET).– AET = Amount of H2O that

evaporates/transpires off landscape per year (depends on temp. and precip.).

• Cold dry ecosystems tend to have low AET.

• Warm wet ecosystems?So, how does AET affect primary

productivity, do you suppose?

What else might affect primary productivity?

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Soil Fertility and Terrestrial Primary Production• Primary production also explained by soil fertility.

– Shaver and Chapin: arctic net primary production (npp) 2X greater on fertilized plots.

– Most well-known limiting nutrients = Nitrogen (N) and Phosphorus (P).

Why N and P?

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Also seen in Aquatic and Marine Primary Production

• Several studies support + relationship between P/N and phytoplankton.

• Perhaps N more limiting in marine, P more in freshwater.

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Global Patterns of Marine Primary Production• Highest nutrient rates and productivity found along

continental margins.– Nutrient run-off from land.– Sediment disturbance

• Open ocean tends to be nutrient poor.– Vertical mixing main nutrient source.

Nutrients, water, and temp still don’t explain all primary productivity, what

else could there be??????

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Consumers also affect primary productivity

• Bottom-Up Controls– Influences of physical and

chemical factors on ecosystem.– Nutrients, water, temperature

• Top-Down Controls– Influences of consumers.– Herbivores eat autotrophs– Carnivores eat herbivores

(trophic cascades)

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Consumers and Primary Production• Carpenter and Kitchell – a trophic cascade e.g.

– Large reduction in planktivorous (herbivore-eating) fish populations by top predator = more herbivores.

– Abundant, large herbivores reduce phytoplankton biomass and rate of primary production.

– Total energy flow reduced

So, consumers affect primary production too – but not always in a negative way

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Primary Production on the Serengeti• Rate of primary production in the Serengeti positively

correlated with rainfall.• McNaughton estimated Serengeti grazers consume 66% of

annual primary production.Do you think consumers have an impact? If so, positive or negative?

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Primary Production in the Serengeti• Grazers can increase primary production at intermediate

levels (say what?).– Growth rates by moderately browsed grasses increased.

• Compensatory Growth– Lower respiration rate due to lower biomass.– Reduced self-shading.– Improved water balance due to reduced leaf area.

Herbivory can increase overall energy flow!!!

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Primary Production in the Serengeti• Light grazing insufficient to produce compensatory growth.

– Limited by intraspecific competition, developmental constraints• Heavy grazing reduces plant’s capacity to recover (exploitation).• Intermediate herbivory = highest energy flow

Would that be exploitation, commensalism, mutualism?

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Trophic levels represent energy flow

• Lindeman suggested grouping organisms into trophic levels; simplifies study of energy transfer.

• Primary producers = 1st level, primary consumers = 2nd level, secondary consumers = 3rd level, etc.

• Each feeds on level immediately below.

So, how does energy actually ‘flow’?

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Trophic Levels and Energy Flow• As energy transferred b/n trophic levels, energy is degraded

(second law of thermodynamics)• Limited assimilation, consumer respiration, heat loss

• Ecological Efficiency = % energy transfer b/n trophic levels (5-20% on average)• Available energy ↓at higher trophic levels.

• Pyramid-shaped energy distribution.

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Secondaryconsumers

(perch)

10

100

1,000

10,000Usable energy

available ateach tropic level(in kilocalories)

Heat

Heat

Heat

Heat

Heat

Producers(phytoplankton)

Tertiaryconsumers(human)

Primaryconsumers

(zooplankton)

Decomposers

• Eventually, not enough energy for population of higher level consumers

• Most ecosystems support 2-5 trophic levels (8 at very most)

Trophic levels are limited as a result

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Energy Flow in a Temperate Deciduous Forest

Only 1% of sun’s energy available to consumers!

Herbivores only consume 1% of net production!

• Gosz studied solar energy flow:– 15% reflected, 41% heat,

42% evapotranspiration– 2.2% fixed by plants as

gross primary production– 1.2% used in plant

respiration– 1% net primary production

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Review• Primary Production

– Temperature and water– Nutrient availability– Consumer effects– Top-down vs. Bottom-up

• Trophic Levels/Dynamics– Ecological efficiency– Limits to trophic levels

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Figure 18_18

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Figure 18_19

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Figure 18_20