Chapter 54Ecosystems

Ecosystem: Overview• An ecosystem consists of

– All the organisms living in a community– all the abiotic factors with which they interact.

• The dynamics of an ecosystem involve two processes: – Energy flow– Chemical cycling.

• Ecosystem ecologists view ecosystems as – Energy machines– Matter processors.

• We can follow the transformation of energy by grouping the species in a community into trophic levels of feeding relationships.

biosphere

Ecosystem inputsconstant inputof energyenergy flowsthrough

nutrients cycle

nutrients can only cycle

inputs energy nutrients

Don’t forgetthe laws of Physics!

Matter cannotbe created ordestroyed

Conservation of Energy• Laws of physics and chemistry apply to ecosystems, particularly

energy flow– The first law of thermodynamics states that energy cannot be

created or destroyed, only transformed• Energy enters an ecosystem as solar radiation from the sun

– The second law of thermodynamics states that every exchange of energy increases the entropy of the universe

• In an ecosystem, energy conversions (transformations) are not completely efficient

– some energy is always “lost” from organisms as heat» This leads to implications at different trophic levels.

– The law of conservation of mass states that matter cannot be created or destroyed

• Mass is never “lost” like energy• Mass continues to change form- atoms rearrange themselves

into new combinations of molecules.– This explains the biogeochemical cycles (C,N,P, etc.)

KeyChemical cyclingEnergy flow

Sun

Heat

Primary producers

Primaryconsumers

Secondary andtertiary consumers

Detritus

Microorganismsand other

detritivores

An ecosystem’s main decomposers are fungi and prokaryotes, which secrete enzymes that digest organic material and then absorb the breakdown products.

Following the Energy Flow Through Ecosystems

• In most ecosystems, primary production is the amount of light energy converted to chemical energy by autotrophs during a given time period

• The extent of photosynthetic production sets the spending limit for an ecosystem’s energy budget

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Gross and Net Primary Production• Total primary production is known as the

ecosystem’s gross primary production (GPP)– GPP is measured as the amount of light energy that is

converted to chemical energy by photosynthesis per unit time.

– Not all of this energy is stored as organic matter by plants because plants need to use this energy to as fuel in their own cellular respiration. Therefore. . .

• Net primary production (NPP) is GPP minus energy used by primary producers for respiration– NPP = GPP - Respiration– NPP is expressed as

Energy per unit area per unit time (J/m2⋅yr), or Biomass added per unit area per unit time

(g/m2⋅yr)

• NPP is the amount of new biomass added in a given time period

• Only NPP is storage of chemical energy that is available to consumers in an ecosystem

• Possible Point of Confusion:• Standing crop is the total biomass of photosynthetic

autotrophs at a given time, NPP is the amount of new biomass added in a given time period– For example: A forest has a large standing crop biomass,

but it’s NPP may be less than grasslands which do not accumulate vegetation because animals eat the plants rapidly.

• Different ecosystems vary widely in their NPP– Terrestrial: Tropical Rainforests are tops– Aquatic: Coral reefs and estuaries are tops

Net Primary Production

Primary Production in Marine and Freshwater Ecosystems

• Light and Nutrients are the two key factors• More than light, nutrients limit primary

production in geographic regions of the ocean and in lakes

• A limiting nutrient is the element that must be added for production to increase in an area

• Nitrogen and phosphorous are the nutrients that most often limit marine production

• Nutrient enrichment experiments confirmed that nitrogen was limiting phytoplankton growth off the shore of Long Island, New York

Inquiry: Which nutrient limits phytoplankton production along the coast of Long Island?

Primary Production in Terrestrial Ecosystems

• Temperature and Moisture are the two key factors

• Tropical forests– Warm and moist– Most productive terrestrial ecosystem on the

planet• Contrast with Deserts and Arctic Tundra

– Dry and extreme temperatures (too hot, too cold)– Not productive

Mean annual precipitation (cm)0 20 200180160140120100806040

Net a

nnua

l prim

ary

prod

uctio

n(a

bove

gro

und,

dry

g/m

2 y

r)1,400

1,200

1,000

800

600

400

200

A global relationship between net primary production and mean annual precipitation for terrestrial ecosystems

Energy transfer between trophic levels is typically less than 20%

efficient• Secondary production of an ecosystem is the

amount of chemical energy in food converted to new biomass during a given period of time

• Examine organic matter transfer from producers -> herbivores (primary consumers)– Herbivores only eat a small amount of the plant

material produced– They can’t digest all the plant material they do

eat– Therefore, much of primary production is not

used by consumers!

Production Efficiency• When a caterpillar feeds on a leaf,

only about one-sixth of the leaf’s energy is used for secondary production, i.e. used for growth (converted to new biomass)– Some of the energy is used for

cellular respiration– Rest is lost as feces

• Energy used for cellular respiration is lost from the ecosystem as heat– This is why energy is said to

flow through, not cycle within, ecosystems.

Trophic Efficiency and Ecological Pyramids

• Trophic efficiency is the percentage of production transferred from one trophic level to the next

• It is usually about 10%, with a range of 5% to 20%• Trophic efficiency is multiplied over the length of a

food chain

Food Chain

Linear Path of Energy Flow

Food WebAll the Possible Ways That Energy Flows

29

Water Cycle

Carbon Cycle

32

Nitrogen Cycle

Phosphorus Cycle