chapter 54 lecture
TRANSCRIPT
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