ECOSYSTEMS AND ENERGY FLOW
CH 55
• Energy flows through ecosystems while matter
cycles through ecosystems
I. Ecosystems and Physics (what?!)
• ecosystems involve matter and energy
transformations
– NRG can’t be created or destroyed, only
transformed
– solar NRG→chemical NRG→heat
– NRG transfers increase entropy therefore they
are inefficient and NRG is lost as heat
– Matter can’t be created
II. Trophic Structure, Food Chains And Food
Webs
• Food chains link trophic levels together
• Food webs are branching food chains with
more complex interactions
III. Energy and Productivity in
Ecosystems
• In most ecosystems, primary production is
the amount of solar energy converted to
chemical energy (the rate of photosynthesis)
• How much photosynthesis occurs, sets how
much energy is available in that ecosystem
A. Gross and Net Production
• Gross primary production (GPP): amount
of photosynthesis in a given time. The
amount of carbon fixed into organic
molecules
• Net primary production (NPP): what is left
over for consumers. It is the amount of new
biomass added by autotrophs
• What do autotrophs do with the organic
molecules produced?
o Use it in cell respiration
o Store it as new biomass
• How do you measure net primary production?
o Gross production – cell respiration = net
production
• Tropical rainforests and coral reefs are the
most productive while marine ecosystems
are the least
• Terrestrial and aquatic ecosystems have
different limitations to their primary
production
Measuring NPP in terrestrial ecosystems
• Terrestrial ecosystem: forest, grassland
• Measure the increase in biomass
Measuring NPP in aquatic ecosystems
• Measure rates of photosynthesis and respiration
B. Primary Production and
Eutrophication
• Excessive addition of nutrients like
phosphorus and nitrogen to aquatic
ecosystems causes eutrophication
– Excess nutrients cause algal bloom
– As algae die, decomposers use up oxygen
decaying them
– This limits oxygen available to fish at
deeper levels
IV. Energy Transfer Between Trophic
Levels
• Is inefficient with only about 10% from one
trophic level is available to next
• Depending on organism, 10-30% of energy
obtained is converted to new biomass =
secondary production
• The rest is used for cell respiration or is lost
as waste
Production efficiency = secondary
production/amount assimilated
• This inefficient energy transfer creates
ecological pyramids
V. Biogeochemical Cycling in Ecosystems
• Life depends on recycling of matter
A. Water cycle
• The water from oceans evaporates into the
atmosphere
• Water from plants evaporates to the
atmosphere by transpiration
• Water precipitates from the atmosphere to
the earth
• Most of the water enters into the oceans
• Some of the water is taken up by living
organisms
B. The Carbon cycle
• CO2 in the atmosphere is fixed by
photosynthetic organisms
• animals eat the photosynthetic organisms
• animals respire and put CO2 back into the
atmosphere
• animals die and their remains become fossil
fuels
• fossil fuels are burned and CO2 is deposited
in the atmosphere
C. The nitrogen cycle
• atmospheric N2 is fixed by nitrogen fixing
bacteria and used by plants
• herbivores eat the plants
• herbivores pee, poop and die returning nitrogen
to the soil which is then fixed by nitrogen fixing
bacteria in plants
• Carnivores eat herbivores
• Carnivores pee, poop, and die returning nitrogen
to the soil to be fixed
• some bacteria in the soil take the soil nitrogen
and restore it to atmospheric nitrogen