Energy Transfer and Losses
March 6, 2009
Pp. 1196-1201
Energy Transfer
Summary Figures
I. Trophic Structure
Considerable energy in autotroph biomass - available to heterotrophs
How much? If biomass is to remain constant, annual productivity must be
harvested.
We expect less energy at the next "trophic level"
That is, primary producer energy > primary consumer energy > etc.
e.g. plants > herbivores > carnivores > secondary carnivores
But, transfer of energy between trophic levels not very efficient
Example: phytoplankton and filter feeding zooplankton of lakes ~ 10%
Why?
II. Ecological Efficiency
A) Consumption or exploitation efficiency = energy consumed/energy available
Primary producers are rich in reduced carbon compounds, but may not
be ingestible.
Physical (thorns, spines, woody, silica) and chemical (toxins, DRS,
distasteful) (Will revisit plant defenses on April 9)
Typically herbivores ingest only 1.5 - 2.5% Net Primary Prod. of forests
10 - 15% net primary prod. of grasslands
60 - 90% net primary prod. of lakes/oceans
B) Assimilation efficiency = energy assimilated/energy consumed
Not everything consumed can be assimilated (poop)
Typically, 30 - 40% ingested material assimilated by grazers/browsers
~80% ingested material assimilated by granivores (seed eaters)
For example, what to do about cellulose?
Cellulase rare among animals
Ruminant
story
C) Production efficiency = biomass energy produced/energy assimilated
Not all energy assimilated results in increased biomass.
Energy required for homeostasis, incl. body heat in endotherms
Typically, homeotherms < 3%
poikilotherms (vertebrates) ~ 10%
poikilotherms (invertebrates) ~ 30.40%
D) Ecological Efficiency = C.E. x A.E. x P.E.
Examples: zooplankton in lakes ~ 9%
buffalo grazing in grasslands ~ 0.13%
Ecological efficiency increases up trophic pyramid
consumption efficiency higher
assimilation efficiency higher
III. The Role of Detritus
Have primary consumers made use of all primary production?
What about uneaten and unassimilated organic matter?
Importance of detritus in lakes, grasslands, forests
Detrital energy may be exported to subsidize another community
Bear Brook as an example
IV. Problems with Trophic Level Concept
Some ecologists find trophic level concept awkward
How do you categorize omnivores? As herbivores or as carnivores?
A significant fraction of animals are omnivorous
V. Food Webs
Food webs as energy schematic diagrams for communities:
Ocean example
Lake George example
Patterns observed from food web analysis?
Food chains are short. Why?
Dissipation of energy? No empirical
support.
Review Questions
1) What advantage does a trophic pyramid of energy have over a trophic pyramid
of numbers?
2) What is consumption efficiency and why might it be low?
3) What is assimilation efficiency and why might it be low?
4) What is production efficiency and why might it be low?
5) Why might assimilation efficiency of granivores (seed eaters) be greater
than that of grazers?
6) Why is consumption efficiency greater in plankton communities than in a
forest?
7) What is detritus, and why is it important in the energy budget of
communities?
8) What is an example of an energy subsidy from one community to another?
9) What does the energy flow hypothesis predict about the length of food
chains?
Key Terms: primary producer, primary consumer = secondary producer, energy pyramid, consumption efficiency, assimilation efficiency, production efficiency, cellulose, ruminant, ecological efficiency, detritus, food web, food chain
