AP® BIOLOGY
SCORING GUIDELINES
Question 9
Interdependence in nature is illustrated by the transfer of energy through trophic levels. The diagram
below depicts the transfer of energy in a food web of an arctic lake located in Alaska.
(a) Choosing organisms from FOUR different trophic levels of this food web as examples, explain how
energy is obtained at each trophic level. (3 point maximum)
* Must include both example and how energy is obtained to earn point
Examples correlated to proper trophic level
Energy Obtained
 T.L #0 - (Detritivores) - Snails - larvae of Chironomids Larvae of Caddis Flies
 T.L #1 - (Producers/autotrophs) Phytoplankton - Algae Aquatic plants - Dead terrestrial plant
 T.L #2 - (First order or primary consumers) Zooplankton
- Snails - Larvae of Chironomids - Larvae of Caddis
Flies
 T.L #3 - (Second order consumers) Grayling - SculpinLake Trout
 T.L #4 - (Third order consumers) Lake trout

Producers obtain energy by
photosynthesis

Consumers obtain energy by digestion
or assimilation (must link to specific
consumer or level)
(b) Describe the efficiency of energy transfer between trophic levels and discuss how the amount of
energy available at each trophic level affects the structure of the ecosystem.(4 point maximum)
*One point must be earned from Efficiency and Ecosystem Structure to earn 4 point max.
Efficiency
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10% rule or overall inefficient
why inefficient (e.g. cellular respiration, never
eaten, wastes, heat, metabolism)
thermodynamics - increase in entropy
primary - net - gross productivity
life history - energy value varies per life stage
Ecosystem Structure
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number of level thus indicated as 4-5 maximum
pyramid of energy
pyramid of biomass
pyramid of numbers
standing crop effect as related to phytoplankton
and zooplankton
AP® BIOLOGY
SCORING GUIDELINES
Question 9 (continued)
(c) If the cells in the dead terrestrial plant material that washed into the lake contained a commercially
produced toxin, what would be the likely effects of this toxin on this food web? Explain. (4 point
maximum)
* 1 point for each effect.... but must link one secondary effect to earn a .............. (Maximum = 4)
* 1 point for each correct interdependent pathway as a secondary effect
(NOTE) this list is not exhaustive
EFFECT and SECONDARY EFFECT(S)
 Death of Snails/Larvae
 Death of snails and larvae lead to death of upper levels
 Death of trout leads to increase of sculpin and grayling
 Magnification/accumulation
 Death of grayling leads to overgrowth of zooplankton
 Death of sculpin leads to increase feeding of trout on grayling
 Sub-lethal effect grayling
 Eutrophication - tied to effect
(1 point maximum) - for any of the following:
 Reason for why toxin is persistent, e.g. lipid soluble, eggshell effect, DDT, indigestible
 Biodegradable
 "left side" not immediately effected
 "Mutagenic" - explanation must be included
 "Create" pathway to phytoplankton (e.g. water soluble)