4. 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.

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.

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.

(A) Trophic Levels (4 different) - energy obtained ...................................... (Maximum = 3)

• List - example of four organisms correlated to proper trophic level

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 - Sculpin- Lake 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) Energy Transfer (efficiency) - effect on ecosystem structure...................... (Maximum = 4)

• (point must be earned from either Efficiency or Ecosystem Structure to earn 4 point maximum)

• 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

• 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

(C) Toxin from dead terrestrial plants (effect) - explanation

(1 pt) for each effect.... but must link one secondary effect to earn a .............. (Maximum = 4)

(1 pt) for each correct interdependent pathway as a secondary effect

EFFECT SECONDARY EFFECT(S)

(NOTE) this list is not exhaustive

• Death of Snails/Larvae • Death of snails and larvae lead to death of upper level

• 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

• 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)