MIT Department of Biology
7.014 Introductory Biology, Spring 2005
Recitation Section 14 Answer Key
March 30-31, 2005
Biogeochemical Cycles—Energy Flows
A. Carbon and Energy in a Food Web. Steady State.
2
Plants
R
R
A
H
6
Herbivore
R
H 11
Carnivore
Pgross
P
1
4
3
A
I
F
B
D
7
10
P 8
B
A
I
P
13
F
B
D
9
D
12
5
B = biomass
D = detritus
F = feces
I = ingestion
A = assimilation
P = production
R = respiration
I=A+F
A=R+P
Detritivore
B
R
14
H
D
15
16
1. What do the arrows in the diagram represent?
The arrows in the diagram represent the flow of carbon and energy through the trophic levels of an ecosystem per unit time. 2. Why are the arrows of different sizes?
The width of the arrow represents the amount of carbon (energy) in the ecosystem at a particular point.
3. How does the energy enter the ecosystem? Is it converted in any way before it is used in the ecosystem?
Energy enters the ecosystem as sunlight which is converted to chemical energy by photosynthesis in plants.
4. What do each of the following numbers represent?
1 = gross plant photosynthesis (production = GPP) 2 = plant respiration (RA)
3 = net plant production (NPP)
4 = plants eaten by herbivores; herbivore ingestion 5 = plants that die before being eaten; plant detritus 6 = herbivore respiration (RH1)
7 = herbivore waste (feces)
8 = net herbivore production
9 = herbivores that die without being eaten
10 = herbivores eaten by carnivores; carnivore ingestion 11 = carnivore respiration (RH2)
12 = carnivore waste (feces)
13 = carnivores that die
14 = respiration of detritivores (RH3)
15 = accumulation of refractory carbon
16 = total refractory C (accumulation = ∫ (15)dt )
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5. Starting with 106 kcal/yr of GPP and given the following values, estimate the values of each of the flows
from part 4 assuming steady-state conditions. NPP
= 50%
For plants : net production efficiency =
GPP
ingestion
production from previous trophic level
assimilation
= 25%
assimilation efficiency =
ingestion
production
= 20%
production efficiency =
assimilation
For the herbivores: exploitation efficiency =
= 20%
For the carnivores: exploitation efficiency = 90% (no reference)
assimilation efficiency = 75%
production efficiency = 5%
b)
500,000
Plants
R
R
A
20,000
3206
R
H
3,375
Herbivore
H
Carnivore
light
10
Pgross
100,000
P
500,000
6
I
25,000
A
F
B
B
P 5,000
4,500
A
I
P
F
D
B
D
D
500
75,000
1125
169
400,000
Detritvore
R
476,794
B
H
D
0
6. The carbon in respiration (R) flows can be used by other organisms, but the energy in the respiration (R)
flows cannot. Both the carbon and energy of all other flows can be used by other organisms. What is
different about the R flows?
The R flows are unique in that they represent energy expended by the organism to move, eat, maintain
homeostasis, etc. This represents the loss of the energy in the form of heat. This is the energy consumed by
the organism; it is therefore unavailable for use by other trophic levels. All other flows represent organic
carbon (“glucose”) which can be “burned” via respiration (aerobic mostly, but anaerobic respiration &
fermentation also use up some) to release energy. This organic carbon can also be used to make biomass,
waste, etc. The R flows represent CO2 which can only be used by autotrophs (oxygenic photosynthesis
mostly, but also anoxygenic photosynthesis and chemosynthesis) to be fixed into organic carbon.
2
B. Non-Steady State.
Now consider the non-steady-state. Suppose you remove 50% of the herbivores at time = A. You then measure
the plant-associated flows and find the following:
5
10 kcal/yr
15
(15)
11
(gross
plant
(gross plant
production)
production)
(12)
10
(7.5)
2
(plant
respiration)
(6)
(5.5)
5
(plant detritus)
5
(2)
(1)
(2)
4
(herbivore
injestion)
0
A
B
C
1. What is going on? Consider the state of the ecosystem at times B and C.
At time = A, the system is at steady state (1 = 2 + 4 + 5) with a low amount of plant biomass. “The cows
are eating a lot of the grass.” Removing the “cows” allows some of the grass to grow back. This is what is
happening at time = B. Here, there is net increase in plant biomass (1 - (2 + 4 + 5) = 3x105 kcal/yr).
Eventually, the grass gets crowded & starts to die at a greater rate, resulting in a new steady state at time =
C.
3