MIT Department of Biology
7.014 Introductory Biology, Spring 2005
Recitation Section 14
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
10
P 8
A
I
13
B
D
B
7
P
F
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
14
B
R
H
D
15
16
1. What do the arrows in the diagram represent?
2. Why are the arrows of different sizes?
3. How does the energy enter the ecosystem? Is it converted in any way before it is used in the ecosystem?
4. What do each of the following numbers represent?
1=
3=
5=
7=
9=
11 =
13 =
15 =
2=
4=
6=
8=
10 =
12 =
14 =
16 =
1
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%
Plants
R
RA
H
R
Herbivore
H
Carnivore
light
Pgross
A
I
P
F
B
B
A
I
P
P
F
B
D
D
D
Detritvore
R
H
B
D
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?
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.
3