Lecture 22 ICA 5 ENERGY FLOW IN THE ECOSYSTEM.
1. What are two over-arching themes of ecosystem ecology?
A. flow of energy B. cycling of matter
2. To what two thermodynamic laws must energy transformations conform? Explain.
A. Law 1: Energy is neither created or destroyed; it is conserved and must be
accounted for (= ‘balance the books’).
B.
Law 2: Energy transfer from one chemical bond to another is ‘inefficient’; energy
is lost as heat as a result (=’heat tax’).
3. Why does amount of energy transfer decrease after each chemical transformation?
Inefficiency on transfer of energy; energy is lost as heat, thus causing the decrease.
4. What does it mean to say: ”Energy flows, while matter cycles in ecosystems?
Energy moves (flows) up the food chain… energy is lost as heat at each biochemical
transfer. Energy is never recycled, so input (flow) of energy from sun is necessary to
sustain life. In contrast, matter moves through food chain, organisms die, are
decomposed, organic matter is transformed to inorganic forms, is returned to abiotic
pools from which it is recycled and used once again by plants and passed up the food
chain. Matter is re-used.
5. What is the last transfer in a ‘plant-based’ food chain?
From predator to top predator
What happens at the last transfer in a ‘decomposer-based’ food chain? Last
remaining chemical bond energy is lost as heat.
6. Figure 1. How + why does energy available decrease at each higher level in food
chains? Energy decreases sharply at each higher trophic level because the lower
trophic level uses much of the energy for its own functions/structures, and much is lost
as heat during these transformations.
7. Figure 2. Is carrying capacity of the earth affected by whether humans are vegetarians
or omnivores? Yes Explain. More humans can be feed at lower trophic levels
because less transformation of chemical bonds is done than if the energy was
transferred to higher trophic levels; hence more energy is available directly for
human consumption.
8. Figure 3. Which pyramid can be inverted and why?
Energy: Never; because energy is lost at each transfer in food chain; can’t have
greater at higher than lower trophic levels
Biomass: Yes: when turnover is high among phytoplankton, no large amount of
biomass is present at any one time, but large amounts of food are available
to support a larger biomass of zooplankton.
Numbers: Yes; if size of organism is smaller at higher trophic levels, then greater
numbers can occur at higher than lower levels (e.g. parasites on a predator).
9. What happens at higher trophic levels to toxins absorbed by plants? They become
‘biomagnified’ = accumulate to higher and higher concentrations.

10. What is Primary Production (PP)? What element does it summarize?
What are its units? Rate of converting light energy to chemical bond energy in
carbohydrates via photosynthesis. Element = C; unit = g C / m2/ time period
Why is its rate important for the ecosystem? It determines limit of chemical bond
energy available to all other organisms in ecosystem.
11. T or F? Photosynthesis occurs by day and respiration by night in plants.
PS = day; respiration = night + day
Figure 4. What is the relationship among Gross PP, Net PP, and Respiration ?
Gross PP = Net PP + Respiration
Imagine two plants with the same GPP, but plant 1 has twice as high a respiration
rate as plant two. Contrast their NPP. Plant 2 will have > NPP than Plant 1.
Which processes would be affected by their difference in NPP? growth,
maintenance, and reproduction
12. Figure 5. Is GPP measured directly or indirectly? Indirectly.
Describe how it is
estimated.
Measure CO2 uptake and CO2 output during light (day). Uptake –
output = NPP. Measure CO2 output during dark (night) = respiration. Add NPP
+ respiration = GPP
13. Figure 6. If measuring CO 2, what is NPP? Explain. NPP = 12 mg CO2/100cm2
leaf/hr NPP is CO2 input – CO2 output; difference = NPP During full sun both
Gross photosynthesis and respiration are occurring; their difference = NPP
What is rate of respiration? Explain. Respiration = 1.5 mg CO2/100cm2/hr.
Only respiration is occurring at night so a direct measure of CO2 output is a
direct measure of
What is GPP? Explain. GPP = 13.5 mg CO2/100cm2/hr = NPP + Respiration
14. How is Photosynthetic Efficiency measured? What are its units? It’s value?
% of light energy from sun converted to NPP = 1-2%
How is Net Production Efficiency measured? What are its units? % of GPP that
remains after respiration = (NPP/GG)x100.
What is one explanation why it is so much lower in tropical than temperate
ecosystems? Respiration occurs year-round, night and day at a higher temperature
in the tropics; it stops in winter and slows at night more in the temperate zone.
15. Which abiotic factors limit NPP? light; temperature, nutrients, precipitation; CO2
Figure 7. Explain the shape of the relationship of NPP vs. temperature. NPP
increases linearly with an increase in temperature. Enzymatic reactions increase
with increasing temperature.
Figure 7. Explain the shape of the relationship of NPP vs. precipitation. NPP
increases up to about 3000 mm rain/yr and then slowly declines. High rainfall
comes from many cloudy days with light limitation. High rainfall also brings low
transpiration, low nutrient uptake, and water-logged soils with less nutrient uptake.
Figure 8. Why is nitrogen important for an ecosystem? It is a part of every
rubisco molecule; the chief molecule responsible for fixing C from the atmosphere
during photosynthesis.

Explain the shape of the relationship of NPP vs. [N]. NPP increases with a
decreasing slope and then levels off with greater N. Other biochemical steps (other
those dependent on rubisco) begin to limit photosynthetic rate.
16. What determines whether NPP increases with >[CO2]? Is photosynthesis rate
limited by concentration of CO2 (e.g. by photorespiration in C3 plants)?
17. Figure 9 + 10. How does the community affect NPP? With a greater number of
species and a greater number of functional types present, NPP (measured as plant
biomass) increases up to some point and then levels off.
18. What limits NPP of freshwater lakes? P; with low N/P, blue-green algae increase
NPP because they are nitrogen fixers; with high N/P, green algal ‘blooms’ occur.
open ocean? near shore = N; open ocean: silica and Fe
19. Design an experiment to test whether Fe limits NPP in the open ocean.
Add iron sulfate to an area of ocean; compare with control area with no addition.
Measure amount of chlorophyll a (a surrogate for NPP).
Figure 12. Describe the results of one such experiment. Chlorphyll a increased at
surface levels with added Fe, but did not increase at 30 m depth with added Fe or in
control areas with ambient Fe. What is the conclusion? Fe limits NPP at surface
levels in the open ocean.
20. Figure 13. Summarize major global patterns of NPP. Would you base such
comparisons on NPP/area or % of total NPP? Based on NPP/area, highest NPP
occurs on land in wetter and warmer biomes and in water in swamps, marshes, algal
beds and coral reefs; it is very low in the open ocean. When take land area of each
biome into account, NPP is highest in open ocean and tropical rain forests. It
declines in other areas generally according to their rank in land area. Which type of
comparison to make depends on the question being raised. Global-level questions
require knowledge as % of total NPP.
21. Define secondary production. Energy fixed in carbon compounds by animals.
Figure 14. What explains pyramid shape of energy for the trophic levels?
Energy transfer to each higher level is limited because of heat loss at each lower
level during chemical transformations.
22. On what factors does amount of energy transfer depend? size of the
‘heat tax’; it is
lower in aquatic than terrestrial systems.
23. Describe the traditional ‘law’ of energy transfer.
10% of production at a given
trophic level is available to transfer to the next higher level.
Figure 15. How is Ecological (food chain) Efficiency quantified?
= (net production of torphic level n / net production of trophic level n-1) X 10
How should the “10% Law” be modified? It ranges from 5-20%; it is higher in
aquatic than terrestrial systems.

24. Fig. 16. What is NPP of Primary Producers? 1114-234 = 880 Prim. consumers?
= 148-44 = 104 Sec. consumers? = 31-18 = 13
What is Ecological Efficiency between level 1 and 2? = (104/880)100 = 11.8%
between 2 and 3? = (13/104)100 = 12.5%
What ultimately happens to:
1) energy: The energy not lost as heat (via respiration) in these 3 trophic levels is
stored in the unconsumed biomass. This energy may flow to decomposers that
extract it in respiration. The remaining energy remains in non-decomposed matter.
2) biomass not consumed in this lake? The unconsumed biomass will likely sink to
the lake bottom where it will either be decomposed, and thus continue immediately
in the BGC cycle, or stored in the sediments on the lake bottom.
25. Figure 17.How many lengths are there in short? 3 long 5-7 food chains?
What are hypotheses that explain the limit on length of food chain? Explain them.
A. Energetics. Energy limits the number of levels because of NPP available,
amount of energy needed by consumers, and ecological efficiency of transfer.
B. Dynamic stability.
Longer chains are less stable because fluctuations at lower
trophic levels are magnified at higher levels and top predators go extinct.
26. Figure 18. Do aquatic or terrestrial ecosystems have more trophic levels? Aquatic
Which factor contributes most to variation in food chain length among these
ecosystems? Ecological efficiency.
27. Figure 19. What processes result in transfer of energy within a trophic level?
Assimilation, respiration, excretion
What are processes that lower energy transfer during secondary production?
Respiration; excretion;
28. How is Exploitation Efficiency quantified:
= ( ingestion by trophic level n / production of trophic level n-1) X100
What are typical values for herbivores? 20% carnivores? 30%
29. What is assimilated energy = ingestion - egestion
How is Assimilation Efficiency quantified? = ( assimilation/ingestion) X 100
What primary factor affects this value? food quality; amount of non-digestible
matter
30. How is Secondary Production quantified? =
Assimilation – respiration - excretion
31. How is Net Production Efficiency quantified? = ( 2 nd
production / assimilation) X100
Why does it depend on metabolic activity? Metabolic activity depends on rate of
respiration. Therefore, NPE depends on metabolic rate.
Why do the most active animals have the lowest values? They have the highest
metabolic (and respiration) rates.
Contrast values for warm- (<1% birds; < 6% mammals) vs. cold-blooded animals
(15-75%).

QUANTIFICATION OF ENERGY FLOW
WITHIN PLANTS
Photosynthetic Efficiency = NPP/energy from sun in same time interval
Gross Primary Production (GPP) = Net Primary Production (NPP) + Respiration
Net Production Efficiency = NPP/GPP
BETWEEN TROPHIC LEVELS
Ecological Efficiency = Net production of level n/net production of level n-1
WITHIN ANIMAL TROPHIC LEVEL
Exploitation Efficiency = Ingestion by trophic level n/production of trophic level n-1
Assimilated energy = Ingestion – Egestion
Assimilation Efficiency = Assimilation / Ingestion
Secondary Production = Assimilated Energy - Respiration - Excretion
Net Production Efficiency = Secondary Production/Assimilation