Lecture 22 ICA 4
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 nor 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. Explain: ”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.
INTRA-TROPHIC LEVEL ENERGY (PLANT-BASED)
6. 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.
7. T or F? Plants photosynthesize; animals respire. F. Plants photosynthesis; all living
cells (plant or animal) respire.
T or F? Photosynthesis occurs by day and respiration by night in plants.
F. PS = day; respiration = night + day
Figure 1. 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
8. Figure 2. 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
9.
Figure 3. 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 respiration.
What is GPP? Explain. GPP = 13.5 mg CO2/100cm2/hr = NPP + Respiration
10. What is Photosynthetic Efficiency? What are its units? Its 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/GPP)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.
11. Which abiotic factors limit NPP? light; temperature, nutrients, precipitation; CO2
Figure 4. Explain this ‘light response curve’. As light intensity increases,
productivity increases up to a point, after which it levels off.
‘Compensation point’ =light intensity at which PS = respiration.
‘Saturation point’ = light intensity above which no further increase in PS occurs.
Figure 5. 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 5. 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.
‘Water use efficiency’ (WUE) = g NPP per kg water transpired
Figure 6. 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
than those dependent on rubisco) begin to limit photosynthetic rate.
‘Nutrient use efficiency’ (NUE) = g production per g N assimilated
Figure 7. How is NUE related to productivity? Biomes with lower productivity tend
to have higher NUE.
Figure 8. Summarize three patterns about nutrients and productivity by these plants.
1. Most species respond to N but not P.
2. Nitrogen-fixing species respond to P but not N.
3. Annual plants respond to combined N + P.
12. What determines whether NPP increases with >[CO2]? Is photosynthesis rate
limited by concentration of CO2 (e.g. by photorespiration in C3 plants)?
Explain how ‘vegetation acts as a C sink. Via photosynthesis plants take CO2 out of
the atmosphere and fix it as C-compounds; they sequester C.
13. Figure 9 + 10. How does community structure 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.
14. 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
Figure 11. Why is productivity higher near the coast than in the open ocean?
Near the coast, Sediments with nutrients are stirred and brought up to higher water
levels with enough light for photosynthesis. The open ocean does not get a
replenished supply of nutrients that settle to the bottom.
15. 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.
16. 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.
Figure 14. Summarize variation in NPP among habitats. Productivity declines
among biomes along a latitudinal gradient from 0 to 90º. It is highest in swamps and
marshes and aquatic beds and estuaries; it is lowest in deserts and the open ocean.
INTER-TROPHIC LEVEL ENERGY TRANSFERS
17. Contrast plant-based vs. decomposer-based food chains. What is the relationship
between them? P-based chains get energy from the sun and transfer it from plants
through a series of animal consumers. Energy remains in organisms as they die
when D-based chains begin. D- based chains get energy from dead plants
and animals – transfer it through a series of consumers, ultimately releasing all
energy as heat.
18. Figure 15. Explain: ‘Food chains represent energy relationships’. Energycontaining food is passed from one feeding level to the next higher level
19. Figure 16. How + why does energy available decrease at each higher level in
food chains? Why does an energy ‘pyramid’ develop?
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. Energy transfer to each higher level is
less and less because of loss at each lower level during chemical transformations.
On what factors does amount of energy transfer depend? size of the ‘heat tax’; NPP,
efficiencies of energy transfer, residence time
20. Figure 17. 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 were transferred to higher trophic levels; hence more energy is available
directly for human consumption.
21. Figure 18. 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).
22. 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 19. 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.
23. Fig. 20. 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.
24. Figure 22. How many lengths are there in food chains? Short = 3; long = 5-7
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.
25. Figure 22. 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; it is higher in aquatic than terrestrial systems.
INTRA-TROPHIC LEVEL ENERGY (ANIMALS)
26. Define secondary production. Energy fixed in carbon compounds by animals.
Figure 23. How is Secondary Production quantified? =
Assimilation – respiration - excretion
27. 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
WITIHIN ECOSYSTEM PRODUCITY
28. How is Net Ecosystem Production (NEP) quantified? C gain – C loss = net C accumulation
How does it relate to carbon sequestration? C assimilated by plants is bound as C in organic
compounds; plants ‘hide’ it, keeping it away from decomposers that return the C as CO2 to
the atmosphere. How does it relate to C sink? The atmosphere is the source of C as CO2; it
moves to the sink, plants, via photosynthesis.
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)100
BETWEEN TROPHIC LEVELS
Ecological Efficiency = (Net production of level n/net production of level n-1)100
WITHIN ANIMAL TROPHIC LEVEL
Exploitation Efficiency = Ingestion by trophic level n/production of trophic level n-1
Secondary Production = Assimilated Energy - Respiration - Excretion
WITHIN ECOSYSTEM LEVEL
Net ecosystem productivitiy (NEP) = carbon gain - carbon lost = net C accumulation