1. Pop Quiz

2. Pop quiz
In absolute silence, answer the following question in writing in 5 sentences – no more and no less. You have 5 minutes.
How do we – as people – solve the environmental challenges affecting Lebanon and the region (Mashreq) and the broader region (Arab region)? Be sure to talk about what species, both plant and animal, (using their latin names) should be incorporated in such a transformation.

4. We’re back Spain? Chapters done so far MCAT? Next chapter
Workshop: “Environment and Security in the Southern Mediterranean” – organized by the European Organization for Cooperation and Security. Focus on Mashreq and Maghreb.
Chapters done so far
Chapters 1, 2, 3, 4, 5, 10, 11, and 16 + extra slides on population (end of chapter 10) + material on climate change on ecology website
Exam 1 – April 8.
If you want more material on Exam 1, let me know 
MCAT?
Next chapter
Chapter 6

5. Chapter 6: Energy in the Ecosystem
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
Chapter 6: Energy in the Ecosystem

6. Background: Organizing Concepts
In 1920s, English ecologist Charles Elton and others promoted a revolutionary concept:
organisms living in the same place not only have similar tolerances of physical factors, but
feeding relationships link these organisms into a single functional entity
This system of feeding relationships is called a food web.

7. The Ecosystem Concept
The English ecologist A.G. Tansley took Elton’s ideas one step further:
in 1935 Tansley coined the term ecosystem, the fundamental unit of ecological organization
the ecosystem concept: “the biological and physical parts of nature together, unified by the dependence of animals and plants on their physical surroundings and by their contributions to maintaining the conditions and composition of the physical world.” -R.E. Ricklefs

8. Alfred J. Lotka, the Thermodynamic Concept, and Lindeman’s concept
Alfred J. Lotka introduced the concept of the ecosystem as an energy-transforming machine:
described by a set of equations representing exchanges of matter and energy among components, and
obeying thermodynamic principles that govern all energy transformations
In 1942, Raymond Lindeman brought Lotka’s ideas of the ecosystem as an energy-transforming machine to the attention of ecologists. He incorporated:
Lotka’s thermodynamic concepts
Elton’s concept of the food web as expression of the ecosystem’s structure
Tansley’s concept of the ecosystem as the fundamental unit in ecology

9. Lindeman’s Foundations of Ecosystem Ecology
The ecosystem is the fundamental unit of ecology.
Within the ecosystem, energy passes through many steps or links in a food chain.
Each link in the food chain is a trophic level (or feeding level).
Inefficiencies in energy transformation lead to a pyramid of energy in the ecosystem.

10. Odum’s Energy Flux Model
Eugene P. Odum popularized ecology to a generation of ecologists.
Odum further developed the emerging framework of ecosystem ecology:
he recognized the utility of energy and masses of elements as common “currencies” in comparative analysis of ecosystem structure and function
Odum extended his models to incorporate nutrient cycling.
Fluxes of energy and materials are closely linked in ecosystem function. However, they are fundamentally different:
energy enters ecosystems as light and is degraded into heat
nutrients cycle indefinitely, converted from inorganic to organic forms and back again
Studies of nutrient cycling provide an index to fluxes of energy.

11. Simple Ecosystem Model
energy input from sun
PHOTOAUTOTROPHS
(plants, other producers)
nutrient
cycling
HETEROTROPHS
(consumers, decomposers)
energy output (mainly heat)

12. Models of ecological energy flow
Eugene Odum’s “universal” model of ecological energy flow. (a) A single trophic level. (b) Representation of a food chain. The net production of one trophic level becomes the ingested energy of the next higher level.
A food chain
A single trophic level

13. An ecological pyramid of energy

14. Only 5% to 20% of energy passes between trophic levels.
Energy reaching each trophic level depends on:
net primary production (base of food chain)
efficiencies of transfers between trophic levels
- More on this later -
Plant use between 15% and 70% of light energy assimilated for maintenance – thus that portion is unavailable to consumers
Herbivores and carnivores expend more energy on maintenance than do plants: production of each trophic level is only 5% to 20% that of the level below it.

15. Energy: how many lbs of grass to support one hawk

16. Ocean food pyramid – roughly 2500 lbs/1136 kg of phytoplankton to support 0.5lb/0.23 kg of tuna

17. Primary Production: reminder
Primary production is the process whereby plants,algae, and some bacteria (primary producers) capture the energy of light and transform it into the energy of chemical bonds in carbohydrate:
its rate is referred to as primary productivity
6CO2 + 6H2O  C6H12O6 + 6O2
for each g of C assimilated, 39 kJ energy stored
The rate of primary production determines the rate of energy supply to the rest of the ecosystem:
gross primary production = total energy assimilated by primary producers
net primary production = energy accumulated (in stored form) by primary producers
gross - net = respiration, the energy consumed by producers for maintenance and biosynthesis

18. Measurement of Primary Production 1
How much energy has been accumulated by net production?
harvest techniques determine dry mass accumulated (net production)
gas exchange techniques determine net uptake of CO2 in light (net production), production of CO2 in dark (respiration) and gross production as their sum
Radioactive carbon (14C) may also determine net uptake of carbon by plants

19. Measurements of Carbon Dioxide flux in dark and light can provide an estimate of GPP

20. Measurement of Primary Production 2
Aquatic systems pose special problems:
harvest approach is not practical for small organisms, such as phytoplankton
carbon is too abundant for practical measurement of small changes
Alternatives for aquatic systems:
light and dark bottles may be used to determine changes in O2
14C approach may also be used in unproductive waters

21. Use of Remote Sensing
Satellites can use spectral bands to infer amount of chlorophyll in water or the near-infrared to red ratio on land (NDVI index)
NDVI=Normalized Difference Vegetation Index

22. Fig , p.850

23. Effects of Light and Temperature 1
Plants are not usually light-limited in full sun.
Shading (by other leaves or plants) may reduce photosynthetic rate below its maximum.
Overall, photosynthetic efficiency of the ecosystem is typically 1-2%:
remaining energy is either reflected or absorbed and dissipated
Leaves reflect 25 to 75%
Molecules other than photosynthetic pigments absorb remainder – converted to heat and radiated, or conducted across leaf surface, or transpired
Photosynthetic efficiency
Percentage of the energy in sunlight that is converted to net primary production during the growing season

24. Effects of Light and Temperature 2
Optimum temperature for photosynthesis varies with system:
about 16oC for many temperate species
as high as 38oC for some tropical species
Rate of photosynthesis increases with temperature, up to a point:
rate of respiration also increases with temperature
net assimilation may thus decrease at high temperatures

25. Water limits primary production (reminder)
Photosynthesis in terrestrial systems is water-limited:
under water stress, stomates close and gas exchange ceases, stopping photosynthesis
Transpiration or water-use efficiency:
typically 2g production per kg of water transpired (4g for drought-tolerant crops)
ecosystem-level efficiency may be an order of magnitude poorer (0.2 g/kg)
Most precipitation is not taken up by plants

26. Nutrients stimulate primary production – terrestrial and aquatic.
Terrestrial production may be nutrient -limited:
fertilizers stimulate crop production
N is the most common limiting element
Aquatic systems are often strongly nutrient - limited:
especially true of open ocean
inadvertent addition of nutrients may stimulate unwanted production

27. Effects of fertilizer on plant growth
Fertilizing stimulates plant growth in natural habitats. Response of the chaparral shrub Adenostoma (a typical chaparral plant), Ceanothus (which harbors nitrogen-fixing bacteria), and annual grasses and forbs to fertilization with nitrogen, phosphorus, or both.

28. Primary production varies among ecosystems.
Primary production is maximum under favorable combinations of:
intense sunlight
warm temperatures
abundant rainfall
ample nutrients
On land, production is highest in humid tropics, lowest in tundra and desert.

29. NPP among ecosystems

30. Credit: © Richard Herrmann/Visuals Unlimited
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Pickelweed saltmarsh.

31. Credit: © Theo Allofs/Visuals Unlimited
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Temperate Rainforest showing moss-covered trees and ferns, Olympic National Park, Washington.

32. Credit: © Adam Jones/Visuals Unlimited
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Fall foliage and view of Mt. LeConte, Great Smokey Mountains National Park, Tennessee.

33. Credit: © Beth Davidow/Visuals Unlimited
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Northern Boreal Forest of Spruce and Aspens and tundra ponds.

34. Credit: © Joe McDonald/Visuals Unlimited
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African Lioness (Panthera leo) and African Elephants, Masai Mara Game Reserve, Kenya.

35. Credit: © Richard Herrmann/Visuals Unlimited
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Chaparral vegetation.

36. Credit: © Steve Maslowski/Visuals Unlimited
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A Bison herd on the prairie.

37. Credit: © Patrick J. Endres/Visuals Unlimited
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Arctic tundra biome in summer, Alaska Range Mountains, Denali National Park, Alaska.

38. Credit: © Richard Thom/Visuals Unlimited
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Sonoran Desert scene with Creosote Bush, Saguaro, Cholla, and Paloverde.

39. Only 5% to 20% of energy passes between trophic levels.
Energy reaching each trophic level depends on:
net primary production (base of food chain)
efficiencies of transfers between trophic levels
Plant use between 15% and 70% of light energy assimilated for maintenance – thus that portion is unavailable to consumers
Herbivores and carnivores expend more energy on maintenance than do plants: production of each trophic level is only 5% to 20% that of the level below it.

40. Ecological Efficiency
Ecological efficiency (food chain efficiency) is the percentage of energy transferred from one trophic level to the next:
range of 5% to 20% is typical, as we’ve seen
to understand this more fully, we must study the use of energy within a trophic level el
Undigested plant fibers in elephant dung

41. Intratrophic Energy Transfers
Intratrophic transfers involve several components:
ingestion (energy content of food ingested)
egestion (energy content of indigestible materials regurgitated or defecated) (the elephant dung)
assimilation (energy content of food digested and absorbed)
excretion (energy content of organic wastes)
respiration (energy consumed for maintenance)
production (residual energy content for growth and reproduction)

42. Fundamental Energy Relationships
Components of an animal’s energy budget are related by:
ingested energy - egested energy = assimilated energy
assimilated energy - respiration - excretion = production

43. Assimilation Efficiency
Assimilation efficiency = assimilation/ingestion
primarily a function of food quality:
seeds: 80%
young vegetation: 60-70%
plant foods of grazers, browsers: 30-40%
decaying wood: 15%
animal foods: 60-90%

44. Net Production Efficiency
Net production efficiency = production/assimilation
depends largely on metabolic activity:
birds: <1%
small mammals: <6%
sedentary, cold-blooded animals: as much as 75%
Gross production efficiency = assimilation efficiency x net production efficiency = production/ingestion, ranges from below 1% (birds and mammals) to >30% (aquatic animals).

45. Active, warm-blooded animals – low net production efficiencies; hummingbird: <1%

46. Production Efficiency in Plants
The concept of production efficiency is somewhat different for plants because plants do not digest and assimilate food:
net production efficiency = net production/gross production; varies between 30% and 85%
rapidly growing plants in temperate zone have net production efficiencies of 75-85%; their counterparts in the tropics are 40-60% efficient

47. Detritus Food Chains Ecosystems support two parallel food chains:
herbivore-based (relatively large animals feed on leaves, fruits, seeds)
detritus-based (microorganisms and small animals consume dead remains of plants and indigestible excreta of herbivores)
herbivores consume:
% of net primary production in temperate forests
12% in old-field habitats
60-99% in plankton communities

48. Exploitation Efficiency
When production and consumption are not balanced, energy may accumulate in the ecosystem (as organic sediments).
Exploitation efficiency = ingestion by one trophic level/production of the trophic level below it.
To the extent that exploitation efficiency is <100%, ecological efficiency = exploitation efficiency x gross production efficiency.

49. Stop here

50. Energy moves through ecosystems at different rates.
Other indices address how rapidly energy cycles through an ecosystem:
residence time measures the average time a packet of energy resides in storage:
residence time (yr) = energy stored in biomass/net productivity
biomass accumulation ratio is a similar index based on biomass rather than energy:
biomass accumulation ratio (yr) = biomass/rate of biomass production

51. Biomass Accumulation Ratios
Biomass accumulation ratios become larger as amount of stored energy increases:
humid tropical forests have net production of 1.8 kg/m2/yr and biomass of 43 kg/m2, yielding biomass accumulation ratio of 23yr
ratios for forested terrestrial communities are typically >20 yr
ratios for planktonic aquatic ecosystems are <20 days

52. Residence Time for Litter
Decomposition of litter is dependent on conditions of temperature and moisture.
Index is residence time = mass of litter accumulation/rate of litter fall:
3 months in humid tropics
1-2 yr in dry and montane tropics
4-16 yr in southeastern US
>100 yr in boreal ecosystems

53. Ecosystem Energetics
Comparative studies of ecosystem energetics now exist for various systems.
Many systems are supported mainly by autochthonous materials (produced within system).
Some ecosystems are subsidized by input of allochthonous materials (produced outside system).
autochthonous production dominates in large rivers, lakes, marine ecosystems
allochthonous production dominates in small streams, springs, and caves (100%)

54. Cedar Bog Lake
Lindeman’s study of a small lake in Minnesota uncovered surprisingly low exploitation efficiencies:
herbivores: 20%
carnivores: 33%
residual production of plants and herbivores accumulates as bottom sediment

55. Some General Rules
Assimilation efficiency increases at higher trophic levels.
Net and gross production efficiencies decrease at higher trophic levels.
Ecological efficiency averages about 10%.
About 1% of net production of plants ends up as production on the third trophic level: the pyramid of energy narrows quickly.
To increase human food supplies means eating lower on food chain!