1. Chapter 11: flux of energy and matter through ecosystems

2. “Like all biological entities, ecological communities require matter for their construction and energy for their activities. We need to understand the routes for which matter and energy enter and leave ecosystems, how they are transformed into plant biomass, and how this fuels the rest of the community – bacteria and fungi, herbivores, detritivores and their consumers.”
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3. 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.

4. 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

5. Some key terms Standing crop Biomass Primary productivity
Bodies of the living organisms within a unit area
Biomass
Mass of organisms per unit area of ground (or water); usually expressed in units of energy or dry organic matter
Primary productivity
Rate at which biomass is produced per unit area by plants
Gross primary productivity
Total fixation of energy by photosynthesis
Net primary productivity = GPP - Respiration
Secondary productivity
Rate of production of biomass by heterotrophs
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6. GPP can be partitioned into respiration and NPP

7. More key terms Live consumer system Decomposer system
Proportion of primary production consumed by herbivores – who are then consumed by carnivores
Decomposer system
Fraction of NPP not eaten by herbivores reaches decomposer system
Two groups responsible for decomposition of detritus
Bacteria and fungi: decomposers
Animals that consume dead matter: detritivores
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8. Geographic patterns in PP
Productivity of forests, grasslands, crops and lakes follows a latitudinal pattern
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9. NPP among ecosystems

10. What limits PP? Terrestrial communities:
Solar radiation, carbon dioxide, water and soil nutrients: resources required for PP
Temperature, a condition, strong influence
IF other resources were in abundant supply, radiation would be used more efficiently [eg: conifer communities only uses between 1 to 3 % of available radiation]
Rainfall strongly correlated with productivity
Of the minerals, the one with strongest influence on community productivity: fixed nitrogen [not atmospheric N]
May be limited by a succession of factors
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11. What limits PP in aquatic environment?
Availability of nutrients (nitrate and phosphate)
Intensity of solar radiation that penetrates water column
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12. Relationship between PP and SP
Positive relationship
Secondary productivity by zooplankton, eat phytoplankton cells, positively related to phytoplankton productivity
Productivity of heterotrophic bacteria – also +ive with phyotplankton
Caterpillars abundance linked to primary productivity (which is linked to annual rainfall)
Seed-eating finch – raises more broods
In wet years (increased plant production)
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13. Where does the energy go?
In aquatic and terrestrial communities: SP is 1/10 of PP
(1) not all of plant biomass is consumed alive by herbivores
(2) not all plant biomass eaten by herbivores is assimilated and available for incorporation into consumer biomass. [what happens to the rest?]
(3) not all energy assimilated is converted to biomass [what happens to the rest?]
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14. 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

15. Thermodynamics and Ecology
1st law of thermodynamics - Energy can be neither created nor destroyed. It can only change forms.
2nd law of thermodynamics - spontaneous natural processes increase entropy overall
the total biomass ALWAYS decreases with increasing trophic levels, as energy is constantly being lost to the atmosphere
So?

16. 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.

17. 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.

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

19. 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

20. An ecological pyramid of energy

22. 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.

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

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

25. 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.

26. 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

27. 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)

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

29. 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%

30. 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).

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

32. 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

33. 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

34. Exploitation Efficiency
When production and consumption are not balanced, energy may accumulate in the ecosystem (as organic sediments).
Exploitation efficiency / trophic transfer 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.

35. Reminder of key terms Consumption efficiency (CE)
Assimilation efficiency (AE)
Production efficiency (PE)

36. 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!
[virtual water]

37. Virtual water

38. Virtual water http://environment. nationalgeographic

39. Virtual water http://environment. nationalgeographic

40. Virtual water http://environment. nationalgeographic

41. Virtual water http://environment. nationalgeographic

42. Virtual water http://environment. nationalgeographic

43. Live consumer and decomposer systems: general patterns of energy flow

44. Live consumer and decomposer systems: general patterns of energy flow
DOM: dead organic matter LCS: live consumer system Relative sizes of boxes and arrows are proportional to the relative magnitude of compartments and flows

45. Process of decomposition
Immobilization – when an inorganic nutrient element is incorporated into organic form, primarily during the growth of green plants [carbon dioxide becoming carbohydrates, eg]
Mineralization – conversion of elements from organic back to an inorganic form
Decomposition – the gradual disintegration of dead organic matter by both physical and biological agents

46. Who decomposes?
Bacteria and fungi: begin the process of decomposition. Use soluble materials (amino acids and sugars)
Microbial specialists: break down residual resources (structural carbohydrates and complex proteins)
Some specialist microbivores feed on bacteria and fungi
Microbivores: group of animals that operate alongside the detritivores; minute animals that specialize at feeding on bacteria or fungi but are able to exclude detritus from their guts

47. What do they eat? Plant detritus
Two of the major components of dead leaves and wood: cellulose and lignin
Lacking cellulase enzymes, majority of detritivores depend on production of cellulases by associated bacteria or fungi or protozoa

48. What do they eat? Feces and carrion [decaying flesh of dead animals]
Carnivorous vertebrates: poor quality dung [feces, manure]. Why?
Elephant dung  within minutes eaten by adult dung beetles feed on the the dung, bury large quantities along with their eggs to provide food for their larvae
Without those beetles, though…
Cattle dung. Cow pop increased from 7 in 1788 to 30 million in 1988 – producing 300 million cowpats/day – in Australia
Lack of native dung beetles  loss of 2.5 million ha/year/ under dung. So introduced 20 species of beetles

49. Elephant dung? Into paper?

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. 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

52. 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