Energy in Agroecosystems

1
Energy in Agroecosystems
2
Carbon and Energy in Ecosystems

• Movement of carbon and energy through ecosystems
  is closely linked.
• Organic compounds
• contain carbon
• sources of potential energy
• Therefore, we can track carbon and energy
  simultaneously.

3
Photosynthesis

Organic molecules contain C Energy Both now
incorporated into living plant system
4
Respiration
Energy is released (for use by organisms) C is
released as CO2
C and energy cycles are completed
5
Energy Tracks Similar to Carbon Flow in Ecosystems
6
Energy in Ecosystems

• Energy is a common denominator to all individuals
  in an ecosystem.
• All living organisms and their products (wood,
  oil, etc.) contain various amounts (kcal) of
  potential energy stored in their tissues.
• Can standardize for comparisons in terms of
  energy.
• Removing water from organisms yields biomass from
  which you can calculate its energy equivalent.

7
1. Energy or Trophic Pyramid
Plant often less than 1-2 efficient at
harvesting light
Production Respiration
Sunlight
8
Primary Producer

• Plant primary producer
• Gross primary production (GPP) total energy
  captured by plant during photosynthesis.
• GPP is used for
• Respiration (R) to maintain life functions of the
  plant.
• Production (P) Net primary production (NPP) for
  plants energy which is stored in plant tissues
  (growth, etc.) -- estimated from dry biomass
  produced over time.
• Note that GPP NPP R.

9
Energy Transfer Efficiency

• Efficiency of transfer of energy from solar
  energy to NPP is low (lt4).

Price, 1997. Insect Ecology see also Tivy, 1992.
10
Energy or Trophic Pyramid
Secondary consumer
Assimilated
Primary consumer
P R NA
Consumed
Production Respiration
Plant
Sunlight
11
Energy Consumed

• Several outcomes from energy consumed (C)
• Non-assimilated (NA) - some energy is never
  assimilated by the animal, but is passed through
  the digestive system and excreted.
• Assimilated (A) - energy actually used by the
  consumer for respiration and production.
• Note that for consumers, A P R, and C A
  NA P R NA.

12
Consumers

• Consumer obtains energy from other organisms.
• Primary consumer - feeds on plants
• Secondary consumer - feeds on primary consumers.
• Consumers obtain energy from P of organisms
  immediately below them.
• Efficiency of transfer between trophic levels
  varies but 10 is rough estimate (see Odum,
  1983).

13
Energy or Trophic Pyramid
Secondary consumer
Assimilated
Primary consumer
P R NA
Consumed
Production Respiration
Plant
Sunlight
14
Allocation of Assimilated Energy

• Plants about 50 for P/A
• Most insects 35-45 for P/A
• Social insects lt 10 for P/A
• Endotherms lt 10 for P/A
• Birds 1-2 for P/A
• Predators have lower P/A than herbivores.
• Endotherms have low P/A

Depends on lifestyle and activity of organism
15
1. Energy or Trophic Pyramid
Secondary consumer
Assimilated
Primary consumer About 10 energy between levels
P R NA
Consumed
Production Respiration
Plant less than 1-2 efficient at harvesting
light
Sunlight
16
Trophic Pyramid

• Trophic pyramid - energy transfers upward through
  layers ( trophic levels) of a pyramid.
• Since energy transfer is never 100 efficient,
  amount of energy for P and R decreases moving up
  to higher levels, so organisms at higher levels
  cannot use more energy than those at lower
  levels.
• NOTE SIMILAR SHAPE FOR CARBON FLOW!

17
Special Considerations for Energy

• Production is measured in kcal/m2/yr, or some
  equiv. units considering area and time.
• Standing crop energy equivalent in living
  tissue available at some specific time (kcal/m2).
• e.g., plant harvest or insects collected at one
  point in time.
• Note that standing crop (energy measured at one
  time) may be different from production (energy
  produced over time).
• e.g., if multiple harvests occur, or if multiple
  generations per year.

18
Special Considerations for Energy

• Units vary in different scientific disciplines
  engineering uses joule 4.19 J 1.0 cal
• Energy is a common denominator
• Same units for photosynthesis to herbivores
  through carnivores in trophic pyramids.
• Various operations (pest control, work, etc.) can
  be estimated so that these can be included with
  energy equivalent of materials in overall energy
  budget.

19
Food Webs

• Trophic pyramid is oversimplified (hot topic in
  1942!). 
• Unrealistic way to track energy flow in
  ecosystems.
• Food web
• Shows multiple connections among organisms within
  ecosystem.
• More realistic.

20
Above-ground Food Webs relatively simple
Note Predator feeding at multiple trophic levels
Price. 1997. Insect Ecology
21
Above-ground Food Webs - very complex
Price - Fig. 22.16
22
Below-ground Food Webs

• This is now known to be a very important
  component of the ecosystem food web that was
  often ignored.
• Much food originates from plant.
• from living plant tissues (into pests).
• from dead plant material (into decomposers).

23
Interconnection of Food Webs

• Above- and below-ground systems are connected
• Food webs from different ecosystems are
  interconnected

24
Energy Follows Carbon Flow in Ecosystems
Solar Direct energy input
Indirect energy
25
Indirect Energy (not given directly to plant)

• Not only fuel, but
• Fertilizers (require energy to make)
• Pesticides (require energy to make)
• Irrigation (energy for pumps, etc.)
• Improved crop varieties (may require more energy
  intensive management)

26
Indirect Energy Input

• Saves plant energy
• Plant can put saved energy into Production

Energy Herbicide Kills weeds
Saves plants from wasting energy on competition
27
Agroecosystems consume energy (because harvest is
removed),

• Especially if external indirect energy is
  supplied to make plant production more efficient

28
Energy and Maximization of YieldYield maximized
by increasing

• Harvest index of biomass actually used can
  be increased with management (over time through
  plant breeding, etc.)
• Plant population (including multiple cropping in
  time and space)
• Plant size (more biomass per unit of land)

29
Plant Size and Yield Increased by Increased
Energy Input Corn Yields in US
Limits to yield per ha -- levels off
30
Fertilizer Use in US
31
Energy Inputs/Outputs for 1 ha of Corn in US
(Carroll, 1990)
(1000 kcal/ha)
32
Energy Inputs/Outputs for 1 ha of Corn in US
(Carroll, 1990)
(1000 kcal/ha)
33
Energy Inputs/Outputs for 1 ha of Corn in US
(Carroll, 1990)
(1000 kcal/ha)
34
Energy Challenge

• Cant increase yields indefinitely through energy
  input
• Maintain yield and production (or even increase
  as population increases)
• Reduce inputs (as less inputs are available)

35
References

• Text, pp. 20-31.
• Carroll et al., 1990. Ch. 5.
• Odum, 1983. Ch. 3.
• Price, 1997. Insect Ecology. John Wiley, New
  York.
• Tivy, 1992. Ch. 6.