1. Energy and Ecosystems
Lecture 03

2. Food chain
Passage of food energy through ecosystem trophic levels in a linear path
A trophic, or feeding, level consists of all organisms feeding at the same energy level

3. A food web is a branching food chain with complex trophic interactions
Species may play a role at more than one trophic level
Food webs can be simplified by isolating a portion of a community that interacts very little with the rest of the community

4. Sea nettle Juvenile striped bass Fish larvae Fish eggs Zooplankton
Figure Partial food web for the Chesapeake Bay estuary on the U.S. Atlantic coast
Fish larvae
Fish eggs
Zooplankton

5. Ecological Pyramids
A plant fixes about 1% of the sun’s energy that falls on its green parts
Successive members of a food chain incorporate ~ 10% of energy available in organisms they consume
Therefore, there are far more individuals at the lower tropic levels
The biomass of the lower trophic levels also tends to be greater
These relationships appear in a diagram as pyramids

6. Limits on Food Chain Length
Each food chain in a food web is usually only a few links long
Two hypotheses attempt to explain food chain length: the energetic hypothesis and the dynamic stability hypothesis
The energetic hypothesis suggests that length is limited by inefficient energy transfer
The dynamic stability hypothesis proposes that long food chains are less stable than short ones
Most data support the energetic hypothesis

7. Biomass available at the next trophic level
About one order of magnitude of available energy is lost from one trophic level to the next
Biomass available at the next trophic level
How heterotrophs use food energy
Reason why food chains generally consist of only 3 or 4 steps
Energy loss in an ecosystem
Cayuga Lake
In NY

8. Energy Loss at Trophic Levels Lindeman efficiency
Provides measure of energy transfer amongst trophic levels
ratio of assimilation between trophic levels
= energy (growth + respiration) of predator
energy (growth + respiration) of food species

9. Laws of Thermodynamics and energy: Kinetic energy
Light, heat
Potential energy – energy in chemical bonds
ATP (adenosine triphosphate)
carbohydrates, lipids, proteins
First Law: energy neither created nor destroyed …
Second Law: energy transformations lead to progressively more dispersed (less funtional) forms of energy - entropy
Light  heat  motion

10. Photosynthesis: (see ch. 6 for details)
Light energy captured by pigments
Used to build bonds forming various complex molecules – anabolic processes
Carbon dioxide absorbed/oxygen waste product
Autotrophs: ‘self feeders’ – algae, certain bacteria, plants
Only certain wavelengths of light effective

11. Light-dependent reactions
1. Pigments capture energy from sunlight
Water is split, O2 released
2. Using energy to make ATP and NADPH
3. Using ATP and NADPH to power the synthesis of carbohydrates from CO2
Light-dependent reactions
Light-independent reactions
The Calvin cycle +
12 H2O
water +
Light energy +
6 H2O
water +
6 O2
oxygen
6 CO2
carbon
dioxide
C6H12O6
glucose

12. Absorption spectra of chlorophylls and carotenoids

13. Global O2 from photosynthesis
80% comes from marine cyanobacteria.
Synechococcus
Synechocystis
20% comes from terrestrial systems.
5% of this comes from tropical rainforests.

14. Primary productivity Gross Primary Productivity (GPP):
total amount of photosynthetic energy captured in a given period of time.
Net Primary Productivity (NPP):
the amount of plant biomass (energy) after cell respiration has occurred in plant tissues.
NPP = GPP – Plant respiration
plant growth/ total photosynthesis/
unit area/ unit area/unit time
unit time

15. Primary productivity – marine ecosystems

16. Impacting Primary Productivity: Light intensity Nutrient availability
N + Fe tends to be limiting in marine systems
P limiting in fresh water system  eutrophication
Lake ecosystem

17. Secondary Productivity
Secondary productivity – the rate at which consumers convert the chemical energy of the food they eat into their own new biomass
Involves heterotrophs
Essentially reverse of photosynthesis - May occur with or without oxygen
Aerobic – most efficient
Anaerobic  fermentative pathways (in anoxic environment)
ATP immediate cellular energy form

18. Glycolysis With Oxygen aerobic Fermentation- anaerobic
Electron Transport most ATP generated here

19. Two food chains: Grazing food chain Detritus food chain
Herbivore  carnivore
Detritus food chain
Dead matter and waste from grazing food chain and primary production
Provides input to grazing food chain

20. Energy Transfer – Loss between trophic levels: trophic efficiency
= energy (growth + respiration) of predator
energy (growth + respiration) of food species

21. Found in larger numbers, but still contain 90% less energy
Ecological pyramids
Fairly large animals
Found in larger numbers, but still contain 90% less energy
Inverted pyramid

22. How does energy efficiency of endotherms vs. ectotherms differ – why?
What is meant by:
Primary productivity?
Secondary ?
What processes occur during photosynthesis/ cellular respiration – how are they essentially complementary processes?
How do the first and second laws of thermodynamics apply to energy transfer in ecosystems?
How does energy efficiency of endotherms vs. ectotherms differ – why?
What is the role of detritivores in energy transfer in ecosystems?