Energy through Ecosystems
A2 Chapter 12
By the end of this session I will:
• (d) define the terms producer, consumer
decomposer and trophic level;
• (e) describe how energy is transferred though
ecosystems;
• (f) outline how energy transfers between trophic
levels can be measured;
• (g) discuss the efficiency of energy transfers
between trophic levels;
• (h) explain how human activities can manipulate
the flow of energy through ecosystems (HSW6b);
The first one is down to you guys
2 MINUTES
(d) define the terms producer, consumer
decomposer and trophic level;
GIVE EXAMPLES
Basic processes to consider!
• 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
How can we calculate the amount of energy at
each trophic level?
Consumers
(heterotrophs)
Producers (autotrophs)
What limits the length of the food chain?
What limits length of food chain?
• H1: Energetics
• Availability of energy limits to 5-7 levels
• Depends on:
NPP
energy needed by consumers
average ecological efficiency
• H2: Dynamic stability
Longer chains less stable because:
Fluctuations at lower trophic levels magnified at
higher levels --->
extinction of top predators.
• 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
Biomass available at
• About one order of
the next trophic level
magnitude of available
energy is lost from one
trophic level to the next
– Reason why food
chains generally consist
of only 3 or 4 steps
How heterotrophs use food
energy
Cayuga Lake
In NY
Energy loss in an ecosystem
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
=
plant growth/
unit area/
unit time
GPP
–
Plant respiration
total photosynthesis/
unit area/unit time
Photosynthesis:
• 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
Light-dependent
reactions
Light-independent
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
The Calvin cycle
6 CO2
carbon
dioxide
+
6 H2O
water
+
Light energy
C6H12O6
glucose
+ 6 O2
oxygen
Absorption
spectra of
chlorophylls
and
carotenoids
Global O2 from photosynthesis
• 80% comes from marine
cyanobacteria.
– Synechococcus
– Synechocystis
• 20% comes from terrestrial
systems.
– 5% of this comes from tropical
rainforests.
Primary productivity – marine ecosystems
Global variation in estimated NPP
Figure 9
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
Challenges We Face
• To feed 9+ billion people in 2050, global food production
must increase by 70% -- 80% to come from yield increases
and 20% from area expansion
• Developing countries must almost double production –
with little possibility of land expansion in many countries
• Yield growth rate for cereals declining: 3.2% in 1960
to 1% in 2050
• Land area (per cap.) declining: from 4.3 ha in 1961 to
1.5 ha in 2050
• Climate change likely to add greater and
unpredictable stresses
• Increased demand for better variety, quality and safety of
agricultural products