Ecosystem Ecology
Ecosystem ecology emphasizes energy flow and
chemical cycling.
An ecosystem consists of all of the organisms in
a community as well as the abiotic components
they interact with.
ECOSYSTEM
ECOSYSTEM
COMMUNITY
PHYSICAL ENVIRONMENT
Energy from the sun fuels global
ecosystems
The Earth is not a closed system. Energy
comes in from outside (i.e., from the sun),
moves through ecosystems and ultimately is
dissipated into space as heat.
Primary Efficiency
How much solar radiation is converted into
biomass?
Tropical Rain Forests : 1-3%
Temperate deciduous forest : 0.5-1%
Hot deserts : 0.01-0.2%
Food Chains
Show energy flow from
one trophic level to
another
Trophic Levels: groups
of organisms that obtain
their energy in a similar
manner
Food Chains
Total number of levels in a food chain
depends upon locality and number of
species
Highest trophic levels occupied by adult
animals with no predators of their own
Secondary Production: total amount of
biomass produced in all higher trophic
levels
Energy
Non-cyclic, unidirectional flow
Losses at each transfer from one trophic
level to another
- Losses as heat from respiration
- Inefficiencies in processing
Total energy declines from one transfer to
another
- Limits number of trophic levels
Energy Flow
Transfer Efficiencies
Efficiency of energy transfer called
transfer efficiency
Units are energy or biomass
Et = Pt
Pt-1
Pt = annual production at
level t
Pt-1 = annual production
at t-1
Transfer Efficiency Example
Net primary production = 150 g C/m2/yr
Herbivorous copepod production = 25 g C/m2/yr
Et = Pt = Pcopepods
= 25 = 0.17
Pt-1 P
150
phytoplankton
Typical transfer efficiency ranges
*Level 1-2 ~20%
*Levels 2-3, …: ~10%
Net Productivity
of species n
Production Efficiency =
Assimilation
of species n
Group
Mammals and birds
Insects
Production
Efficiency
Respiration
1 – 3%
97 – 99%
10 – 41%
59 – 90%
Why the difference?  Homeothermy (it takes a lot of
energy to keep warm)
Why Is Energy Transfer Not 100%
Efficient?
When energy is changed from one form to
another, some is degraded in the form of heat.
This law of thermodynamics is true for all
physical-chemical systems, including living
ones.
One ecologist summarized this fact by the
statement: “There is no such thing as a free
lunch.” All physical transactions come at a cost
of energy.
Secondary Efficiency
Worldwide : mean = 10% per level
Energy and Biomass Pyramids
Kaneohe Bay
Tertiary
consumers
10 J
Secondary
consumers
100 J
Primary
consumers
1000 J
Primary
producers
10,000 J
1,000,000 J of sunlight
Biomass
Why does the ocean have such a low biomass of
primary producers?
Inverted Pyramids
In lakes & open sea algae are the main primary
producers.
Algae multiple fast but also consumed at a higher rate
therefore the pyramid appears inverted.
Food Webs
Food chains don’t exist in real
ecosystems
Almost all organisms are eaten by
more than one predator
Food webs reflect these multiple and
shifting interactions
Antarctic Food Web
Nutrients
Unlike energy, nutrients are constantly
recycled from one form to another and
pass through multiple trophic levels to
decomposers to abiotic forms and back to
living organisms again.
Heat
First Trophic
Level
Second Trophic
Level
Third Trophic
Level
Fourth Trophic
Level
Producers
(plants)
Primary
consumers
(herbivores)
Secondary
consumers
(carnivores)
Tertiary
consumers
(top carnivores)
Heat
Heat
Heat
Solar
energy
Heat
Heat
Detritvores
(decomposers and detritus feeders)
Heat
Detritivores
Main decomposers are fungi and bacteria.
These break down organic matter and release
chemical elements into the soil, water and air
where producers can recycle them into organic
compounds.
Without the action of decomposers life would
cease because essential nutrients would
remain locked up in detritus and unavailable to
organisms.
Detritivores
2O & HIGHER LEVEL
CONSUMERS
DECOMPOSERS
1O
CONSUMERS
PRODUCERS
Detritivores (species
that feed on the dead
remains of organisms)
are also known as
decomposers. These
include bacteria, fungi
and invertebrate
animals of numerous
species.
Fungal decomposition
of a tree stump
Primary Production
About 1% of the visible light that strikes
earth is converted by photosynthesis into
chemical energy.
This is enough energy to create about
170 billion tons of organic matter annually.
Gross and net primary
productivity
In an ecosystem gross primary
productivity (GPP) is the amount of light
energy converted into chemical energy
per unit time.
Net primary productivity (NPP) is gross
primary productivity minus the energy
used by the primary producers for
respiration.
Net primary productivity
Net primary productivity indicates how
much energy is available for use by other
trophic levels.
It is measured as biomass of vegetation
added to the ecosystem per unit area per
unit time (g/m2/yr).
Net primary productivity
NPP is influenced by light and nutrient
availability and differs among
ecosystems.
Tropical rain forests have high NPP as do
estuaries and coral reefs. Lakes, tundra
and the open ocean have relatively low
NPP.
Global net primary productivity
Tropical rainforests contribute about 22% of the
Earth’s total NPP and open ocean about 24%.
The open ocean has a much lower rate of NPP,
but covers a far larger area.
Various temperate forests and grasslands and
the continental shelf (shallow continental
waters) contribute most of the rest.
Global net primary productivity
Overall, terrestrial ecosystems contribute
about 66% of NPP and marine
ecosystems the remainder.
NET PRODUCTION IN SOME
MAJOR BIOMES
BIOME
Net Production / Unit Area
( grams / meter 2 / year)
RANGE
AVERAGE
Desert & Semi-desert
0-250
40
Artic & Alpine Tundra
10-400
140
Coniferous Forest
400-2,000
800
Deciduous Forest
600-2,500
1,250
Grassland
200-1,500
600
Tropical Forest
1,000-3,500
2,000
Limits on primary productivity
In marine and freshwater ecosystems both light
and nutrients are important in controlling NPP.
The inability of light to penetrate the water limits
photosynthesis to the upper layers. More than
50% of solar radiation is absorbed in the first
meter. Even in clear water, only about 5-10% of
radiation reaches a depth of 20m.
Regional annual net primary production for Earth
Limits on primary productivity
Nutrient limitation is a major factor
affecting NPP in aquatic biomes.
In marine environments the nutrients
limiting primary productivity are usually
nitrogen and phosphorus, which are
scarce in the photic zone.
Decomposition and nutrient
cycling rates
The rates at which nutrients cycle is
strongly affected by the rates at which
decomposers work. In the tropics,
warmer temperatures and abundant
moisture cause organic material to
decompose 2-3 times faster than it does
in temperate regions.
Decomposition and nutrient
cycling rates
High rate of decomposition means little
organic material accumulates as leaf litter.
In tropical forest about 75% of nutrients
are in woody trunks of trees and only
about 10% in soil. In temperate forest
about 50% of nutrients are in the soil
because decomposition is slower.
An Ecological Mystery
Keystone Species
Kelp Forests
An Ecological Mystery
Long-term study of sea otter populations
along the Aleutians and Western Alaska
1970s: sea otter populations healthy and
expanding
1990s: some populations of sea otters were
declining
Possibly due to migration rather than
mortality
1993: 800km area in Aleutians surveyed
- Sea otter population reduced by 50%
Vanishing Sea Otters
1997: surveys repeated
Sea otter populations had declines by 90%
- 1970: ~53,000 sea otters in survey area
- 1997: ~6,000 sea otters
Why?
- Reproductive failure?
- Starvation, pollution disease?
An Ecological Mystery
Long-term study of sea otter populations
along the Aleutians and Western Alaska
1970s: sea otter populations healthy and
expanding
1990s: some populations of sea otters were
declining
Possibly due to migration rather than
mortality
1993: 800km area in Aleutians surveyed
- Sea otter population reduced by 50%
Vanishing Sea Otters
1997: surveys repeated
Sea otter populations had declines by 90%
- 1970: ~53,000 sea otters in survey area
- 1997: ~6,000 sea otters
Why?
- Reproductive failure?
- Starvation, pollution disease?
Cause of the Decline
1991: one researcher observed an orca
eating a sea otter
Sea lions and seals are normal prey for orcas
Clam Lagoon inaccessible to orcas- no
decline
Decline in usual prey led to a switch to sea
otters
As few as 4 orcas feeding on otters could
account on the impact
- Single orca could consume 1,825
otters/year
• Carrying Capacity – the number or weight of
animals of a single or mixed population that
can be supported permanently on a given
area
• Ecological Carrying Capacity – maximum
density of animals that can be sustained
without inducing negative effects on
ecosystem
• Economic Carrying Capacity – density of
animals that enables maximal sustained
harvesting and is always lower than the
ecological carrying capacity