Transcript Productivity - College of Forestry, University of Guangxi

An introduction to Ecosystems
Lecture 11
Principles of Ecology
College of Forestry, Guangxi University
Eben Goodale
Primary reading for this class
•
Jackson et al. 2001. Questions:
– What kind of scientists wrote this article? What do
they study and what kind of data did they collect.
– What kind of marine habitats do they review?
– What’s their conclusion as to what has been the most
important human activity that has harmed these
ecosystems?
Primary reading for this class
• Groups of 4.
– Talk about for 10 minutes (title, abstract, figures).
Circle words in abstract that are difficult and translate
them in phone.
– Each person write by themselves answers to
questions for 10 minutes.
– Switch papers with someone else in your group. Read
what they wrote and criticize their paper. 10 minutes.
Moving up a Layer:
Definition of an Ecosystem
Ecosystem
Characteristics of the community:
Number of species
Species relative abundances
Species diversity (1 + 2)
Last lecture, we finished communities asking
why different communities have different
species diversities.
Definition of an Ecosystem
Arthur Tansley
Ecosystem:
Biological community
with all the abiotic
factors influencing
that community
ECOSYSTEM
Flow of Energy
through the Ecosystem;
Cycling of Chemical Nutrients
Biological realm
high energy organic compounds
Light
Photosynthesis
Respiration
Heat
Energy required
energy released
Motion
Physical realm
low energy inorganic compounds
Notice similarities
To a machine（机器）
Moving up a Layer:
Definition of an Ecosystem
Ecosystem
Characteristics of the ecosystem
That we’ll talk about:
- Productivity (biomass)
- Energy flow
- Nutrient flow
Today’s lecture
• Productivity
– Gross and Net Primary Productivity
• What is it?
• How to measure it?
• How does it vary across the globe
• Food web and Energy transfer
– Efficiencies of Transfer
– Biomass Pyramids
– Food webs and Toxins
Productivity（生产力）
• Productivity of an ecosystem = accumulation
of carbon/biomass（生物量）/energy over
time (flow)
– Produced by the autotrophs, in most cases the
photoautotrophs: plants or plankton.
– Gross primary productivity 总初级生产力(GPP):
total carbon fixed by plants. Never > 4% available
sunlight.
– Net primary productivity 净初级生产力(NPP):
fixation – plant respiration. Usually ~ 1% available
sunlight.
Primary Production
• Measured by:
A) biomass (gr.)
B) energy (kcal)
Convert biomass to
Energy through caliometry
（热量）
Bomb caliometer: burn it and see how much temp raised
Ways of estimating NPP
Satellite（卫星）
imagery makes use
of different
vegetation types
having different
reflectance（反射率）
patterns
Ways of estimating NPP
Most common index
called NDVI:
normalized
difference vegetation
index.
Available for 1km2
for whole world
( NIR  red )
NDVI 
( NIR  red )
Ways of estimating NPP
• More local
techniques include
measuring amount
of CO2 given off by
forest.
• “Eddy（涡度）
covariance
technique”
Productivity of ecosystems
Limits to productivity:
abiotic factors
AET
Actual
evapotranspiration
Limits to Primary Production: Terrestrial ecosystems
- Soil Moisture and Evapotranspiration rate
- Length of Growing Season (depends on
average temperature)
- Nutrient Limitation
Must be in correct chemical form and
must be soluble
Nitrogen (as NO3, NH4)
Phosphorous (as PO4)
Limits to Primary Production: Terrestrial ecosystems
• Soil moisture and
evapotranspiration rate also
greatly effects decomposition
（分解）rate.
• Decomposition rates very
low in cold climates.
• Very high in tropics ->
tropical soils tend to have
very little top soil（表土）;
once disturbed, tropical
forests more difficult to grow
back.
Limits to Primary Production: Marine Ecosystems
-- Light - Penetrates less in turbid waters
-- Nutrients
Much of the sea only as
productive as a desert
Enriched w/ Phosphorus,
Carbon
and Nitrogen
Nitrogen
very important
Upwellings
bring nutrients
to the surface
near coasts,
and highly
influence
fish stocks
in marine ecosystems
Enriched w/ Carbon and Nitrogen
And iron can be, too.
Limits to Primary Production: Freshwater Ecosystems
-- Light
-- Temperature – Can be important in small bodies of water
-- Nutrients
Enriched w/ Phosphorus,
Carbon and Nitrogen
Phosphorus strongly effects primary production
Enriched w/ Carbon and Nitrogen
of freshwater bodies: famous lake study by Schindler
and colleagues.
Eutrophication（富营养化） of lakes may be brought on by the addition of
phosphate-detergents
Eutrophication
Nutrient inputs ->
Algae, bacterial bloom ->
Oxygen depletion ->
Die-off of animals
A freshwater example..
Where have you seen something like this?
Eutrophication also a problem
for marine systems
Dead zones problems
For more than 400 rivers
A “dead zone”
(light blue)
at mouth of
Mississippi River
Before we go on…. Case study of
Chapter 20: Deep sea trenches
• The majority of life
supported by the photoautotrophs
• But in deep sea vents, no
light. Chemical plumes
coming up from ocean
floor.
• First explored in
1976…amazing
discovery: so much life!
• Most of these large
organisms filled with
chemosynthetic bacteria.
Today’s lecture
• Productivity
– Gross and Net Primary Productivity
• What is it?
• How to measure it?
• How does it vary across the globe
• Food web and Energy transfer
– Efficiencies of Transfer
– Biomass Pyramids
– Food webs and toxins
Food webs
An early foodweb
by Raymond Lindeman
(1942)
Food webs
Animals not just grouped by size or relatedness. Grasshoppers and deer at same level.
Food webs
Reality is complex and many
species are not really just on
one level (“omnivores（杂食动物）”).
A food web determined by
stable isotope data
Food webs have some repeated
patterns
There are fewer
species on top
than there are
on bottom
There are fewer
individuals
of each species
on top than on
bottom
This is called
the trophic
pyramid.
Why does this happen?
Energy Transfer
Energy transfer is not efficient
（有效率的）
Secondary production
• Detritus（碎屑） feeders + Herbivores
• Gross assimilation（同化作用） (amount
of plant/algal material eaten) reduced by
the amount of energy used for
maintenance (respiration etc.) = net
assimilation (biomass of detritus +
herbivores) = 1% of net production.
Energy transfer is not efficient
• Energy dissipation:
energy is “wasted” with
each trophic transfer
• Ecological efficiency is
typically only 10%,
ranges from 5-20%
• Ecol. efficiency depends
on: metabolic efficiency,
energy spent in other
activities (e.g.
endothermy)
Much of not primary production is not
consumed….
Energy transfer is not efficient
• …and what is consumed
is then lost into excreta
(feces/urine) or
respiration.
• Only what is
“assimilated” (turned to
body tissue) is not lost.
• At each transfer step,
only ~5-20% of the
energy left; the rest lost.
Energy pyramids
• Basic problem with energy dissipation（损耗）: not enough
energy left to go around at the top levels
• Biomass/energy content vs. numbers: energy available gets
smaller and energy/individual tends to get larger as we go up
the chain
Biomass pyramids
• Biomass (dry weight), carbon, energy, number are all
correlated (approximately).
• Why is the English Channel biomass pyramid
inverted（倒金字塔型）?
High turnover… plankton（浮游生物） small and die soon,
But very high rate of reproduction
Question: Is the food web
controlled by the top or bottom?
What’s an example of “top-down”
control that we’ve already talked about?
Question: Are there limits to the
number of trophic levels?
Number of levels
rarely goes beyond
4. Limited by:
•Total amount of
energy
•Amount of
disturbance
•Size of the
ecosystem
Question: Are more complex
food webs more stable?
The experiments
of Tilman et al.
In Minnesota
Top predators
increase
Specialist
herbivores
decrease, but
Total #
herbivores
increase
Plant
biodiversity
increases
Not just plants. In same system have looked at insects
Food webs and toxins
• Some toxins such as
heavy metals and
persistent organic
pollutants (POPs) are
not metabolized
(broken down) or
excreted.
• These pollutants build
up in higher trophic
level organisms.
Food webs and toxins
• Well-known example
of a POP is DDT, an
insecticide. Made
eggshells birds thin,
and many high trophic
species like hawks
became rare.
• Major influence of
Rachel Carson’s
Silent Spring.
Food webs and toxins
• Well-known example
of a POP is DDT, an
insecticide. Made
eggshells birds thin,
and many high trophic
species like hawks
became rare.
• Major influence of
Rachel Carson’s
Silent Spring.
1962
Food webs and toxins
• Well-known example
of a POP is DDT, an
insecticide. Made
eggshells birds thin,
and many high trophic
species like hawks
became rare.
• Major influence of
Rachel Carson’s
Silent Spring.
The power of one
scientist’s voice …
Can it be you?
Rachel Carson
Food webs and toxins
• Mercury is a
heavy metal than
bioaccumulates.
• It is a neurotoxin,
and affects the
behavior and
reproduction of
high trophic
organims.
Food webs and toxins
• It is put into air by
any kind of
industrial process.
• Could be problem
for large Asian
countries like
China and India
Mercury emissions in 2005
From Pacyna et al. 2010
Homework
• Chapter 22, 23 summaries
• Myers et al. 2000 “Biodiversity hotspots of
conservation priorities”
Questions:
- How do the authors define a hotspot?
- Are there any in China and where?
- How do the authors want their hotspot list
to be used?
Key concepts
• An ecosystem is a
biological community with
all the abiotic factors
influencing that
community.
• Net primary productivity
(NPP) is the energy fixed
minus energy used = amt
of biomass per year.
• NPP varies with
temperature and rainfall
in terrestrial ecosystems.
• At each step on a food
web, energy is lost. The
efficiency of the transfer
of energy is low.
• Trophic pyramids show
there are fewer species
and animals at higher
trophic levels.