Chapter 2
The Ecosystem
Ecosystem: General Model
Ecosystem: Internal Dynamics
S = Storage
A = Autotrophs
H = Heterotrophs
Ecosystem Trophic Structure
• Autotrophic Stratum
– ‘Green’ upper level (chlorophyll containing
plants)
– Fixation of light energy; use of simple
inorganic substances  complex organic
substances
• Heterotrophic Stratum
– ‘Brown’ lower lever (soil, sediment, decaying
matter, etc.)
– Use, rearrangement, and decomposition of
complex materials
Ecosystem Components
1) Inorganic substances – C, N, CO2, H2O,
and others involved in material cycles
2) Organic Compounds – proteins,
carbohydrates, lipids, humic substances,
and others that link the biotic and abiotic
components
3) Physical Environment – air, water,
substrate, climate regime, other physical
factors
Ecosystem Components
4) Producers – (autotroph) organisms, usually
green plants, that manufacture food from simple
inorganic substances
5) Heterotrophs – (phagotroph) organisms that
ingest other organisms or particulate organic
matter
6) Decomposers – (saprotroph) mostly bacteria
and fungi that obtain energy by breaking down
dead tissue or absorbing dissolved organic
matter
1) Saprophages – organisms that feed on dead organic
material; important for nutrient cycles
Designations are not species specific. A species may be intermediate between
heterotroph and decomposer. Consider decomposition as a process involving
several organisms.
Organic Detritus
• Detritus – all the organic
matter involved in the
decomposition of dead
organisms
– Important link between abiotic
and biotic components
• Products of decomposition
– POM – particulate organic
matter
– DOM – dissolved organic
matter
– VOM – volatile organic matter
Ecosystem Function
• Interaction between the autotrophic and
heterotrophic components
Simple
Material
(C,H,O)
Autotrophs
Heterotrophs
Complex
Material
(Carbohydrate)
Most vital elements are
in a constant state of
flux. Others, like ATP,
are never found outside
of the cell. Humic
substances never found
inside of a living cell.
Gradients and Ecotones
• The biosphere is characterized by a series
of gradients (zonation)
• Temperature:
– Equator to the poles, mountaintop to valley.
• Moisture:
– Wet to dry along major weather systems
• Depth:
– Shore to bottom in aquatic environments
Vegetation
based zones
Horizintal and
vertical zones
Metabolic
zonation
Temperature
stratification
zonation
Ecosystem Boundaries?
• It is easy to picture ecosystems as having
distinct boundaries.
• The area of transition from one ecosystem
to another is considered to be an ecotone.
• Ecotones have a mixture of species from
both ecosystems.
– A marsh between a freshwater lake and dry
land.
– Zone of grasses, shrubs, and scattered small
trees between forests and grasslands.
Where does one
ecosystem end and the
other begin?
Two examples of
ecotones.
Species Overlap in Ecotones
Land zone
Transition zone
Number
of species
Species in land zone
Species in aquatic zone
Species in transition
zone only
Aquatic zone
Edge Effect
• Higher species diversity found on the
edge of an ecosystem (ecotone) than in
the interior
– Marsh and open water (shrimp, crabs, juvenile
finfish)
• Edge species – those species that are
concentrated in ecotones
• Sharp edge usually a poor habitat
– Clear cut – forest edge
Old Field versus Pond
• Two systems compared to understand
ecosystem structure.
• Majority of inorganic and organic
compounds are in storage.
– New Hampshire Forest nitrogen: 90% in soil
organic matter, 9.5% in biomass, 0.5%
available in soluble form.
• Rate of ecosystem function is controlled
by:
– Rate of nutrient release from solids, solar
input, temperature change, day length, other
climate conditions
Primary Producers
• Pond: macrophytes and phytoplankton
– Macrophytes can be important in some cases
– Phytoplankton important in oceans
• Old Field (Grassland): macrophytes
– Rooted plants dominate
– Algae, mosses, lichens can be present
Consumer Organisms
• Herbivores  Primary consumers
– Pond: zooplankton (animal plankton) or
benthos (bottom forms)
– Field: Small insects, large hooved animals
• Carnivores  Secondary and tertiary
consumers
– Pond: predaceous insects and small fish
(nekton)
– Field: predatory insects, spiders, birds,
mammals
Detritivores and Decomposers:
• Found throughout ecosystems, but mostly
at the mud-water or soil-leaf litter
interface.
– Nongreen bacteria, fungi, flagellates
• Decomposition increases with
temperature
– Cellulose, lignin, and humus impart a spongy
texture to soil
Food Webs (chain) and Energy Flow
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
Community Metabolism
• Production:Respiration
(P/R ratio)
• If > 1, then excess
biomass is being
produced
• If < 1, then more
biomass is being
consumed than
produced
• If = 1, then
compensation point
Watershed Concept
• Ponds and grasslands
are actually open
systems
• Watershed =
catchment basin
• Often, the entire
drainage basin must
be considered as the
unit of management
You will be required to draw a map of the major rivers
of the Mississippi River Basin as part of exam 1.
Ecosystem Diversity
• We can look at genetic diversity, species
diversity, habitat diversity, and diversity of
functional properties
• Two components of interest:
– The richness (total # of species)
– Relative abundance of each species
(evenness)
Diversity Indices
• A mathematical measure of species
diversity in a community.
• Reveals important information regarding
rarity and commonness of species in a
community.
Shannon-Wiener Diversity Index (H)
H = - pi(lnpi)  Larger H = more diversity
• Variables associated with the ShannonWeiner Diversity index:
 S – total number of species in the community
(richness)
 pi – proportion of S made up of the ith species
 Hmax = ln(S)
 EH – equitability (evenness; b/t 0 and 1) = H / Hmax
Species
1
2
3
4
1.386294
#
12
562
8
1
583
pi
0.020583
0.963979
0.013722
0.001715
ln(pi )
(pi )(lnpi )
-3.88328 -0.07993
-0.03669 -0.03536
-4.28875 -0.05885
-6.36819 -0.01092
-0.18507
H= 0.18507
E = 0.18507/1.386297 =
0.1335
#
12
12
12
12
48
pi
0.25
0.25
0.25
0.25
ln(pi 2)
-1.38629
-1.38629
-1.38629
-1.38629
-0.34657
Species Richness =
4
-0.34657 ln (species richness) = Hmax = 1.386294
-0.34657
H = - (sum of (pi)(lnpi)) = 1.38629
-0.34657
E = H/Hmax 0.999997
-1.38629
#
pi
12
0.05
12
0.05
12
0.05
12
0.05
12
0.05
12
0.05
12
0.05
12
0.05
12
0.05
12
0.05
12
0.05
12
0.05
12
0.05
12
0.05
12
0.05
12
0.05
12
0.05
Species Richness =
4
12
0.05
ln (species richness) = Hmax = 1.386294
12
0.05
H = - (sum of (pi)(lnpi)) = 0.69881
12
0.05
E = H/Hmax 0.504085
240
Species Richness =
20
ln (species richness) = Hmax = 2.995732
H = - (sum of (pi)(lnpi)) = 2.99573
E = H/Hmax 0.999999
Species richness and equitability
affect the Shannon Wiener index.
#
3
38
1
6
48
pi
ln(pi 2)
0.0625
-2.77259
0.791667 -0.23361
0.020833 -3.8712
0.125
-2.07944
-0.17329
-0.18495
-0.08065
-0.25993
-0.69881
Species
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
ln(pi 2)
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-2.99573
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-0.14979
-2.99573
29 species,
fairly evenly
distributed
11 species,
dominated
by 1 species
Relative Abundance
Global Production and Decomposition
• Approximately 1017 grams of organic matter
produced by photosynthesis annually
– Approximate equivalent oxidized back to CO2 and H2O
(but not exactly).
• Since Precambian, a small fraction of
photosynthetic material was incompletely
decomposed and sequestered  fossil fuels
– Led to decrease in atmospheric CO2 and increase in O2.
– Release of the sequestered CO2 has led to increased
atmospheric levels
Production Within an Ecosystem
• Allochthonous input – organic material
transferred into the ecosystem from an
outside source
• Autochthonous input – organic material
produced within the ecosystem
Photosynthesis:
Solar energy converted to chemical energy
CO2 converted to Carbohydrate
Solar energy + 6CO2 + 6H2O → C6H12O6 + 6O2
Happy Rays
of Sunshine
You need to
know this
CO2
O2
(from air)
(to air)
C6H12O6
H2O
Radiant Energy
• Photosynthesis converts solar energy
into the chemical energy of a
carbohydrate by two sets of reactions:
• Solar energy + 6CO2 + 6H2O → C6H12O6 + 6O2
Reduced
Oxidized
Electrons from H2O are energized by the sun.
Carbohydrate
(glucose)
Oxidation-Reduction
• Oxidation is the loss of electrons (energy)
and reduction is the gain of electrons
(energy).
• In covalent rxn’s, oxidation also refers to the
loss of hydrogen atoms, and reduction refers
to the gain of hydrogen atoms.
Bacterial Photosynthesis
CO2 + 2H2A + light energy  (CH2O) + 2A
A could be Sulfur (2H2S  4H + 2S; not oxygen)
or an organic compound.
Photosynthetic bacteria generally play a minor
role in the production of organic matter, but are
important in nutrient cycling.
Photosynthetic Bacteria
• Photosynthetic bacteria that release oxygen are
largely aquatic cyanobacteria.
• Obligate anaerobes – function only in the
absence of oxygen (green and purple sulfur
bacteria).
– Occur between the reduced and oxidized boundary layer
in sediments or water where the light intensity is low.
– Important for sulfur cycle
• Facultative anaerobes – able to function with or
without oxygen.
– Generally are non-sulfur photosynthetic bacteria
Photosynthesis Overview
• Composed of light-dependent and lightindependent reactions
• Light-dependent reactions
– Capture solar energy and excite electrons
– Water molecule is split and electrons and H+
enter the electron transport system
– O2, NADPH, and ATP are produced
• Light-independent reactions
– CO2 is reduced to a carbohydrate
– NADPH and ATP are consumed
Light-dependent Reactions
Solar energy is
used excite
electrons
(increases
potential energy).
ADP and NADP+
are reduced to
ATP and NADPH.
Water is split  H+, e-, and O2
*Considered an electron donor*
ATP and NADPH
are then used to
power the lightindependent
reactions.
Light-independent Reactions
• Calvin Cycle – three stages
– CO2 fixation, CO2 reduction, RuBP
regeneration
– Reactions require energy, which is supplied by
ATP and NADPH
Light-independent Reactions
-Calvin Cycle
Fixation of CO2
From lightdependant
reactions
From lightdependant
reactions
C3, C4, and CAM plants
• Carbon fixation so far has been described
as C3.
– Initial carbon fixation and Kreb’s cycle occur at
the same time in the same place.
– Rubisco oxidizes RuBP in the presence of a
high oxygen concentration
– High rates of photosynthesis also lead to high
rates of photorespiration
• C4 and CAM plants have adapted the
photosynthesis process to reduce
photorespiration
Rubisco is the
enzyme that
carboxylates RuBP
with CO2.
However, in the
presence of high
O2, it will oxidize
RuBp and release a
CO2.
Because Carbon is fixed as a 3
carbon molecule, this is called C3
photosynthesis.
This represents a
loss of CO2 that was
already ‘fixed’ – this
is called
photorespiration.
In C4 plants, bundle sheath cells also contain chloroplasts,
and mesophyll cells are arranged concentrically around
bundle sheath cells.
Oxygen is
produced in the
Mesophyll cells, so
it does not
accumulate in the
bundle sheath
cells when the
stomata are
closed.
High concentration
of CO2 in the
bundle-sheath cell
Reduces
photorespiration
Partition by space
Partition by time
Decomposition (respiration)
• Type 1. Aerobic respiration – gaseous
oxygen is the electron acceptor (oxidant)
• Type 2. Anaerobic respiration – gaseous
oxygen is not the electron acceptor
• Type 3. Fermentation – anaerobic, but the
organic compound oxidized is also the
electron acceptor
Aerobic Respiration
C6H12O6 + 6O2  6CO2 + 6H2O + energy (ATP)
Glycolysis and Kreb’s Cycle lead to complete
breakdown of carbohydrate to CO2 and H2O
Overview of Glycolysis
Glucose (6-C sugar)
2 ATP
2 ADP
6-C sugar diphosphate
3-C sugar-phosphate
2 ADP
3-C sugar-phosphate
2 ADP
2 ATP
2 NAD+
2 ATP
2 NAD+
2 NADH
2 NADH
3-C pyruvate
3-C pyruvate
The NAD+ cycle
Remember:
When NAD+  NADH it
has been reduced.
Remember:
When NADH  NAD+ it
has been oxidized.
Pyruvate
(Oxygen present)
Cellular Respiration
(Oxygen not present)
Fermentation
Pyruvate oxidation: if oxygen is present
2 NAD+
2Pyruvate + 2CoA
2 NADH + H+
2 Acetyl-CoA + 2CO2
Pyruvate is converted to a C2 acetyl group
attached to coenzyme A (CoA), and CO2 is
released. This occurs in the cytoplasm if
oxygen is present.
Acetyl-CoA
(2 C)
C6
C4
NAD+
NADH
CO2
NADH
NAD+
Krebs cycle
C5
FADH2
NAD+
NADH
FADH
C4
CO2
ATP
ADP
+P
Oxygen receives energyspent electrons at the end
of the electron transport
system then combines
with hydrogen to form
water:
½ O2 + 2 e - + 2 H+ → H 2O
Glycolysis
NAD+
Transition
Electron
Reaction
Transport
NADH
Chain
Krebs
Cycle
Remember: Electrons = Energy
Anaerobic Respiration
• Usually saprophages (bacteria, yeasts,
molds, and protozoa)
– Can occur in some muscle tissue
• Methane bacteria – obligate anaerobes;
produce methane by decomposing
organic matter
– Marsh gas
• Desulfovibrio – important sulfur reducing
bacteria that reduce SO4 and produce H2S.
Chapter 2 Continued………….
Decomposition
• The breakdown of large molecules to it’s
basic components
– Abiotic (forest fires) and biotic process
– Organic material is an energy source for
decomposer organisms
• Decomposition is physical and chemical
– Leaf shredders  particulate organic matter
• Increase surface area
– Bacteria and fungi use enzymes to break apart
large molecules
• Left over nutrients are reabsorbed by primary
producers
Decomposition Rate
• Composition of
organic material
– For example: Lignin vs.
protein, lipid,
carbohydrates
• Presence of
macroinvertebrates
such as shredders
Decomposition Rate
• Can be affected by
temperature and water
– Remember: enzymes
work faster at high
temperatures
Decomposition Rate
• Depends on a variety
of organisms
– Bacteria, fungi, insects,
nematodes
Ecosystem Function
• A combination of production, respiration,
and decomposition
• What are the anthropogenic impacts on
ecosystem function?
• Ecological Footprint – a measure of the
anthropogenic effect on the environment
Ecological Footprint
•
•
Ecological footprint – amount of land needed to produce the
resources needed by the average person in a country
Methods:
1. Correct consumption data for trade imports and exports
Consumptionwheat= production + imports – exports
2. Convert to land area needed to produce the item
Awheat = Cwheat / ywheat
A=total area needed, C=consumed, Y=yield
3. Obtain per capita ecological footprint by dividing by
population size
fwheat = awheat/population size
Ecological footprint in relation to
available ecological capacity.
It would take about 3 times the current land area of
Earth if all 6.1 billion people consumed the same as
the 276 million people in the US
Per Captia Ecological Footprint
(Hectares of land per person)
Country
10.9
United States
5.9
The Netherlands
India
Country
1.0
Total Ecological Footprint
(Hectares)
3 billion
hectares
United States
The Netherlands
India
94 million hectares
1 billion
hectares