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

Autotrophs Heterotrophs

Simple Material (C,H,O)

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.

Land zone Transition zone Aquatic zone

Numberof species

Species in land zoneSpecies in aquatic zone

Species in transition zone only

Species Overlap in Ecotones

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 Heat Heat Heat

Heat

Heat

Heat

First TrophicLevel

Second TrophicLevel

Third TrophicLevel

Fourth TrophicLevel

Solarenergy

Producers(plants)

Primaryconsumers(herbivores)

Tertiaryconsumers

(top carnivores)

Secondaryconsumers(carnivores)

Detritvores(decomposers and detritus feeders)

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)

• Variables associated with the Shannon-Weiner 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

H = - pi(lnpi) Larger H = more diversity

Species # pi ln(pi) (pi)(lnpi)1 12 0.020583 -3.88328 -0.079932 562 0.963979 -0.03669 -0.035363 8 0.013722 -4.28875 -0.058854 1 0.001715 -6.36819 -0.01092

1.386294 583 -0.18507

H= 0.18507E = 0.18507/1.386297 = 0.1335

# pi ln(pi2)

12 0.25 -1.38629 -0.34657 Species Richness = 412 0.25 -1.38629 -0.34657 ln (species richness) = Hmax = 1.38629412 0.25 -1.38629 -0.34657 H = - (sum of (pi)(lnpi)) = 1.3862912 0.25 -1.38629 -0.34657 E = H/Hmax 0.99999748 -1.38629

Species # pi ln(pi2)

1 12 0.05 -2.99573 -0.149792 12 0.05 -2.99573 -0.149793 12 0.05 -2.99573 -0.149794 12 0.05 -2.99573 -0.149795 12 0.05 -2.99573 -0.149796 12 0.05 -2.99573 -0.149797 12 0.05 -2.99573 -0.149798 12 0.05 -2.99573 -0.14979

9 12 0.05 -2.99573 -0.1497910 12 0.05 -2.99573 -0.1497911 12 0.05 -2.99573 -0.1497912 12 0.05 -2.99573 -0.1497913 12 0.05 -2.99573 -0.1497914 12 0.05 -2.99573 -0.1497915 12 0.05 -2.99573 -0.1497916 12 0.05 -2.99573 -0.1497917 12 0.05 -2.99573 -0.1497918 12 0.05 -2.99573 -0.1497919 12 0.05 -2.99573 -0.1497920 12 0.05 -2.99573 -0.14979

240 -2.99573Species Richness = 20

ln (species richness) = Hmax = 2.995732H = - (sum of (pi)(lnpi)) = 2.99573

E = H/Hmax 0.999999

# pi ln(pi2)

3 0.0625 -2.77259 -0.17329 Species Richness = 438 0.791667 -0.23361 -0.18495 ln (species richness) = Hmax = 1.3862941 0.020833 -3.8712 -0.08065 H = - (sum of (pi)(lnpi)) = 0.698816 0.125 -2.07944 -0.25993 E = H/Hmax 0.50408548 -0.69881

Species richness and equitability affect the Shannon Wiener index.

29 species, fairly evenly distributed

11 species, dominated by 1 species

Rel

ativ

e A

bu

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ance

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:

CO2 (from

air)

H2O

O2 (to

air)

C6H12O6

Solar energy converted to chemical energy

CO2 converted to Carbohydrate

Solar energy + 6CO2 + 6H2O → C6H12O6 + 6O2

You need to know this

Happy Rays of Sunshine

Radiant Energy

• Photosynthesis converts solar energy into the chemical energy of a carbohydrate by two sets of reactions:

• Solar energy + 6CO2 + 6H2O → C6H12O6 + 6O2

Carbohydrate (glucose)

OxidizedReduced

Electrons from H2O are energized by the sun.

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 light-independent 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

Water is split H+, e-, and O2

*Considered an electron donor*

Solar energy is used excite electrons (increases potential energy).

ADP and NADP+ are reduced to ATP and NADPH.

ATP and NADPH are then used to power the light-independent 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

From light-dependant reactions

From light-dependant reactions

Fixation of CO2

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.

This represents a loss of CO2 that was already ‘fixed’ – this is called photorespiration.Because Carbon is fixed as a 3

carbon molecule, this is called C3 photosynthesis.

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

Partition by space Partition by time

Reduces photorespiration

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

Glucose (6-C sugar)

3-C sugar-phosphate 3-C sugar-phosphate

3-C pyruvate 3-C pyruvate

Overview of Glycolysis

6-C sugar diphosphate2 ADP

2 ATP

2 NADH

2 ATP

2 NADH

2 ATP

2 ADP 2 ADP

2 NAD+ 2 NAD+

Remember:When NAD+ NADH it has been reduced.

Remember:When NADH NAD+ it has been oxidized.

The NAD+ cycle

Pyruvate

(Oxygen present) (Oxygen not present)

Cellular Respiration Fermentation

2Pyruvate + 2CoA 2 Acetyl-CoA + 2CO2

2 NAD+ 2 NADH + H+

Pyruvate oxidation: if oxygen is present

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.

C4

Acetyl-CoA (2 C)

C6NADH

CO2

C5

NADH

CO2

ATP

C4

FADH2

NADH

Krebs cycleNAD+

NAD+

NAD+

FADH

ADP + P

Oxygen receives energy-spent electrons at the end of the electron transport system then combines with hydrogen to form water:

½ O2 + 2 e- + 2 H+ → H2O

NAD+

NADH

Glycolysis

Transition

Reaction

Krebs

Cycle

Electron

Transport

Chain

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 – exports2. 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

United States

The Netherlands

India

CountryPer Captia Ecological Footprint(Hectares of land per person)

10.9

5.9

1.0

CountryTotal Ecological Footprint

(Hectares)

United States

The Netherlands

India

3 billion hectares

94 million hectares

1 billion hectares