Ecosystems and Physical Laws Ecologists view ecosystems as transformers of energy and processors of matter Laws of physics and chemistry apply to ecosystems,

Ecosystems and Physical Laws

• Ecologists view ecosystems as transformers of energy and processors of matter

• Laws of physics and chemistry apply to ecosystems, particularly energy flow

• Energy is conserved but degraded to heat during ecosystem processes

Trophic Relationships

• Energy and nutrients pass from primary producers (autotrophs) to primary consumers (herbivores) and then to secondary consumers (carnivores)

• Energy flows through an ecosystem, entering as light and exiting as heat

• Nutrients cycle within an ecosystem

Microorganismsand other

detritivores

Tertiaryconsumers

Secondaryconsumers

Detritus Primary consumers

Sun

Primary producers

Heat

Key

Chemical cycling

Energy flow

Decomposition

• Decomposition connects all trophic levels

• Detritivores, mainly bacteria and fungi, recycle essential chemical elements by decomposing organic material and returning elements to inorganic reservoirs

Physical and chemical factors limit primary production in ecosystems

• Primary production in an ecosystem is the amount of light energy converted to chemical energy by autotrophs during a given time period

• The extent of photosynthetic production sets the spending limit for an ecosystem’s energy budget

• The amount of solar radiation reaching the Earth’s surface limits photosynthetic output of ecosystems

• Only a small fraction of solar energy actually strikes photosynthetic organisms

Gross and Net Primary Production

• Total primary production is known as the ecosystem’s gross primary production (GPP)

• Net primary production (NPP) is GPP minus energy used by primary producers for respiration

• Only NPP is available to consumers

• Ecosystems vary greatly in net primary production and contribution to the total NPP on Earth

• Overall, terrestrial ecosystems contribute about two-thirds of global NPP and marine ecosystems contribute about one-third

Open oceanContinental shelf

Upwelling zonesExtreme desert, rock, sand, ice

Swamp and marshLake and stream

Desert and semidesert scrubTropical rain forest

Temperate deciduous forestTemperate evergreen forest

Tropical seasonal forest

SavannaCultivated land

EstuaryAlgal beds and reefs

Boreal forest (taiga)Temperate grassland

Woodland and shrublandTundra

0.40.4

1.01.31.51.61.71.82.42.72.93.33.54.7

0.30.10.1

5.265.0

Freshwater (on continents)

Terrestrial

Marine

Key Percentage of Earth’ssurface area

Average net primaryproduction (g/m2/yr)

6050403020100 2,5002,0001,5001,0005000

Percentage of Earth’s netprimary production

2520151050

125

2,500

3601,500

5003.090

900600

800

2,200

600

250

1,6001,2001,300

2,000

700140

0.3

7.99.19.6

5.43.5

0.67.1

4.93.8

2.3

24.45.6

1.20.9

0.10.040.9

22

Light Limitation

• Depth of light penetration affects primary production in the photic zone of an ocean or lake

Nutrient Limitation

• More than light, nutrients limit primary production in geographic regions of the ocean and in lakes

• A limiting nutrient is the element that must be added for production to increase in an area

• Nitrogen and phosphorous are typically the nutrients that most often limit marine production

• Nutrient enrichment experiments confirmed that nitrogen was limiting phytoplankton growth in an area of the ocean

Atlantic Ocean

ShinnecockBay

Moriches Bay

Long Island

2

45

30

1115

19

21

Coast of Long Island, New York

Great South Bay

Phytoplankton

Inorganicphosphorus

GreatSouth Bay

MorichesBay

ShinnecockBay

Station number2119153011542

8

5

4

3

21

0

6

78

5

4

3

21

0

6

7

Phytoplankton biomass and phosphorus concentration

Ph

yto

pla

nk

ton

(mil

lio

ns

of

cel

ls/m

L)

Ino

rga

nic

ph

os

ph

oru

s(µ

m a

tom

s/L

)

Ammonium enriched

Station number2119153011542

30

Ph

yto

pla

nk

ton

(mil

lio

ns

of

cel

ls p

er m

L)

Startingalgal

density

Phytoplankton response to nutrient enrichment

24

18

12

6

0

Phosphate enrichedUnenriched control

• The addition of large amounts of nutrients to lakes has a wide range of ecological impacts

• In some areas, sewage runoff has caused eutrophication of lakes, which can lead to loss of most fish species

Primary Production in Terrestrial and Wetland Ecosystems

• In terrestrial and wetland ecosystems, climatic factors such as temperature and moisture affect primary production on a large scale

• Actual evapotranspiration can represent the contrast between wet and dry climates

• Actual evapotranspiration is the water annually transpired by plants and evaporated from a landscape

• It is related to net primary production

Control

August 1980JulyJune00

100

200

300L

ive,

ab

ove

-gro

un

d b

iom

ass

(g d

ry w

t/m

2 )

50

150

250N + P

N only

P only

Energy transfer between trophic levels is usually less than 20% efficient

• Secondary production of an ecosystem is the amount of chemical energy in food converted to new biomass during a given period of time

• When a caterpillar feeds on a leaf, only about one-sixth of the leaf’s energy is used for secondary production

• An organism’s production efficiency is the fraction of energy stored in food that is not used for respiration

Growth (new biomass)

Cellularrespiration

Feces100 J

33 J

67 J

200 J

Plant materialeaten by caterpillar

Trophic Efficiency and Ecological Pyramids

• Trophic efficiency is the percentage of production transferred from one trophic level to the next

• It usually ranges from 5% to 20%

• A pyramid of net production represents the loss of energy with each transfer in a food chain

1,000,000 J of sunlight

10,000 J

1,000 J

100 J

10 JTertiaryconsumers

Secondaryconsumers

Primaryconsumers

Primaryproducers

Trophic level Dry weight(g/m2)

Tertiary consumers

Secondary consumers

Primary consumers

Primary producers

1.5

11

37

809

Most biomass pyramids show a sharp decrease in biomass at successively higher trophic levels, as illustrated by data from a bog at Silver Springs, Florida.

Trophic level Dry weight(g/m2)

Primary consumers (zooplankton)

Primary producers (phytoplankton)

21

4

In some aquatic ecosystems, such as the English Channel, a small standing crop of primary producers (phytoplankton) supports a larger standing crop of primary consumers (zooplankton).

Trophic level Number ofindividual organisms

Tertiary consumers

Secondary consumers

Primary consumers

Primary producers

3

354,904

708,624

5,842,424

• The green world hypothesis proposes several factors that keep herbivores in check:

– Plant defenses

– Limited availability of essential nutrients

– Abiotic factors

– Intraspecific competition

– Interspecific interactions

A General Model of Chemical Cycling

• Gaseous carbon, oxygen, sulfur, and nitrogen occur in the atmosphere and cycle globally

• Less mobile elements such as phosphorus, potassium, and calcium cycle on a more local level

• A model of nutrient cycling includes main reservoirs of elements and processes that transfer elements between reservoirs

• All elements cycle between organic and inorganic reservoirs

Fossilization

Reservoir a Reservoir b

Reservoir c Reservoir d

Organicmaterialsavailable

as nutrients

Organicmaterialsunavailableas nutrients

Inorganicmaterialsavailable

as nutrients

Inorganicmaterialsunavailableas nutrients

Livingorganisms,detritus

Coal, oil,peat

Atmosphere,soil, water

Mineralsin rocks

Assimilation,photosynthesis Burning

of fossil fuels

Weathering,erosion

Formation ofsedimentary rock

Respiration,decomposition,excretion

Biogeochemical Cycles

• In studying cycling of water, carbon, nitrogen, and phosphorus, ecologists focus on four factors:

1. Each chemical’s biological importance

2. Forms in which each chemical is available or used by organisms

3. Major reservoirs for each chemical

4. Key processes driving movement of each chemical through its cycle

Transportover land

Precipitationover landEvaporation

from oceanPrecipitationover ocean

Net movement ofwater vapor by wind

Solar energy

Evapotranspirationfrom land

Runoff andgroundwater

Percolationthroughsoil

Cellularrespiration

Burning offossil fuelsand wood

Carbon compoundsin water

Photosynthesis

Primaryconsumers

Higher-levelconsumers

Detritus

Decomposition

CO2 in atmosphere

Assimilation

N2 in atmosphere

DecomposersNitrifyingbacteria

Nitrifyingbacteria

Nitrogen-fixingsoil bacteria

Denitrifyingbacteria

NitrificationAmmonification

Nitrogen-fixingbacteria in rootnodules of legumes

NO3–

NO2–NH4

+NH3

Sedimentation

Plants

Rain

Runoff

Weatheringof rocks

Geologicuplift

SoilLeaching

Decomposition

Plant uptakeof PO4

3–

Consumption

Decomposition and Nutrient Cycling Rates

• Decomposers (detritivores) play a key role in the general pattern of chemical cycling

• Rates at which nutrients cycle in different ecosystems vary greatly, mostly as a result of differing rates of decomposition

Concrete dams and weirs built across streams at the bottom of watersheds enabled researchers to monitor the outflow of water and nutrients from the ecosystem.

One watershed was clear cut to study the effects of the loss of vegetation on drainage and nutrient cycling.

The concentration of nitrate in runoff from the deforested watershed was 60 times greater than in a control (unlogged) watershed.

Deforested

Control

Completion oftree cutting

Nit

rate

co

nce

ntr

atio

n in

ru

no

ff(m

g/L

)

1965 19681966 1967

80.0

60.0

40.0

20.0

4.0

3.0

2.0

1.0

0

Agriculture and Nitrogen Cycling

• Agriculture removes nutrients from ecosystems that would ordinarily be cycled back into the soil

• Nitrogen is the main nutrient lost through agriculture; thus, agriculture greatly impacts the nitrogen cycle

• Industrially produced fertilizer is typically used to replace lost nitrogen, but effects on an ecosystem can be harmful

Contamination of Aquatic Ecosystems

• Critical load for a nutrient is the amount that plants can absorb without damaging the ecosystem

• When excess nutrients are added to an ecosystem, the critical load is exceeded

• Remaining nutrients can contaminate groundwater and freshwater and marine ecosystems

• Sewage runoff causes cultural eutrophication, excessive algal growth that can greatly harm freshwater ecosystems

Acid Precipitation

• Combustion of fossil fuels is the main cause of acid precipitation

• North American and European ecosystems downwind from industrial regions have been damaged by rain and snow containing nitric and sulfuric acid

Field pH

5.35.2–5.35.1–5.25.0–5.14.9–5.04.8–4.94.7–4.84.6–4.74.5–4.64.4–4.54.3–4.4<4.3

5.3

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5.35.3

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5.6

5.5

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5.65.2

5.1

5.15.74.9

5.7

5.0

5.0

5.0

5.04.9

4.9

4.9

4.9

4.14.3

4.3

4.34.4

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4.4

4.5

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4.64.6

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4.64.6

4.64.64.6

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4.6

4.7

4.7

4.74.74.7

4.7

4.7

4.74.7

4.7

4.74.7

4.7

4.7

4.7

4.84.8

4.8

4.8

4.8

4.8

4.8

4.8

4.8

5.0

5.0

5.3

5.2

5.1

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5.25.2

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5.5

4.74.7

4.7

4.7

4.7

4.7

4.7

4.9

4.8

4.8

4.64.7

4.7

4.7

4.84.8

4.8

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4.9

4.9

4.9

5.0

5.0

5.0

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5.7

Toxins in the Environment

• Humans release many toxic chemicals, including synthetics previously unknown to nature

• In some cases, harmful substances persist for long periods in an ecosystem

• One reason toxins are harmful is that they become more concentrated in successive trophic levels

• In biological magnification, toxins concentrate at higher trophic levels, where biomass is lower

Zooplankton0.123 ppm

Phytoplankton0.025 ppm

Lake trout4.83 ppm

Smelt1.04 ppm

Herringgull eggs124 ppm

Co

nc

en

tra

tio

n o

f P

CB

s

Chlorine atoms

O3Chlorine

Cl2O2

CIO

O2

O2

CIO

Chlorine from CFCs interacts with ozone (O3), forming chlorine monoxide (CIO) and oxygen (O2).

Sunlight causes Cl2O2 to break down into O2 and free chlorine atoms. The chlorine atoms can begin the cycle again.

Two CIO molecules react, forming chlorine peroxide (Cl2O2).

Sunlight

Animations and Videos• Chapter Quiz Questions – 1

• Chapter Quiz Questions – 2

• An Idealized Energy Pyramid

• Energy Flow and The Water Cycle

• The Global Hydrological Cycle

• The Carbon Cycle

• The Global Carbon Cycle

• Nitrogen Cycle

• The Global Nitrogen Cycle

Animations and Videos• Phosphorus Cycle

• Sulfur Cycle

• Nitrogen Fixation Animation