PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Chapter 54Chapter 54

Ecosystems

Overview: Ecosystems, Energy, and Matter

• An ecosystem consists of all the organisms living in a community, as well as the abiotic factors with which they interact

• Ecosystems range from a microcosm, such as an aquarium, to a large area such as a lake or forest

• Regardless of an ecosystem’s size, its dynamics involve two main processes: energy flow and chemical cycling

• Energy flows through ecosystems while matter cycles within them

Concept 54.1: Ecosystem ecology emphasizes energy flow and chemical cycling

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

Ecosystems and Physical Laws

• 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

LE 54-2

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

Concept 54.2: 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

Ecosystem Energy Budgets

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

The Global 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

LE 54-4

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)TerrestrialMarine

Key Percentage of Earth’ssurface area

Average net primaryproduction (g/m2/yr)

6050403020100 2,5002,0001,5001,0005000Percentage 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

• Overall, terrestrial ecosystems contribute about two-thirds of global NPP

• Marine ecosystems contribute about one-third

LE 54-5

North Pole

60°N

30°N

South Pole

Equator

60°S

30°S

60°W 60°E 120°E120°W180° 0° 180°

Primary Production in Marine and Freshwater Ecosystems

• In marine and freshwater ecosystems, both light and nutrients control primary production

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

LE 54-6

Atlantic Ocean

ShinnecockBay

Moriches Bay

Long Island

2

4 5

30

1115

19

21

Coast of Long Island, New York

Great South Bay

Phytoplankton

Inorganicphosphorus

GreatSouth Bay

MorichesBay

ShinnecockBay

Station number2119153011542

8

543210

67

8

543210

67

Phytoplankton biomass and phosphorus concentration

Phyt

opla

nkto

n(m

illio

ns o

f cel

ls/m

L)

Inor

gani

c ph

osph

orus

(µm

ato

ms/

L)

Ammonium enriched

Station number2119153011542

30

Phyt

opla

nkto

n(m

illio

ns o

f cel

ls p

er m

L)

Startingalgal

density

Phytoplankton response to nutrient enrichment

24

18

12

6

0

Phosphate enrichedUnenriched control

• Experiments in another ocean region showed that iron limited primary production

• 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

LE 54-8

Mountain coniferous forest

Temperate forest

Tropical forest

Temperate grassland

Arctic tundra

Desertshrubland

1,5001,00050000

1,000

2,000

3,000

Actual evapotranspiration (mm/yr)

Net

prim

ary

prod

uctio

n (g

/m2 /y

r)

• On a more local scale, a soil nutrient is often the limiting factor in primary production

LE 54-9

Control

August 1980JulyJune00

100

200

300Li

ve, a

bove

-gro

und

biom

ass

(g d

ry w

t/m2 )

50

150

250 N + P

N only

P only

Concept 54.3: 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

Production Efficiency

• 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

LE 54-10

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%

Pyramids of Production

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

LE 54-11

1,000,000 J of sunlight

10,000 J

1,000 J

100 J

10 JTertiaryconsumers

Secondaryconsumers

Primaryconsumers

Primaryproducers

Pyramids of Biomass

• In a biomass pyramid, each tier represents the dry weight of all organisms in one trophic level

• Most biomass pyramids show a sharp decrease at successively higher trophic levels

LE 54-12a

Trophic level Dry weight(g/m2)

Tertiary consumersSecondary consumers

Primary consumersPrimary producers

1.51137809

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.

• Certain aquatic ecosystems have inverted biomass pyramids: Primary consumers outweigh the producers

LE 54-12b

Trophic level Dry weight(g/m2)

Primary consumers (zooplankton)Primary producers (phytoplankton)

214

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).

Pyramids of Numbers

• A pyramid of numbers represents the number of individual organisms in each trophic level

LE 54-13

Trophic level Number ofindividual organisms

Tertiary consumersSecondary consumers

Primary consumersPrimary producers

3354,904708,6245,842,424

• Dynamics of energy flow in ecosystems have important implications for the human population

• Eating meat is a relatively inefficient way of tapping photosynthetic production

• Worldwide agriculture could feed many more people if humans ate only plant material

LE 54-14

Trophic level

Secondary consumers

Primary consumers

Primary producers

The Green World Hypothesis

• Most terrestrial ecosystems have large standing crops despite the large numbers of herbivores

• 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

Concept 54.4: Biological and geochemical processes move nutrients between organic and inorganic parts of the ecosystem

• Life depends on recycling chemical elements

• Nutrient circuits in ecosystems involve biotic and abiotic components and are often called biogeochemical cycles

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

LE 54-16

Fossilization

Reservoir a Reservoir b

Reservoir c Reservoir d

Organicmaterialsavailable

as nutrients

Organicmaterials

unavailableas nutrients

Inorganicmaterialsavailable

as nutrients

Inorganicmaterials

unavailableas 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

LE 54-17a

Transportover land

Precipitationover landEvaporation

from oceanPrecipitationover ocean

Net movement ofwater vapor by wind

Solar energy

Evapotranspirationfrom land

Runoff andgroundwater

Percolationthroughsoil

LE 54-17b

Cellularrespiration

Burning offossil fuelsand wood

Carbon compoundsin water

Photosynthesis

Primaryconsumers

Higher-levelconsumers

Detritus

Decomposition

CO2 in atmosphere

LE 54-17c

Assimilation

N2 in atmosphere

DecomposersNitrifyingbacteria

Nitrifyingbacteria

Nitrogen-fixingsoil bacteria

Denitrifyingbacteria

NitrificationAmmonification

Nitrogen-fixingbacteria in rootnodules of legumes

NO3–

NO2–NH4

+NH3

LE 54-17d

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

LE 54-18

Nutrientsavailable

to producers

Decomposers

Geologicprocesses

Abioticreservoir

Consumers

Producers

Vegetation and Nutrient Cycling: The Hubbard Brook Experimental Forest

• Vegetation strongly regulates nutrient cycling

• Research projects monitor ecosystem dynamics over long periods

• The Hubbard Brook Experimental Forest has been used to study nutrient cycling in a forest ecosystem since 1963

• The research team constructed a dam on the site to monitor loss of water and minerals

LE 54-19

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

Nitr

ate

conc

entr

atio

n in

runo

ff(m

g/L)

1965 19681966 1967

80.060.040.020.0

4.03.02.01.0

0

• In one experiment, the trees in one valley were cut down, and the valley was sprayed with herbicides

• Net losses of water and minerals were studied and found to be greater than in an undisturbed area

• These results showed how human activity can affect ecosystems

Concept 54.5: The human population is disrupting chemical cycles throughout the biosphere

• As the human population has grown, our activities have disrupted the trophic structure, energy flow, and chemical cycling of many ecosystems

Nutrient Enrichment

• In addition to transporting nutrients from one location to another, humans have added new materials, some of them toxins, to ecosystems

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

LE 54-21

North America

Europe

4.3 4.6

4.14.34.6

4.34.6

4.6

• By the year 2000, acid precipitation affected the entire contiguous United States

• Environmental regulations and new technologies have allowed many developed countries to reduce sulfur dioxide emissions

LE 54-22

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

5.3

5.35.3

5.3 5.3

5.3

5.2

5.3

5.6

5.9

5.4

5.2

5.2

5.2

5.2

5.4

5.56.0

5.0

5.4

6.3

5.3

5.3

6.15.5

5.4

5.4

5.4

5.4

5.6

5.5

5.5

5.6

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

4.44.44.4

4.4

4.5

4.54.5

4.54.54.54.5

4.54.5

4.54.5

4.5

4.5

4.54.54.5

4.5

4.54.54.5

4.5

4.64.6

4.64.6

4.6

4.6

4.64.6

4.64.64.6

4.64.6

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

5.1

5.1

5.1

5.25.2

5.2

5.3

5.4

5.4

5.5

5.5

4.74.7

4.7

4.7

4.7

4.7

4.7

4.9

4.84.8

4.64.7

4.7

4.7

4.84.8

4.8

4.8

4.9

4.9

4.9

5.0

5.0

5.0

5.1

5.1

5.0

5.0 5.05.1

5.2

5.3

5.45.4

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

LE 54-23

Zooplankton0.123 ppm

Phytoplankton0.025 ppm

Lake trout4.83 ppm

Smelt1.04 ppm

Herringgull eggs124 ppm

Con

cent

ratio

n of

PC

Bs

Atmospheric Carbon Dioxide

• One pressing problem caused by human activities is the rising level of atmospheric carbon dioxide

Rising Atmospheric CO2

• Due to the burning of fossil fuels and other human activities, the concentration of atmospheric CO2has been steadily increasing

How Elevated CO2 Affects Forest Ecology: The FACTS-I Experiment

• The FACTS-I experiment is testing how elevated CO2 influences tree growth, carbon concentration in soils, and other factors over a ten-year period

The Greenhouse Effect and Global Warming

• The greenhouse effect caused by atmospheric CO2 keeps Earth’s surface at a habitable temperature

• Increased levels of atmospheric CO2 are magnifying the greenhouse effect, which could cause global warming and climatic change

Depletion of Atmospheric Ozone

• Life on Earth is protected from damaging effects of UV radiation by a protective layer or ozone molecules in the atmosphere

• Satellite studies suggest that the ozone layer has been gradually thinning since 1975

LE 54-26

Ozo

ne la

yer t

hick

ness

(Dob

son

units

)

350

300

250

200

150

100

50

01960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Year (Average for the month of October)1955

• Destruction of atmospheric ozone probably results from chlorine-releasing pollutants produced by human activity

LE 54-27

Chlorine atoms

O3Chlorine

Cl2 O2

CIO

O2

O2

CIO

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

Sunlight causes Cl2 O2 to break down into O2 and free chlorine atoms. The chlorine atoms can begin the cycle again.

Two CIO molecules react, forming chlorine peroxide (Cl2 O2 ).

Sunlight

• Scientists first described an “ozone hole” over Antarctica in 1985; it has increased in size as ozone depletion has increased

LE 54-28

October 1979 October 2000