Chapter 55

Ecosystems

An ecosystem consists of all the organisms living in a community– As well as all the abiotic factors with which

they interact– They can be very small or very large

Aquarium Coniferous Forest

Energy Flow and Chemical Cycling

Ecosystem ecology emphasizes energy flow and chemical cycling

Ecosystem ecologists view ecosystems– As transformers of energy and processors of

matter

Ecosystems and Physical Laws

The laws of physics and chemistry apply to ecosystems– Particularly in regard to the flow of energy

Energy is conserved– But degraded to heat during ecosystem

processes (energy transformations are inefficient…some energy is always lost as heat)

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

Microorganismsand other

detritivores

Detritus

Primary producers

Primary consumers

Secondaryconsumers

Tertiary consumers

Heat

Sun

Key

Chemical cycling

Energy flow

Detritus is dead organic material

Nutrient cycling

Nutrients cycle within an ecosystem

Decomposition– Connects all trophic levels

– Detritivores, mainly bacteria and fungi, recycle essential chemical elements

– By decomposing organic material and returning elements to inorganic reservoirs

Decomposition

Primary Production

Primary production in an ecosystem– Is the amount of light energy converted

to chemical energy by autotrophs during a given time period

Physical and chemical factors limit primary production in ecosystems

Ecosystem Energy Budgets

The extent of photosynthetic production– Sets the “spending limit” for the energy budget

of the entire ecosystem

The Global Energy Budget

The amount of solar radiation reaching the surface of the Earth– Limits the photosynthetic output of ecosystems

Only a small fraction of solar energy– Actually strikes photosynthetic organisms– And only ~1% of that is converted to chemical

energy by photosynthesis– …still that’s a lot of energy

Gross and Net Primary Production

Total primary production in an ecosystem– Is known as that ecosystem’s gross primary

production (GPP)– Not all of this production is stored as organic material

in the growing plants (The plants use some of the energy for the fuel of day-to-day living)

Net primary production (NPP)– Is equal to GPP minus the energy used by the primary

producers for respiration– It is the amount of new biomass added in a given time

period– Only NPP is available to consumers

Different ecosystems vary considerably in their net primary production and in their contribution to the total NPP on Earth

Lake and stream

Open ocean

Continental shelf

Estuary

Algal beds and reefs

Upwelling zones

Extreme desert, rock, sand, ice

Desert and semidesert scrub

Tropical rain forest

Savanna

Cultivated land

Boreal forest (taiga)

Temperate grassland

Tundra

Tropical seasonal forestTemperate deciduous forest

Temperate evergreen forest

Swamp and marsh

Woodland and shrubland

0 10 20 30 40 50 60 0 500 1,000 1,500 2,000 2,500 0 5 10 15 20 25

Percentage of Earth’s netprimary production

Key

Marine

Freshwater (on continents)

Terrestrial

5.2

0.3

0.1

0.1

4.7

3.53.3

2.9

2.7

2.41.8

1.7

1.6

1.5

1.3

1.0

0.4

0.4

125

360

1,500

2,500

500

3.0

90

2,200

900

600

800

600

700

140

1,600

1,2001,300

2,000

250

5.6

1.2

0.9

0.1

0.040.9

22

7.99.1

9.6

5.4

3.50.6

7.1

4.9

3.8

2.3

0.3

65.0 24.4

Percentage of Earth’ssurface area

(a) Average net primaryproduction (g/m2/yr)(b) (c)

The open ocean contributes relatively little per unit of area…but there is an awful lot of area.

Forested areas contribute a great deal per unit area

180 120W 60W 0 60E 120E 180

North Pole

60N

30N

Equator

30S

60S

South Pole

Regional annual NPP (light violet lowestred highest)

Primary Production in Marine and Freshwater EcosystemsIn marine and freshwater ecosystems– Both light and nutrients are important in

controlling primary production

Light Limitation

The depth of light penetration– Affects primary production throughout the

photic zone of an ocean or lake

Nutrient Limitation

More than light, nutrients limit primary productionA limiting nutrient is the element that must be added– In order for production to increase in a particular area

Nitrogen and phosphorous– Are typically the nutrients that most often limit marine

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 (overnourishment?) of lakes, which can lead to the eventual loss of most fish species from the lakes

The overnourished upper lake has a tremendous cyanobacterial bloom

Phosphorus is often the limiting nutrient for cyanobacterial growth. Oversupply leads to blooms

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 geographic scale

Actual EvapotranspirationThe contrast between wet and dry climates can be represented by a measure called actual evapotranspiration

Actual evapotranspiration

– Is the amount of water

annually transpired by

plants and evaporated from

a landscape

– Is related to net primary

production (warm and moist

is better than cold and dry)Actual evapotranspiration (mm H2O/yr)

Tropical forest

Temperate forest

Mountain coniferous forest

Temperate grassland

Arctic tundra

Desertshrubland

Net

prim

ary

prod

uctio

n (g

/m2 /

yr)

1,000

2,000

3,000

0500 1,000 1,5000

On a local scale– A soil nutrient is often the limiting factor in

primary production

EXPERIMENT Over the summer of 1980, researchers added phosphorus to some experimental plots in the salt marsh, nitrogento other plots, and both phosphorus and nitrogen to others. Some plots were left unfertilized as controls.

RESULTS

Experimental plots receiving just phosphorus (P) do not outproduce the unfertilized control plots.

CONCLUSION

Live

, ab

ove-

grou

nd b

iom

ass

(g d

ry w

t/m

2)

Adding nitrogen (N) boosts net primaryproduction.

300

250

200

150

100

50

0June July August 1980

N P

N only

Control

P only

These nutrient enrichment experiments confirmed that nitrogen was the nutrient limiting plant growth in this salt marsh.

Secondary Production

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

The secondary production of an ecosystem– Is the amount of chemical energy in

consumers’ food that is converted to their own new biomass during a given period of time

Production EfficiencyWhen a caterpillar feeds on a plant leaf– Only about one-sixth

of the energy in the leaf is used for secondary production

The production efficiency of an organism– Is the fraction of

energy stored in food that is not used for respiration

Plant materialeaten by caterpillar

Cellularrespiration

Growth (new biomass)

Feces100 J

33 J

200 J

67 J

67J of the 100J of assimilated energy is used for respiration.

33J of 100J is new biomass…so the PE is 33%Nondigested materials aren’t figured into PE

Production efficiency

Endotherms (birds and mammals) have low production efficiencies. In the range of 1-3%– A lot of energy is used to maintain that constant body temp.

Ectotherms have higher production efficiencies– Fish at ~10%– Insects at around 40%

Trophic Efficiency and Ecological Pyramids

Trophic efficiency– Is the percentage of production transferred from

one trophic level to the next– Usually ranges from 5% to 20%

Pyramids of ProductionThe loss of energy with each transfer in a food chain can be represented by a pyramid of net production

Tertiaryconsumers

Secondaryconsumers

Primaryconsumers

Primaryproducers

1,000,000 J of sunlight

10 J

100 J

1,000 J

10,000 J

Pyramids of BiomassMost biomass pyramids– Show a sharp decrease at successively higher

trophic levels

Trophic level Dry weight(g/m2)

Primary producers

Tertiary consumers

Secondary consumers

Primary consumers

1.5

11

37809

Certain aquatic ecosystems can have inverted biomass pyramids– Here the phytoplankton (algae) grow very rapidly and are very

productive but their consumption by zooplankton holds the population size down

Trophic level

Primary producers (phytoplankton)

Primary consumers (zooplankton)

(b) 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).

Dry weight(g/m2)

21

4

Pyramids of Numbers

A pyramid of numbers– Represents the number of individual organisms

in each trophic levelTrophic level Number of

individual organisms

Primary producers

Tertiary consumers

Secondary consumers

Primary consumers

3

354,904

708,624

5,842,424

The dynamics of energy flow through ecosystems– Have important implications for the

human population

Eating meat– Is a relatively inefficient way of tapping

photosynthetic production

Worldwide agriculture could successfully feed many more people– If humans all fed more efficiently, eating only

plant material (processing the plant material through another food animal decreases efficiency of energy transfer)

Trophic level

Secondaryconsumers

Primaryconsumers

Primaryproducers

Relative food energy available to the human population at different trophic levels

The Green World HypothesisMost terrestrial ecosystems have large standing crops despite the large numbers of herbivores

According to the green world hypothesis

– Terrestrial herbivores consume relatively little plant biomass because they are held in check by a variety of factors

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

– Plants have defenses against herbivores

– Nutrients, not energy supply, usually limit herbivores (paucity of essential nutrients in their diet)

– Abiotic factors limit herbivores (temperature, water availability)

– Intraspecific competition can limit herbivore numbers (territoriality, competition, etc.)

– Interspecific interactions check herbivore densities (predators, parasites, disease, etc)

Chemical CyclingBiological and geochemical processes move nutrients between organic and inorganic parts of the ecosystemLife on Earth– Depends on the recycling of essential chemical

elements

Nutrient circuits that cycle matter through an ecosystem– Involve both biotic and abiotic components

and are often called biogeochemical cycles

Biogeochemical Cycles

The water cycle and the carbon cycleTransportover land

Solar energy

Net movement ofwater vapor by wind

Precipitationover ocean

Evaporationfrom ocean

Evapotranspirationfrom land

Precipitationover land

Percolationthroughsoil

Runoff andgroundwater

CO2 in atmosphere

Photosynthesis

Cellularrespiration

Burning offossil fuelsand wood Higher-level

consumersPrimaryconsumers

DetritusCarbon compounds in water

Decomposition

THE WATER CYCLE THE CARBON CYCLE

Water moves in a global cycle driven by solar energyThe carbon cycle reflects the reciprocal processes of photosynthesis and cellular respiration

More Biogeochemical Cycles

N2 in atmosphere

Denitrifyingbacteria

Nitrifyingbacteria

Nitrifyingbacteria

Nitrification

Nitrogen-fixingsoil bacteria

Nitrogen-fixingbacteria in rootnodules of legumes

Decomposers

Ammonification

Assimilation

NH3 NH4+

NO3

NO2

Rain

Plants

Consumption

Decomposition

Geologicuplift

Weatheringof rocks

Runoff

SedimentationPlant uptakeof PO4

3

Soil

Leaching

THE NITROGEN CYCLE THE PHOSPHORUS CYCLE

The nitrogen cycle and the phosphorous cycle

Decomposition and Nutrient Cycling Rates

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

The rates at which nutrients cycle in different ecosystems are extremely variable, mostly as a result of differences in rates of decomposition

Consumers

Producers

Nutrientsavailable

to producers

Abioticreservoir

Geologicprocesses

Decomposers

Human activities

The human population is disrupting chemical cycles throughout the biosphere

As the human population has grown in size– Our activities have disrupted the trophic

structure, energy flow, and chemical cycling of ecosystems in most parts of the world

Agriculture and Nitrogen CyclingAgriculture constantly removes nutrients from ecosystems that would ordinarily be cycled back into the soil

Nitrogen is the main nutrient lost through agriculture– Thus, agriculture has a great impact on the

nitrogen cycle

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

Nutrient EnrichmentIn addition to transporting nutrients from one location to another…Humans have added entirely new materials, some of them toxins, to ecosystems

Contamination of Aquatic Ecosystems

The critical load for a nutrient– Is the amount of that nutrient that can be absorbed by

plants in an ecosystem without damaging it

When excess nutrients are added to an ecosystem, the critical load is exceeded– And the remaining nutrients can contaminate

groundwater and freshwater and marine ecosystems

Sewage runoff contaminates freshwater ecosystems– Causing eutrophication, excessive algal growth,

which can cause significant harm to these 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

4.6

4.64.3

4.14.3

4.6

4.64.3

Europe

North America

Numbers indicate the average pH of precipitation in that area

By the year 2000 the entire contiguous United States was affected by acid precipitation

(defined as precipitation less than pH 5.6)

Field pH5.35.2–5.35.1–5.25.0–5.14.9–5.04.8–4.94.7–4.84.6–4.74.5–4.64.4–4.54.3–4.44.3

Toxins in the EnvironmentHumans release an immense variety of toxic chemicals– Including thousands of synthetics previously

unknown to nature (Xenobiotic)

One of the reasons such toxins are so harmful– Is that they become more concentrated in

successive trophic levels of a food web

In some cases, harmful substances– Persist for long periods of time in an ecosystem and

continue to cause harm

In biological magnification– Toxins concentrate at higher trophic levels because

at these levels biomass tends to be lower

Con

cent

ratio

n of

PC

Bs

Herringgull eggs124 ppm

Zooplankton 0.123 ppm

Phytoplankton 0.025 ppm

Lake trout 4.83 ppm

Smelt 1.04 ppm

Small concentrations of toxin spread among many individuals

Fewer individuals each with high concentration of toxin

Atmospheric Carbon Dioxide

The rising level of atmospheric carbon dioxide is human in origin

Rising Atmospheric CO2Due to the increased burning of fossil fuels and other human activities the concentration of atmospheric CO2 has been steadily increasing

CO

2 c

onc

en

trat

ion

(pp

m)

390

380

370

360

350

340

330

320

310

3001960 1965 1970 1975 1980 1985 1990 1995 2000 2005

1.05

0.90

0.75

0.60

0.45

0.30

0.15

0

0.15

0.30

0.45

Te

mp

era

ture

va

ria

tion

(C

)

Temperature

CO2

Year

The Greenhouse Effect and Global Warming

The greenhouse effect is caused by atmospheric CO2

– But is necessary to keep the surface of the Earth at a habitable temperature

Increased levels of atmospheric CO2 are magnifying the greenhouse effect– Which could cause global warming and

significant climatic change

Depletion of Atmospheric Ozone

Life on Earth is protected from the damaging effects of UV radiation– By a protective layer or ozone molecules

present in the atmosphere

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

Ozo

ne la

yer

thic

knes

s (D

obso

n un

its)

Year (Average for the month of October)

350

300

250

200

150

100

50

01955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

The destruction of atmospheric ozone– Probably results from chlorine-releasing

pollutants produced by human activity

1

2

3

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

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

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

Sunlight

Chlorine O3

O2

ClO

ClO

Cl2O2

O2

Chlorine atoms

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

increased