Chapter 51

Ecosystems

Chapter 51

Ecosystems

Many global environmental problems have emerged recently.

Ecosystems consist of all the organisms that live in an area along with the nonbiological components.

Energy and nutrient flows link the biotic and abiotic environments.

Energy Flow and Trophic Structure

All ecosystems consist of four components that are linkedby the flow of energy:

• Primary producers

• Consumers

• Decomposers

• Abiotic environment (Fig. 51.1)

External energy source

PRIMARYPRODUCERS

CONSUMERS DECOMPOSERS

ABIOTIC ENVIRONMENT

Figure 51.1

External energy source

PRIMARYPRODUCERS

CONSUMERS DECOMPOSERS

ABIOTIC ENVIRONMENT

Figure 51.1

Energy Flow and Trophic Structure

Key points about energy flow through ecosystems.

• Energy enters ecosystems in the form of sunlight that is usedin photosynthesis by producers.

• Plants use only a tiny fraction of the total radiation that isavailable to them.

• Only a tiny fraction of fixed energy actually becomes availableto consumers.

Energy Flow and Trophic Structure

Key points about energy flow through ecosystems.

• Most net primary production that is consumed enters the decomposer food web.

• From there, only a small fraction is used for secondaryproduction by herbivores and carnivores.

• Most energy fixed during photosynthesis is used for respiration, not synthesis of new tissues. (Fig. 51.2)

Energy source:1,254,000kcal/m2/year

0.8% energy captured by photosynthesis. Of this...

…45% supports growth(Net primary production)

…11% entersgrazing food web

…34% entersdecomposer food webas dead material

…55% lostto respiration

Figure 51.2

Energy source:1,254,000kcal/m2/year

…11% enters grazing food web

…34% entersdecomposer food webas dead material

0.8% energy captured by photosynthesis. Of this...

…45% supports growth(Net primary production)

…55% lost to respiration

Figure 51.2

0–100100–200200–400400–600600–800>800

Productivity ranges (g/m2/yr)

Figure 51.3a

Terrestrial productivity

<3535–5555–90>90

Productivity ranges (g/m2/yr)

Figure 51.3b

Marine productivity

80.7% respiration

17.7% excretion1.6% growth and reproduction

Energy derived from plants

Figure 51.4

Predators of decomposers:

Spider

Centipede

MushroomMushroom

EarthwormEarthworm

Primary Primary decomposers:decomposers:

Bacteria and archaeaBacteria and archaeaMillipedeMillipede

NematodesNematodesPillbugsPillbugs

Salamander

305 nm 49.4 µm

Figure 51.5

PuffballPuffball

Energy Flow and Trophic Structure

Trophic structure

• Organisms that obtain their energy from the same type ofsource occupy the same trophic level.

• Each feeding level within an ecosystem represents a trophiclevel.

Energy Flow and Trophic Structure

Trophic structure

• Organisms at the top trophic level are not eaten by any other organisms.

• Productivity is highest at the lowest trophic level. (Fig. 51.6a,b)

Trophic

level4

3

2

1

Feeding strategySecondary carnivore

Carnivore

Herbivore

Autotroph

Grazing food chain Decomposer food chain

Cricket

Maple tree leaves

Owl

Shrew

Earthworm

Dead maple leaves

Cooper’shawk

Robin

Figure 51.6a

Trophic levels

4Secondary carnivore

3

Carnivore

2

Herbivore

1

Autotroph

Productivity

Figure 51.6b

Pyramid of productivity

Energy Flow and Trophic Structure

Food chains and food webs

• Food chains are typically embedded in more complexfood webs. (Fig. 51.7a,b)

Pisaster(a sea star)

Thais(a snail)

Bivalves(clams, mussels)

Figure 51.7a

Food chain

Pisaster

Thais

ChitonsLimpets

BivalvesAcornbarnacles

Gooseneckbarnacles

Figure 51.7b

Food web

Energy Flow and Trophic Structure

Food chains and food webs

• The maximum number of links in any food chain or web ranges from 1 to 6. (Fig. 51.7c)

• Hypotheses offered to explain this:

Energy transfer may limit food-chain length.

Long food chains may be more fragile.

Food-chain length may depend on environmental complexity.

Nu

mb

er o

f o

bse

rvat

ion

s

Number of links in food chain

10

8

6

4

2

01 2 3 4 5 6

Streams

Lakes

Terrestrial

Figure 51.7c

Food chains tend to have few links.

Average number of links = 3.5

Biogeochemical Cycles

The path an element takes as it moves from abiotic systems through living organisms and back again is referred to asits biogeochemical cycle. (Fig. 51.8)

Ass

imila

tio

n

Loss to erosion or leaching into groundwater

Soil nutrient pool

Decomposerfood web

Detritus

Death

Herbivore

Uptake

Plants

Feces or urine

Figure 51.8

Boreal forest

Figure 51.9 upper

Tropical rain forest

Figure 51.9 lower

Biogeochemical Cycles

A key feature in all cycles is that nutrients are recycledand reused.

The overall rate of nutrient movement is limited most by decomposition of detritus.

The rate of nutrient loss is a very important characteristic inany ecosystem. (Fig. 51.10a,b)

Devegetation experiment

Choose two similar watersheds.Document nutrient levels in soil organic matter, plants, and streams.

Figure 51.10a upper

Figure 51.10a lower

Clearcut Control

Devegetate one watershed and leave the other intact.Monitor the amount of dissolved substances in streams.

Devegetated

Net

dis

solv

ed s

ub

stan

ce (

kg/h

a)

1965–66 1966–67 1967–68 1968–69 1969–70

Control

1000

800

600

400

200

0

Year

Figure 51.10b

Nutrient runoff results

Biogeochemical Cycles

Nutrient flow among ecosystems links local cycles into one massive global biogeochemical cycle.

• The carbon cycle and the nitrogen cycle are examples of major, global biogeochemical cycles. (Fig. 51.11, 51.13a)

• Humans are now disrupting almost all biogeochemical cycles. This can have very harmful effects. (Fig. 51.12a,b; 51.13b)

THE GLOBAL CARBON CYCLEAll values in gigatons of carbon per year

Physicaland chemical processes: 92

2Ocean: 40,000 Rivers: 1

Land, biota, soil, litter, peat: 2000

Decomposition:50

Respiration:50

Photosynthesis:102

Physicaland chemical processes: 90

Deforestation:1.5

Fossilfuel use:

6.0

Atmosphere: 750 (in 1990)+3.5 per year

Aquatic ecosystems Terrestrial ecosystems Human–inducedchanges

Figure 51.11

THE GLOBAL NITROGEN CYCLE

Nitrogenfixing cyanobacteria

MudDecomposition of detritus into ammonia

Nitrogen-fixing bacteria in roots and soil

Industrial fixation

Protein andnucleic acid synthesis

Atmospheric nitrogen (N2)

Bacteria in muduse N-containing molecules as energy sources, excrete (N2)

Run–off

Lightning and rain

Figure 51.13a

Land use

Fossil fuel use

Year

An

nu

al f

lux

of

carb

on

(10

15g

)

6

5

4

3

2

1

01860 1880 1900 1920 1940 1960 1980

Figure 51.12a

Human-induced increases in CO2 flux over time

Year

CO

2 co

nce

ntr

atio

n (

pp

m)

360

350

340

330

320

3101960 1970 1980 1990

Figure 51.12b

Atmospheric CO2

Natural sources Human sources

Am

ou

nt

of

nit

rog

en (

gig

ato

ns/

year

)

160

140

120

100

80

60

40

20

0

Sources of nitrogen fixation

Lightning

Biologicalfixation

Fossil fuels

Nitrogenfertilizer

Nitrogen-fixing crops

Figure 51.13b