Chapter 51

Ecosystems

Ecosystems

Population: all the individuals of a certain species that live in a particular area

Community: all the different species that interact together within a particular area

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

Ecosystems

Many global environmental problems have emerged recently.

Ecosystem ecology follows the flow of energy and nutrients through ecosystems

Humans have artificially affected the flow of these components

Energy Flow within Ecosystems

Energy enters an ecosystem primarily though sunlight:

Energy Flow and Trophic Structure

Species within an ecosystems are classified into different trophic levels:

• Primary producers: autotrophs, photosynthetic- plants, algae, some bacteria

• Consumers

• Primary consumers: herbivores that eat producers (plants)- deer, rabbits, etc.

• Secondary consumers: carnivores that eat herbivores: wolf eating a deer

• Tertiary consumers: carnivores that eat carnivores: a hawk eating a snake that ate a mouse

• Decomposers: fungi, bacteria that break down organic material (dead plants and animals)

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

Different Trophic Levels in an Ecosystem

External energy source

PRIMARYPRODUCERS

CONSUMERS DECOMPOSERS

ABIOTIC ENVIRONMENT

Energy Flow in an ecosystem

Predators of decomposers:

Spider

Centipede

MushroomMushroom

EarthwormEarthworm

Primary Primary decomposers:decomposers:

Bacteria and archaeaBacteria and archaeaMillipedeMillipede

NematodesNematodesPillbugsPillbugs

Salamander

305 nm 49.4 µm

PuffballPuffball

Decomposers

Energy Flow and Trophic Structure

Key points about energy flow through ecosystems.

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

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

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

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

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

Ecological Efficiency: percent of energy transferred from one trophic level to the next

Ecosystem Processes

Production: rate at which energy/nutrients are converted into growth

• Includes Primary Production: growth by autotrophs

• Includes Secondary Production - growth by heterotrophs

Consumption - the intake and use of organic material by heterotrophs

Decomposition - the chemical breakdown of organic material

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

Very little of the energy consumed by primary consumers are used for secondary production

4Secondary carnivore

3

Carnivore

2

Herbivore

1

Autotroph

Productivity

Example: 100g of plant becomes 5-20g of

grasshopper then 0.25-1g of mouse

Pyramid of productivity

Pisaster(a sea star)

Thais(a snail)

Bivalves(clams, mussels)

The Different Trophic levels in an ecosystem is often pictured as a Food chain

Energy Flow and Trophic Structure

Food chains and food webs

• Food chains are typically embedded in more complexfood webs.

• Many organisms feed at more than one trophic level

Pisaster

Thais

ChitonsLimpets

BivalvesAcornbarnacles

Gooseneckbarnacles

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.

• 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

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.

Examples: nitrogen cycle, carbon cycle, phosphorus cycle

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

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.

Boreal forest: nutrients are put back into the soil slowly, so organic material builds up

Tropical rain forest: decomposition is rapid so there is very little organic build up

Result: if living material is removed from tropical rain forests, the soil is nutrient poor to support new growth

Devegetation experiment

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

The rate of nutrient loss is a very important characteristic inany ecosystem.

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

Nutrient runoff results

Nutrient export increases dramatically in devegetated plot

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.

• Humans are now disrupting almost all biogeochemical cycles. This can have very harmful effects.

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

Humans are adding significant amounts of carbon into the atmosphere

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

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

Industrial fixationNitrogen

fixing cyanobacteria

MudDecomposition of detritus into ammonia

Nitrogen-fixing bacteria in roots and soil

Protein andnucleic acid synthesis

Atmospheric nitrogen (N2) =78%

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

Lightning and rain

Only nitrogen-fixing bacteria can use N2

make ammonia (NH3) or nitrate (NO3) limiting nutrient (demand exceeds supply) for plants

All organisms require nitrogen to make protein Animals get nitrogen from their diets, not the air

THE GLOBAL NITROGEN CYCLE

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

Human activities now fix almost as much nitrogen each year as natural sources