Ecosystems

Chapter 27

Chapter 28 At a Glance

• 28.1 How Do Energy and Nutrients

Move Through Ecosystems?

• 28.2 How Does Energy Flow Through

Ecosystems?

• 28.3 How Do Nutrients Cycle Within and

Among Ecosystems?

• 28.4 What Happens When Humans

Disrupt Nutrient Cycles?

Ecosystem

An association of organisms and their

physical environment, interconnected by

ongoing flow of energy and a cycling of

materials

Modes of Nutrition

• Autotrophs

– Capture sunlight or chemical energy

– Producers

• Heterotrophs

– Extract energy from other organisms or

organic wastes

– Consumers, decomposers, detritivores

O2

CO2

H2O

sugar

plant tissues

other

nutrients

energy

from

sunlight

photosynthesis

Producers

Fig. 28-1

Simple

Ecosystem

Model

Autotrophs (plants,

other self-

feeding organisms)

Heterotrophs

(animals, most

fungi,

many protists,

many bacteria)

(mainly metabolic heat)

Energy Flow, Nutrient Cycling,

and Feeding Relationships in

Ecosystems

Fig. 28-2

solar energy

heat

heat

heat

heat

nutrients

heat energy

energy stored in chemical bonds

detritus feeders

and decomposers

primary consumers

higher-level

consumers

producers

energy from sunlight

nutrients

O S P

N

Mg H

Ca H

2O

Diet of an omnivore (red fox)

• Herbivores

• Carnivores

• Parasites

• Omnivores

• Decomposers

• Detritivores

Seasonal variation in the diet of an

omnivore (red fox)

rodents,

rabbits

fruits insects

birds

rodents,

rabbits

fruits

insects

birds

rodents,

rabbits

rodents,

rabbits

birds

birds

insects

insects

fruits

fruits

WINTER

FALL

SUMMER

SPRING

Trophic Levels

• All the organisms at a trophic level are

the same number of steps away from

the energy input into the system

• Producers are closest to the energy

input and are the first trophic level

Trophic Levels in Prairie

5th

4th

3rd

2nd

1st

Fourth-level consumers (heterotrophs):

Top carnivores, parasites,

detritivores, decomposers

Third-level consumers (heterotrophs): Carnivores, parasites, detritivores,

decomposers

Second-level consumers (heterotrophs):

Carnivores, parasites, detritivores,

decomposers

First-level consumers

(heterotrophs):

Herbivores, parasites, detritivores,

decomposers

Primary producers (autotrophs):

Photoautotrophs, chemoautotrophs

Food Chain

• A straight line

sequence of who

eats whom

• Simple food chains

are rare in nature

marsh hawk

upland sandpiper

garter snake

cutworm

flowering plant

Food Chains on Land

Fig. 28-4a

(a) A simple terrestrial food chain

tertiary consumer

(4th trophic level)

producer

(1st trophic level)

primary consumer

(2nd trophic level)

secondary consumer

(3rd trophic level)

Food Web marsh hawk

crowupland

sandpiper

garter snake

frog

spider

weasel badger coyote

ground squirrelpocket

gopherprairie vole

clay-colored

sparrow

earthworms, insects (e.g.,

grasshoppers, cutworms)

grasses, composites

HIGHER TROPHIC LEVELS

Complex array of carnivores,

omnivores and other consumers.

Many feed at more than one trophic level continually,

seasonally, or when an

oppportunity presents itself

SECOND TROPHIC LEVEL Primary

consumers (e.g., herbivores,

detritivores, and decomposers)

FIRST TROPHIC LEVEL

Primary producers

Primary Productivity

• Gross primary productivity is

ecosystem’s total rate of photosynthesis

• Net primary productivity is rate at which

producers store energy in tissues in

excess of their aerobic respiration

Energy Losses

• Energy transfers are never 100 percent

efficient

• Some energy is lost at each step

• Limits the number of trophic levels in an

ecosystem

Primary Productivity Varies

• Seasonal variation

• Variation by habitat

• The harsher the environment, the

slower plant growth, the lower the

primary productivity

28.2 How Does Energy Flow

Through Ecosystems? • Energy pyramids illustrate energy

transfer between trophic levels

– The net energy transfer between trophic levels is roughly 10% efficient and is known as the “10% law”

• An energy pyramid, which shows maximum energy available at the base and steadily diminishing amounts at higher levels, illustrates the general energy relationships between tropic levels

Silver Springs:

Annual Energy

Flow

ENERGY INPUT

17,000,000 kilocalories

energy transfers through ecosystem

incoming solar energy not harnessed:

producers

herbivores

carnivores

top carnivores

decomposers, detritivores

1,679,190 (98.8%)

20,810 (1.2%)

transferred to the next trophic level:

energy still in organic wastes and remains:

energy losses as metabolic heat and as net export from the ecosystem:

ENERGY OUPUT: TOTAL ANNUAL ENERGY FLOW:

4,245 3,368

13,197

383

21

720

272

16 5

5,060

2,265

90

1,700,000 (100%)

20,810 + 1,679,190

Silver Springs Study

• Aquatic ecosystem in Florida

• Site of a long-term study of a grazing food web

5

decomposers, detritivores

(bacteria, crayfish)

1.5

11

37

third-level carnivores

(gar, large-mouth bass)

second-level consumers

(fishes, invertebrates)

first-level consumers

(herbivorous fishes,

turtles, invertebrates)

primary producers (algae,

eelgrass, rooted plants) 809

All Heat in the End

• At each trophic level, the bulk of the

energy received from the previous level

is used in metabolism

• This energy is released as heat energy

and lost to the ecosystem

• Eventually, all energy is released as

heat

Biogeochemical Cycles

• The flow of a nutrient from the

environment to living organisms and

back to the environment

• Main reservoir for the nutrient is in the

environment

Nutrient Flow:

Land Ecosystem

Three Categories

• Hydrologic cycle

– Water

• Atmospheric cycles

– Nitrogen and carbon

• Sedimentary cycles

– Phosphorus and sulfur

Hydrologic Cycle

evaporation

from ocean

425,000

precipitation

into ocean

385,000

evaporation from land

plants

(evapotranspiration)

71,000

precipitation

onto land

111,000

wind-driven water vapor

40,000

surface and

groundwater flow

40,000

Atmosphere

Oceans Land

Hubbard Brook Experiment

• A watershed was experimentally

stripped of vegetation

• All surface water draining from

watershed was measured

• Removal of vegetation caused a six-fold

increase in the calcium content of the

runoff water

Hubbard Brook Experiment

losses from disturbed

watershed plot

time of deforestation

Aquifer Depletion

• Green signifies high

overdrafts

• Gold, moderate

overdrafts

• Yellow, insignificant

withdrawals

• Shaded areas show

groundwater pollution

• Blue squares: saltwater

intrusion

Carbon Cycle

• Carbon moves through the atmosphere

and food webs on its way to and from

the ocean, sediments, and rocks

• Sediments and rocks are the main

reservoir

Carbon Cycle

photosynthesis TERRESTRIAL

ROCKS

volcanic action

weathering

diffusion

Bicarbonate,

carbonate

Marine food webs

Marine Sediments

Atmosphere

Terrestrial

rocks

Soil water Peat, fossil

fuels

Land food

webs

Carbon in the Oceans

• Most carbon in the ocean is dissolved

carbonate and bicarbonate

• Ocean currents carry dissolved carbon

Greenhouse Effect

• Greenhouse gases impede the escape

of heat from Earth’s surface

Carbon Dioxide Increase

• Carbon dioxide levels fluctuate

seasonally

• The average level is steadily increasing

• Burning of fossil fuels and deforestation

are contributing to the increase

Carbon in Atmosphere

• Atmospheric carbon is mainly carbon

dioxide

• Carbon dioxide is added to

atmosphere

– Aerobic respiration, volcanic action,

burning fossil fuels

• Removed by photosynthesis

Other Greenhouse Gases

• CFCs - synthetic gases used in plastics

and in refrigeration

• Methane - produced by termites and

bacteria

• Nitrous oxide - released by bacteria,

fertilizers, and animal wastes

Greenhouse Gases

carbon

dioxide

methane

CFCs

nitrous oxide

Climate Change

• Long-term increase in the temperature

of Earth’s lower atmosphere

Global Warming Parallels

Atmospheric CO2 Increases

Fig. 28-15

Biological Magnification

A nondegradable or slowly degradable

substance becomes more and more

concentrated in the tissues of

organisms at higher trophic levels of a

food web

DDT in Food Webs

• Synthetic pesticide banned in the United

States since the 1970s

• Birds that were top carnivores

accumulated DDT in their tissues

• Mercury in the fish today

Minamata Bay Disease

(mercury poisoning)

Nitrogen Cycle

• Nitrogen is used in amino acids and

nucleic acids

• Main reservoir is nitrogen gas in the

atmosphere

Nitrogen Cycle

gaseous nitrogen in atmosphere

food webs on landnitrogen fixation

fertilizers

uptake by autotrophs uptake by autotrophsexcretion, death,

decomposition

ammonia, ammonium in soil nitrate in soil

nitrate in soil

nitrification

nitrification

ammonification

nitrogen-rich wastes,

remains in soil

loss by

leaching

loss by leaching

loss by

dentrification

Nitrogen Fixation

• Plants cannot use nitrogen gas

• Nitrogen-fixing bacteria convert

nitrogen gas into ammonia (NH3)

• Ammonia and ammonium can be

taken up by plants

Ammonification & Nitrification

• Bacteria and fungi carry out

ammonification, conversion of

nitrogenous wastes to ammonia

• Nitrifying bacteria convert ammonium to

nitrites and nitrates

Nitrogen Loss

• Nitrogen is often a limiting factor in

ecosystems

• Nitrogen is lost from soils via leaching

and runoff

• Denitrifying bacteria convert nitrates

and nitrites to nitrogen gas

Human Effects

• Humans increase rate of nitrogen loss

by clearing forests and grasslands

• Humans increase nitrogen in water and

air by using fertilizers and by burning

fossil fuels

• Too much or too little nitrogen can

compromise plant health

Phosphorus Cycle

• Phosphorus is part of phospholipids and

all nucleotides

• It is the most prevalent limiting factor in

ecosystems

• Main reservoir is Earth’s crust; no

gaseous phase

Phosphorus Cycle

GUANO

FERTILIZER

ROCKS

LAND

FOOD

WEBS

DISSOLVED

IN OCEAN

WATER

MARINE

FOOD

WEBS

MARINE SEDIMENTS

excretion

weathering

mining

agriculture

uptake by

autotrophs

death,

decomposition

sedimentation setting out leaching, runoff

weathering

uplifting over

geologic time

DISSOLVED IN

SOILWATER,

LAKES, RIVERS

uptake by

autotrophs

death,

decomposition

Human Effects

• In tropical countries, clearing lands for

agriculture may deplete phosphorus-

poor soils

• In developed countries, phosphorus

runoff is causing eutrophication of

waterways