Ecosystems

Chapter 30

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

Simple Ecosystem

Modelenergy input from sun

nutrientcycling

PHOTOAUTOTROPHS(plants, other producers)

HETEROTROPHS(consumers, decomposers)

energy output (mainly heat)

Consumers

• Herbivores

• Carnivores

• Parasites

• Omnivores

• Decomposers

• Detritivores

SPRING

rodents, rabbits

fruits

insects

birds

SUMMER

rodents, rabbits

fruits

insects

birds

Seasonal variation in the diet of an omnivore (red fox)

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

FoodWeb

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

Two Types of Food Webs

Producers (photosynthesizers)

Producers (photosynthesizers)

herbivores

carnivores

decomposers

decomposers

detritivores

ENERGY OUTPUT ENERGY OUTPUT

Grazing Food Web Detrital Food Web

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

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

Primary Productivity Varies

• Seasonal variation

• Variation by habitat

• The harsher the environment, the

slower plant growth, the lower the

primary productivity

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

1.1

37

third-level carnivores(gar, large-mouth bass)

second-level consumers(fishes, invertebrates)

first-level consumers(herbivorous fishes,turtles, invertebrates)

809 primary producers (algae,eelgrass, rooted plants)

Biomass pyramid

Pyramid of Energy Flow

• Primary producers trapped about 1.2 percent of the solar energy that entered the ecosystem

• 6-16% passed on to next level

detritivores

21

383

3,368

20,810 kilocalories/square meter/year

top carnivores

carnivores

herbivores

producers

decomposers

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 Cycle

• 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

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 vapor40,000

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 fromdisturbed watershed

time ofdeforestation

losses fromundisturbed watershed

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 geolgic 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

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

photosynthesisTERRESTRIAL

ROCKS

volcanic action

weathering

diffusion

Bicarbonate, carbonate

Marine food webs

Marine Sediments

Atmosphere

TerrestrialRocks

Soil WaterPeat, Fossil

Fuels

Land Food Webs

Carbon in the Oceans

• Most carbon in the ocean is dissolved carbonate and bicarbonate

• Ocean currents carry dissolved carbon

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

Greenhouse Effect

• Greenhouse gases impede the escape

of heat from Earth’s surface

Global Warming

• Long-term increase in the temperature

of Earth’s lower atmosphere

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

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

Nitrogen Cycle

• Nitrogen is used in amino acids and

nucleic acids

• Main reservoir is nitrogen gas in the

atmosphere

Nitrogen Cycle

NO3-

IN SOIL

NITROGEN FIXATION

by industry for agriculture

FERTILIZERS

FOOD WEBS ON LAND

NH3-, NH4

+

IN SOIL

1. NITRIFICATION

loss by leaching

uptake by autotrophs

excretion, death,

decomposition

uptake by autotrophs

NITROGEN FIXATION

loss by leaching

AMMONIFICATION

2. NITRIFICATION

NITROGENOUS WASTES, REMAINS IN SOIL

GASEOUS NITROGEN (N2)

IN ATMOSPHERE

NO2-

IN SOIL

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