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
Model
energy
input from
sun
PHOTOAUTOTROPHS
(plants, other producers)
nutrient
cycling
HETEROTROPHS
(consumers, decomposers)
energy output (mainly heat)
Consumers
• Herbivores
SPRING
fruits
insects
• Carnivores
rodents,
rabbits
birds
• Parasites
• Omnivores
• Decomposers
• Detritivores
SUMMER
fruits
rodents,
rabbits
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
Fourth-level consumers (heterotrophs):
Top carnivores, parasites,
detritivores, decomposers
Third-level consumers (heterotrophs):
Carnivores, parasites, detritivores,
decomposers
Second-level consumers (heterotrophs):
3rd
Carnivores, parasites, detritivores,
decomposers
First-level consumers
(heterotrophs):
2nd
Herbivores, parasites, detritivores,
decomposers
Primary producers (autotrophs):
1st
Photoautotrophs, chemoautotrophs
Food Chain
MARSH
HAWK
• A straight line
sequence of who
UPLAND
SANDPIPER
eats whom
• Simple food chains
GARTER SNAKE
are rare in nature
CUTWORM
Food
Web
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
Grazing Food Web
Detrital Food Web
Two Types of Food Webs
Producers
(photosynthesizers)
Producers
(photosynthesizers)
herbivores
decomposers
carnivores
detritivores
decomposers
ENERGY OUTPUT
ENERGY OUTPUT
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
Biomass
pyramid
decomposers,
detritivores
5
(bacteria,
crayfish)
third-level carnivores
(gar, large-mouth bass)
1.5
1.1
second-level consumers
(fishes, invertebrates)
37
first-level consumers
(herbivorous fishes,
turtles, invertebrates)
809
primary producers (algae,
eelgrass, rooted plants)
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
top carnivores
21
carnivores
herbivores
383
decomposers
3,368
producers
20,810 kilocalories/square meter/year
detritivores
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
Atmosphere
wind driven water vapor
40,000
evaporation
from ocean
425,000
precipitation
into ocean
385,000
precipitation
onto land
111,000
evaporation from
land plants
(evapotranspiratio
n) 71,000
surface and
groundwater flow
40,000
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
time of
deforestation
losses from
undisturbed 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
mining
excretion
FERTILIZER
GUANO
agriculture
uptake by
autotrophs
MARINE
FOOD
WEBS
weathering
DISSOLVED
IN OCEAN
WATER
uptake by
autotrophs
weathering
DISSOLVED IN
SOILWATER,
LAKES, RIVERS
death,
decomposition
sedimentation
death,
decomposition
leaching, runoff
setting out
uplifting over
geolgic time
MARINE SEDIMENTS
ROCKS
LAND
FOOD
WEBS
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
diffusion
Atmosphere
Bicarbonate,
volcanic action
carbonate
Marine
food
TERRESTRIAL
webs ROCKS
Terrestrial
Rocks
photosynthesis
Land Food
Webs
Soil Water
Marine Sediments
weathering
Peat, Fossil
Fuels
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
GASEOUS N ITROGEN
(N2) IN ATMOSPHERE
NITROGEN
FIXATION
by industry for
agriculture
FOOD WEBS
ON LAND
FERTILIZERS
NITROGEN FIXATION
uptake by
autotrophs
excretion,
death,
decomposition
NITRO GENOUS WASTES,
REMAINS IN SOIL
NH3-, NH4+
IN SOIL
AMMONIFICATION
loss by
leaching
1. NITRIFICATION
uptake by
autotrophs
NO3IN SOIL
2. NITRIFICATION
NO2IN SOIL
loss by
leaching
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