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Communities
biodiversity, issues
keystone species
habitats, niches
Interactions:
competition, predation,
mutualism, commensalism
co-evolution
Dining In
• Wasps and Pieris caterpillars form a food chain
• Wasp eggs layed inside caterpillar
– Caterpillar eaten up by larvae
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• A second wasp can detect wasp larvae inside
these caterpillars
– and will deposit her own eggs inside of the
first wasp’s larvae
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• Finally, yet another wasp, a chalcid, may lay its
eggs inside the second wasp’s larvae
• Only the chalcid wasp’s larvae emerge from the
caterpillar carcass
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Community
• = All the organisms in a particular area
• Description includes:
– Trophic structure
(feeding relationships)
-- Vegetation
-- Biodiversity
-- Response to disturbances
Figure 36.1
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TROPHIC LEVEL
What’s for
lunch?
Quaternary
consumers
Carnivore
Carnivore
Tertiary
consumers
Carnivore
Carnivore
Secondary
consumers
Carnivore
Carnivore
Primary
consumers
Herbivore
Zooplankton
Producers
Plant
Phytoplankton
A TERRESTRIAL FOOD CHAIN
AN AQUATIC FOOD CHAIN
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Figure 36.9A
Food webs often
more real than
food chains
Food webs reveal
the flow of energy
Wastes and
dead organisms
Tertiary
and
secondary
consumers
Secondary
and
primary
consumers
Primary
consumers
Producers
(Plants, algae,
phytoplankton)
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Detritivores
(Prokaryotes, fungi,
certain animals)
Figure 36.10
Energy supply limits the length of food chains
• 170 billion tons of biomass per year
Tertiary
consumers
10 kcal
Secondary
consumers
100 kcal
Primary
consumers
1,000
kcal
Producers
10,000 kcal
Avg. 10%
conversion of
biomass
to next level
Endothermic animals
convert only 2%
1,000,000 kcal of sunlight
Plants convert
30-85%
Figure 36.11
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• Consequences:
• Low density of large carnivores
• a field of corn can support more vegetarians
than carnivores.
TROPHIC LEVEL
Secondary
consumers
Primary
consumers
Human
meat-eaters
Human
vegetarians
Cattle
Corn
Corn
Producers
Figure 36.12
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• Chemicals are
concentrated in
food chains by
biological
magnification
DDT concentration:
increase of
10 million times
DDT in
fish-eating birds
25 ppm
DDT in
large fish
2 ppm
DDT in
small fish
0.5 ppm
DDT in
zooplankton
0.04 ppm
DDT in water
0.000003 ppm
Figure 38.3B
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• Biodiversity is the variety of organisms that
make up a community
• Components:
– Species variety: total number of different
species in the community
– relative abundance of different species
– Genetic variation within each species
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Biodiversity - current issues
1. Another mass extinction in progress?
2. Wildlife preserves - do they work?
3. Biodiversity hot spots: Should conservation
efforts focus only on areas of high biodiversity?
4. Alien species
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Communities
• Habitat is the environment in which an
organism lives.
• A population's niche is its role in the
community
– How it uses the biotic and abiotic resources of
its habitat
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Community interactions
• There are four main types of relationships
among species within communities
– Competition
– Parasitism, predation
– Commensalism
– Mutualism
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Community interactions
Interspecific competition occurs between
two populations if they both require the same
limited resource
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• The competitive exclusion principle
– Populations of two species cannot coexist in a
community if their niches are nearly identical
High
tide
Chthamalus
Balanus
Ocean
Low
tide
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Figure 36.2
• Competition between species with identical
niches has two possible outcomes
– One population will eventually eliminate the
other
– Natural selection may lead to resource
partitioning (division)
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• Predation is an interaction where one species
eats another
– consumer = predator
– food species = prey
• Parasitism is a form of predation
– Parasite, host
– Not immediately lethal
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Other modes of nutrition for plants:
• Some plants have
evolved parasitic ways
of obtaining food from
other plants
– Dodder taps into the
host’s vascular tissue
Figure 32.12A
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– Mistletoe siphons sap from vascular tissue of its
host plants
Figure 32.12B
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• As predators adapt to prey, natural selection
also shapes the prey's defenses.
• This reciprocal
adaptation =
coevolution
– Example:
Heliconius and
the passionflower
vine
Eggs
Sugar
deposits
Figure 36.3A
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• Carnivorous plants obtain nutrients from
animal tissues
– Sundew and Venus
flytrap use insects as
a source of minerals
– enables them to
thrive in highly
acidic soil
Figure 32.12C, D
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• Prey gain protection against predators through
a variety of defense mechanisms
1. Mechanical defenses, such as the quills of a
porcupine
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2. Chemical defenses
– Animals are often brightly colored to warn
predators
– Example: the poison-arrow frog
Figure 36.3B
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3. Camouflage
Figure 36.3C
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4. Batesian mimicry occurs when a harmless
species mimics a harmful one
– mimicry can involve behavior
– hawkmoth larva puffs up its head to mimic the
head of a snake
Figure 36.3D
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What are effects of predation?
• Eliminates weaker individuals
• keystone predator maintains
diversity by reducing
numbers of the
strongest competitors
in a community
- Ex. sea star is a
keystone predator
Figure 36.4A
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• Predation by killer whales
on sea otters, allowing sea
urchins to overgraze on kelp
– Sea otters represent the
keystone species
Figure 36.4B
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• Commensalism - one partner benefits and
the other is unaffected
• Examples
- Algae that grow on the shells of sea turtles
– Barnacles that attach to whales
– Birds that feed on insects flushed out of the
grass by grazing cattle
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• Mutualism:
both partners benefit
Example:
– Acacia trees and the
ants of the genus
Pseudomyrmex
Figure 36.5B
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• Mutualism
• mycorrhizae
– fungal threads increase
plant's absorption
of nutrients and water
– fungus receives
nutrients from
the plant
Figure 32.11
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Legumes and certain other plants house nitrogenfixing bacteria
• nodules in the
plant roots
contain
bacteria that
convert N2 gas
to soluble
NO3-, NH4+
Shoot
Nodules
Roots
Figure 32.14A
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Nitrogen-fixing
bacteria and
legumes
ATMOSPHERE
N2
Amino
acids
Nitrogen-fixing
bacteria
N2
NH4+
NH4+
(ammonium)
Soil
Ammonifying
bacteria
Organic
material
Nitrifying
bacteria
NO3–
(nitrate)
Root
Figure 32.13
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Disturbance is a prominent feature of most
communities
• Disturbances include events such as storms,
fires, floods, droughts, overgrazing, and human
activities
– damage
biological
communities
– remove
organisms
– alter the
availability of
resources
Figure 36.6
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