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Communities
biodiversity, issues
keystone species
habitats, niches
Interactions:
competition, predation,
mutualism
co-evolution
Dining In
• Wasps and Pieris caterpillars form an unusual
three-step 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:
– Biodiversity
-- Vegetation
– Response to disturbances
– Trophic structure
(feeding relationships)
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
Wastes and
dead organisms
Tertiary
and
secondary
consumers
Food webs often
more real than
food chains
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
10% conversion
to biomass
at next level
1,000,000 kcal of sunlight
Figure 36.11
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• Consequence: a field of corn can support more
vegetarians than meat-eaters
TROPHIC LEVEL
Secondary
consumers
Primary
consumers
Human
meat-eaters
Human
vegetarians
Cattle
Corn
Corn
Producers
Figure 36.12
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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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• 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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• 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
– Example: gray tree frog
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
Examples:
- Nitrogen-fixing
bacteria and legumes
– Acacia trees and the
ants of the genus
Pseudomyrmex
Figure 36.5B
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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
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings