1. Community Interactions
Chapter 39:
Community Interactions

2. Biotic component of area
Chapter 39: Community Interactions
Community: All interacting populations of species within a
defined area
Biotic component of area
Functions of Community Interactions:
1) Limit population sizes (via interspecific competition)
2) Enhance population sizes (via interspecific mutualisms)
3) Shape the bodies / behaviors of interacting species:
Coevolution: One species acts as major force driving
the evolution on another species
Toxic
Resistant
More toxic
More resistant

3. Types of Community Interactions (Table 39.1):
Chapter 39: Community Interactions
Types of Community Interactions (Table 39.1):
Interaction Type
Effect on
Organism A
Organism B
Competition
Harm
Harm
Predation
Benefits
Harm
Symbiosis
Parasitism
Benefits
Harm
Commensalism
Benefits
No Effect
Mutualism
Benefits
Benefits

4. Interspecific Competition: Two or more species attempt to use
Chapter 39: Community Interactions
Interspecific Competition: Two or more species attempt to use
the same limited resources (e.g. food)
Intensity of competition equal to how similar requirements
of each species are
Ecological Niche: Resources required by a species
Physical home / habitat
Temperature range necessary for survival
Moisture / shade requirements
Organism's role in ecosystem (e.g. predator / prey)

5. If placed together, one species will outcompete the other
Chapter 39: Community Interactions
Interspecific Competition:
Competitive Exclusion Principle: No two species can inhabit the
exact same niche simultaneously
and continuously
(Dr. G. F. Gause)
If placed together, one species will outcompete the other
(Figure 39.1)

6. (+) Interspecific Competition: requirements will each occupy a smaller
Chapter 39: Community Interactions
Interspecific Competition:
Resource Partitioning: Two interacting species with similar
requirements will each occupy a smaller
niche than if left alone (i.e. they share)
(Dr. R. MacArthur)
(Figure 39.2)
Population
Size & Distribution
Intraspecific
competition
(+)
Interspecific
competition
(+)

7. Types of Community Interactions (Table 39.1):
Chapter 39: Community Interactions
Types of Community Interactions (Table 39.1):
Effect on
Organism A
Effect on
Organism B
Interaction Type
Competition
Harm
Harm
Predation
Benefits
Harm
Symbiosis
Parasitism
Benefits
Harm
Commensalism
Benefits
No Effect
Mutualism
Benefits
No Benefits

8. Predator: Organism that eats other organisms, killing them in
Chapter 39: Community Interactions
Predator: Organism that eats other organisms, killing them in
the process
To survive, predators must feed and prey must avoid being food!
Each exerts intense pressure on the other = Coevolution

9. Evolutionary Results of Predator - Prey Interactions:
Chapter 39: Community Interactions
Evolutionary Results of Predator - Prey Interactions:
1) Counteracting Strategies:
Predator / prey change behavior in attempt to “outwit” other
Example:
Bat vs. Moth

10. Evolutionary Results of Predator - Prey Interactions:
Chapter 39: Community Interactions
Evolutionary Results of Predator - Prey Interactions:
2) Camoflage:
Predator / prey blends in with background (colors / shapes)
Predator Examples
Cheetah
Prey Examples
Sand dab
Angling
fish
Treehopper

11. “Mess With Me At Your Own Risk!”
Chapter 39: Community Interactions
Evolutionary Results of Predator - Prey Interactions:
3) Warning Coloration:
Brightly-colored prey warn predator of danger (e.g. toxic)
“Mess With Me At Your Own Risk!”

12. Evolutionary Results of Predator - Prey Interactions:
Chapter 39: Community Interactions
Evolutionary Results of Predator - Prey Interactions:
4a) Warning Mimicry:
Palpable prey copy coloration of toxic prey
Coral Snake (toxic)
King Snake (non-toxic)
Black on yellow - kill a fellow;
Black on red - never dread

13. Evolutionary Results of Predator - Prey Interactions:
Chapter 39: Community Interactions
Evolutionary Results of Predator - Prey Interactions:
4b) “Confusion” Mimicry:
Prey closely mimics shape / behavior of predator
Jumping Spider
(Predator)
Snowberry Fly
(Prey)

14. Swallowtail Caterpillar
Chapter 39: Community Interactions
Evolutionary Results of Predator - Prey Interactions:
4c) “Startle” Mimicry:
Prey have eye-spots that mimic “larger” animal
Peacock Moth
Swallowtail Caterpillar

15. Evolutionary Results of Predator - Prey Interactions:
Chapter 39: Community Interactions
Evolutionary Results of Predator - Prey Interactions:
4d) Aggressive Mimicry:
Predator baits prey to lure prey in
Angling
fish

16. Evolutionary Results of Predator - Prey Interactions:
Chapter 39: Community Interactions
Evolutionary Results of Predator - Prey Interactions:
5) Chemical Warfare:
Predator / prey use toxic chemicals for attack / defense
Bombardier Beetle
Milkweed
Monarch
Butterflies

17. Types of Community Interactions (Table 39.1):
Chapter 39: Community Interactions
Types of Community Interactions (Table 39.1):
Effect on
Organism A
Effect on
Organism B
Interaction Type
Competition
Harm
Harm
Predation
Benefits
Harm
Symbiosis
Parasitism
Benefits
Harm
Commensalism
Benefits
No Effect
Mutualism
Benefits
Benefits

18. Symbiosis: Close interaction between organisms of different
Chapter 39: Community Interactions
Symbiosis: Close interaction between organisms of different
species for extended periods
Parasatism:
One species benefits;
other species harmed
Commensalism:
One species benefits;
other species unaffected
Mutualism:
One species benefits;
other species benefits

19. Keystone Species Strongly Influence Community Structure:
Chapter 39: Community Interactions
Keystone Species Strongly Influence Community Structure:
Keystone Species: A species that plays a major role in
community structure that is out of proportion
to its abundance in the community
If removed, the community will drastically change
Starfish
Elephant
 starfish =  mussels =  algae
 elephant =  forest =  grassland

20. Communities Change Over Time:
Chapter 39: Community Interactions
Communities Change Over Time:
Succession: Structural change in a community and its non-
living environment over time
Biotic assemblages replace one another in sequence
Pioneers: Hardy invaders to an area
Climax Community: Relatively stable community

21. Major Forms of Succession:
(Figure 39.16)
Chapter 39: Community Interactions
Major Forms of Succession:
Primary Succession: Community colonizes bare rock / clear pool
towards climax
Larger plants
grow in soil
(Pioneers)
Rock
weathers
Lichen / moss
forms soil

22. Primary Succession: Mount St. Helens
Chapter 39: Community Interactions
Primary Succession: Mount St. Helens
May 18th 1980

23. Major Forms of Succession:
(Figure 39.16)
Chapter 39: Community Interactions
Major Forms of Succession:
Secondary Succession: New community develops from previous
disturbed community
towards climax
Perennial plants
shade out weeds
(Pioneers)
Fast-growing
weeds root in soil

24. Secondary Succession: Forest Fires
Chapter 39: Community Interactions
Secondary Succession: Forest Fires

25. Succession Culminates in a Climax Community:
Chapter 39: Community Interactions
Succession Culminates in a Climax Community:
Climax Community: Community that will perpetuate itself
indefinitely if not disturbed
Separate ecological niches allow populations to coexist
Have more species / community interactions than early
succession communities (longer-lived species too)
Climax communities vary from region to region (biomes)
Some ecosystems are maintained at a sub-climax state:

26. Exhibit unchecked population growth (no predators / naive prey)
Chapter 39: Community Interactions
Exotic Species: Species introduced into an area where they
did not evolve
Exhibit unchecked population growth (no predators / naive prey)
Zebra Mussel
Water Hyacinth
Brown Tree Snake
Solution: Biocontrol
Virus
(Rabbits)
Mongoose (Rats) - Oops!