1. Community Interactions
Chapter 7
Community Interactions

2. 7-1 COMMUNITY STRUCTURE AND SPECIES DIVERSITY

3. Community
Community: populations of all species living and interacting in an area at a particular time

4. Four Characteristics of Community Structure
Physical Appearance: size and distribution of its population and species
Species Diversity/Richness: number of different species
Species Abundance: number of individuals of each species
Niche Structure: number of niches, how they compare, and how they interact

5. Differing Physical Appearances
Patch Effects: most large communities usually consist of a mosaic of vegetation “patches”
Edge Effects: differences in physical appearance at boundaries between ecosystems

6. Species Diversity Species Rich Environments
Tropical Rain Forests
Coral Reefs
Deep Sea
Large Tropical Lakes
Tend to have high species diversity but low species abundance
Factors that Affect Diversity
Latitude (terrestrial) – distance from equator
Depth (aquatic)
Pollution (aquatic)

7. Species Abundance Determined by: Rate at which new species immigrate
Rate at which species become extinct

8. Ecological Niche vs. Habitat
Niche: role an organism plays in an ecosystem
niche is like an "occupation“ – a species’ interactions with habitat and other organisms (their role in food web)
Habitat: actual location where an organism lives
habitat is like an "address"

9. 7-2 GENERAL TYPES OF SPECIES

10. GENERAL TYPES OF SPECIES
Generalist Species
Specialist Species
Native Species
Nonnative Species
Indicator Species
Keystone Species

11. Generalist vs. Specialist
Generalist Species have broad niches, can live many places, use a variety of resources
e.g., dandelions, cockroaches, coyotes, humans
Specialist Species have narrow niches, live only in specific places
e.g., spotted owls, giant pandas
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12. Native vs. Nonnative Species
Native Species: species that normally live & thrive in a particular ecosystem
Nonnative Species:
also called exotic, invasive, or alien species
originate in other ecosystem
deliberate or accidental introduction by humans causes problems
may thrive and crowd out native species

13. The Case of the Killer Bees
1957 Brazil imported wild African bees to help increase honey production
Displaced domestic honeybees
Actually reduced honey production
Moved north in Central America
Established populations in Texas, Arizona, New Mexico, Puerto Rico, and California

14. Snakehead Fish

15. CANE TOADS!

16. The Simpsons!
Bart vs. Australia

17. Indicator Species
Indicator Species: species that serve as early warnings that a community or ecosystem is being damaged
Birds are good indicator species
Found everywhere
Respond to environmental change quickly
northern spotted owls are indicators of healthy old–growth forest
Fish are good indicator species in aquatic ecosystems (i.e. trout)

18. “The loss of a keystone species is like a drill accidentally striking a power line. It causes lights to go out all over.”
– E.O. Wilson
Keystone Species
Keystone Species: species that play a critical role in an ecosystem
Ex: sea otters are keystone species because they prevent sea urchins from depleting kelp beds
Ex: flying foxes are keystone species because they pollinate tropical trees and disperse seeds, such as durian fruit trees

19. 7-3 SPECIES INTERACTIONS: COMPETITION AND PREDATION

20. Species Interactions
The effects of one species on another may be negative, positive, or neutral
Five kinds of interactions:
Interspecific competition
Predation
Parasitism
Mutualism
Commensalism

21. Intra- vs. Inter- specific competition
Intraspecific Competition – members of the SAME species compete for resources
Interspecific Competition – members from 2 or more DIFFERENT species compete for resources
When two or more species use the same limited resource (food, space, etc.) they may adversely affect each other
niche overlap
Ex: fire ants & native ants in North America

22. Relative population density Each species grown alone
No Competition
High
Low
Relative population density 2 4 6 8 10 12 14 16 18
Days
Each species grown alone
Paramecium
aurelia
Paramecium
caudatum

23. Interspecific Competition
High
Paramecium
aurelia
Relative population density
Paramecium
caudatum
Low 2 4 6 8 10 12 14 16 18
Days
Both species grown together

24. Resource Partitioning
Species with similar resource requirements can coexist because they use limited resources:
at different times
in different ways
in different places
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25. Resource Partitioning
Where are the 2 species competing?
Why is there no competition here?

26. Resource Partitioning
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27. Resource Partitioning Example
Five species of insect–eating warblers coexist in spruce forests of Maine:
feed in different portions of trees
consume somewhat different insects
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28. Predation
Members of one species (predator) feed on another species (prey);
+ / –
Ex: lion feeding on zebra
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29. Predator-Prey Relationship
What do predators do to increase their chances of getting a meal?
What do prey do to avoid being eaten?

30. Predator-Prey Relationship
Predators get better at catching prey
Prey get better at avoiding capture
Similar to an “arms race”
Ex: During the Cold War, the US and the USSR tried to intimidate the other with bigger and better weapons

31. Predators Pursuit Faster (cheetahs) Better eyesight (eagles)
Hunting in packs (wolves)
Ambush
Camouflage (praying mantis)
Mimicry (alligator snapping turtles)

32. Prey Camouflage Ex: walking stick Warning colors Ex: poison dart frog
Chemical warfare
Ex: skunk
Mimicry
Ex: king snake
Behavior
Ex: blowfish

33. 7-4 SPECIES INTERACTIONS: PARASITISM, MUTUALISM, AND COMMENSALISM

34. Parasitism
One organism (parasite) lives on part of another organism (host)
+ / –
Ex: flea living on a dog
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35. Mutualism Two species interact in a way that benefits both + / +
Ex: lichens (algae & fungi)
Ex: clownfish & anemones
Ex: ants & acacias
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36. Commensalism
One organism benefits from another, but neither helps nor harms the other organism
+ / 0
Ex: epiphyte growing on a tree
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37. Species Interactions - +
The effects of one species on another may be negative, positive, or neutral
five kinds of interactions:
POPULATION A
POPULATION B
COMPETITION -
PREDATION +
PARASITISM
COMMENSALISM
MUTUALISM

38. 7-5 ECOLOGICAL SUCCESSION: COMMUNITIES IN TRANSITION

39. Ecological Succession
Succession: gradual & fairly predictable change in species composition over time
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40. Primary Succession
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41. Primary Succession
Type of succession that occurs where there was no ecosystem before
Occurs on rocks, cliffs, and sand dunes
Pioneer species: the first organism to colonize any newly available area and begin the process of ecological succession

42. Primary Succession
Primary Succession: gradual establishment of biotic communities in an area where no life existed before
Ex: succession on newly formed islands & after the retreat of a glacier
Early Communities: lichens & mosses colonize bare rock
Mid Communities: small herbs & shrubs colonize
Late Communities: tree species colonize

43. Lichens
Parking Lot
Mosses

44. Secondary Succession
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45. Secondary Succession
Secondary Succession: gradual reestablishment of biotic communities in an area where a biotic community was previously present
Ex: "old field succession"

46. Secondary Succession: Mount St. Helens
Erupted in 1980
44,460 acres were burned and flattened
After the eruption, plants began to colonize the debris

52. Disturbance
Disturbance: a distinct event that disrupts an ecosystem or community
disturbance initiates secondary succession
Natural disturbances: fires, hurricanes, tornadoes, droughts, & floods
Human-caused disturbances: deforestation, overgrazing, plowing

53. Intermediate Disturbance Hypothesis: moderate disturbances in communities promote greater species diversity than small or major disturbances

54. Climax Community
Climax community: the final and stable community in an ecosystem
Will continue to change in small ways, but will remain primarily the same over time (unless disturbed)

55. The Circle of Life in Secondary Succession

56. 7-6 ECOLOGICAL STABILITY AND SUSTAINABILITY
Stability - complex networks of positive and negative feedbacks loops

57. Ecosystem Stability
Inertia/Persistence: the ability of an ecosystem to resist being disturbed
Constancy: the ability of a living sysmte such as a population to keep its numbers within the limits imposed by available resources
Resilience: the ability of an ecosystem to “bounce back” after it has been disturbed

58. Precautionary Principle
We should try to prevent potential harm to an ecosystem even though we don’t understand all of the cause-and-effects
“better safe than sorry”