1. COMMUNITY ECOLOGY I: BIODIVERSITY
Any assemblage of populations [of plants
and/or animals] in a given area or habitat.

2. Community Biodiversity:
Number of species, relative abundance of
each species, kinds of species present in that habitat.

3. Why is community biodiversity so important?
Communities with high biodiversity are more productive.
Communities with high biodiversity are more stable.
Communities with high biodiversity are better able to withstand and recover from environmental stresses.
Higher-diversity communities are more resistant to invasive species.

4. How can we describe biodiversity quantitatively?
Species Richness: number of different species
present in the community
Species Evenness: relative abundance of the different species present
Species Diversity: number and relative abundance
of each species

5. Which Forest is More Diverse?
Each forest has the same
four tree species
(same species richness),
but they differ in
species evenness
(relative abundance of
each species).

6. How can we describe biodiversity quantitatively?
Shannon-Weiner Diversity Index
Ranges from 0 (only one species present) to infinity
(though usually less than 3 in temperate habitats).

7. How can we describe biodiversity qualitatively?

8. Marine Food Web

9. Bottom-Up and Top-Down Controls
V  H: increasing V increases H, but not vice versa (Bottom-Up)
V  H: increasing H decreases V, but not vice versa (Top-Down)
V  H  P: “Trophic Cascade”

10. Consider a grassland with five trophic levels: plants, grasshoppers, snakes, raccoons, and bobcats. If you released additional bobcats into the grassland, how would biomass change if the Top-Down model applied?
Plants would increase
Plants would decrease
Raccoons would increase
Snakes would decrease
Grasshoppers would increase

11. What Factors Affect Biodiversity?
1) Certain types of species have major influences on community composition...
a) Foundation species have major positive or negative influences because of their physical effects on the environment.
Beaver
Beaver dam

12. b) Dominant species have major (usually negative) influences because of their high abundance.
e.g. Ponderosa Pine

13. Effect is positive (enhances biodiversity)
c) Keystone species influence ecological communities more than would be expected from their abundances.
Effect is positive (enhances biodiversity)

15. Keystone predator: - a species whose predatory behavior has regulating
effects on other species in the community
maintains higher species richness by altering
competitive relationships

16. Classic Example: Rocky Intertidal Zone

17. Sessile invertebrates
Acorn barnacle
Gooseneck barnacle
Mussel

18. Mobile invertebrates
Whelk
Starfish
Chiton

19. Intense competition for space among sessile
invertebrates; one is dominant competitor
One mobile invertebrate species keeps
dominant competitor in check and maintains
biodiversity: Keystone predator

20. Expt: Removed and excluded different mobile
invertebrate species to see the effect on biodiversity
Keystone - # species dropped from 17 to 2.5 in 3 years
Keystone
(Expt’l)
Keystone

21. Recent Example: Sea Otters
Like the rocky intertidal zone,
kelp forests are communities
of extremely high biodiversity.
Sea otters feed on sea urchins,
which, in turn, feed on kelp.

22. However, orcas have
recently turned to
feeding on sea otters
along west coast.
As a result, sea urchins
have increased and kelp
forests have declined.

23. What Factors Affect Biodiversity?
2) Disturbance
Events that damage communities, remove organisms
from them, and alter resource availability
(storms, floods, fire, glaciers, volcanoes)

24. Succession i) Primary Succession ii) Secondary Succession
The change in species richness and species
composition of a community over time,
usually after a disturbance of some kind.
i) Primary Succession
ii) Secondary Succession

25. i) Primary Succession - succession on newly exposed, soil-free areas
e.g. glacial retreat ( community gradients)

26. Primary Succession at Glacier Bay

27. e.g. volcanic eruption
1981
1999
Primary succession around Mount St. Helens

28. ii) Secondary Succession
succession in disturbed areas, where at least soil remains
(usually due to clear-cutting or fire)
e.g. old field succession
Old growth
Spoil banks
Biodiversity can
increase with forest age.

29. 3) “Species-Area Relationship” What Factors Affect Biodiversity?
Species richness increases
with the size of the habitat.