Transcript Some species have major influences on community composition

COMMUNITY ECOLOGY I:
BIODIVERSITY
Community:
Any assemblage of populations [of plants
and/or animals] in a given area or habitat.
Community Biodiversity:
Number of species, relative abundance of
each species, kinds of species present in
that habitat.
Why is community biodiversity so
important?
A) Communities with high biodiversity are more
productive.
B) Communities with high biodiversity are more stable.
C) Communities with high biodiversity are better able to
withstand and recover from environmental stresses.
D) Higher-diversity communities are more resistant to
invasive species.
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
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).
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).
How can we describe biodiversity qualitatively?
Marine Food Web
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”
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 TopDown model applied?
A) Plants would increase
B) Plants would decrease
C) Raccoons would increase
D) Snakes would decrease
E) Grasshoppers would increase
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
b) Dominant species have major (usually negative)
influences because of their high abundance.
e.g. Ponderosa Pine
c) Keystone species influence ecological
communities more than would be
expected from their abundances.
• Effect is positive (enhances biodiversity)
Keystone predator:
- a species whose predatory behavior has regulating
effects on other species in the community
-maintains higher species richness by altering
competitive relationships
Classic Example: Rocky Intertidal Zone
Sessile invertebrates
Acorn barnacle
Gooseneck barnacle
Mussel
Mobile invertebrates
Whelk
Starfish
Chiton
• 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
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
Keystone
(Expt’l)
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.
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.
What Factors Affect Biodiversity?
2) Disturbance
Events that damage communities, remove organisms
from them, and alter resource availability
(storms, floods, fire, glaciers, volcanoes)
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
i) Primary Succession
- succession on newly exposed, soil-free areas
e.g. glacial retreat ( community gradients)
Primary Succession at Glacier Bay
e.g. volcanic eruption
1981
1999
Primary succession around Mount St. Helens
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.
What Factors Affect
Biodiversity?
3) “Species-Area
Relationship”
Species richness increases
with the size of the habitat.