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
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).
GREEN OAKS LAB
Green Oaks Field Station
Two forest habitats were sampled:
Old Growth: relatively undisturbed, but
selectively logged in the 50’s and 60’s
(avoided cutting
White Oaks)
H’ = 2.499
Spoil Banks: completely
clear-cut for strip-mining
purposes in 1940-1941
- however, some non-local trees planted during
60-year recovery period
H’ = 2.191
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”
What Factors Affect Biodiversity?
Some species have major influences on
community composition...
• Foundation species have major positive or
negative influences because of their physical
effects on the environment.
Beaver
Beaver dam
• Dominant species have major (usually negative)
influences because of their high abundance.
e.g. Ponderosa Pine
• 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.
Disturbance
Events that damage communities, remove
organisms from them, and alter resource
availability (storms, floods, fire, etc.)
Intermediate Disturbance Hypothesis:
Moderate levels of disturbance can create
conditions that foster greater species diversity
than low or high levels of disturbance.
Example of Intermediate Disturbance
Succession
The change in species richness and species
composition of a community over time,
usually after a disturbance of some kind.
Primary Succession
Secondary Succession
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
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.
Another factor that influences
biodiversity: LATITUDE
Latitudinal Gradients
Species richness increases
along polar-equatorial
gradients.
Bird Species
WHY?
Evolutionary History
-tropics are older than
temperate zones
Climate
-solar energy, water,
temperature all higher
in tropics
Tree Species
Another factor that influences
biodiversity: AREA
“Species-Area
Relationship”
Species richness increases
with the size of the habitat.