COMMUNITY ECOLOGY I:

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COMMUNITY ECOLOGY I BIODIVERSITY
Community Any assemblage of populations of
plants and/or animals in a given area or
habitat.
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Community Biodiversity Number of species,
relative abundance of each species, kinds of
species present
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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
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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).
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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).
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GREEN OAKS LAB
Green Oaks Field Station
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Two forest habitats were sampled
Old Growth relatively undisturbed,
but selectively logged in the 50s and
60s (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

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How can we describe biodiversity qualitatively?
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Marine Food Web
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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

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What Factors Affect Biodiversity?
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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 dam
Beaver
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• Dominant species have major (usually negative)
  influences because of their high abundance.

e.g. Ponderosa Pine
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• Keystone species influence ecological
  communities more than would be expected from
  their abundances.
• Effect is positive (enhances biodiversity)

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

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Classic Example Rocky Intertidal Zone
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Sessile invertebrates
Acorn barnacle
Gooseneck barnacle
Mussel
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Starfish
Mobile invertebrates
Whelk
Chiton
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• 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

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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
(Exptl)
Keystone
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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.
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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.
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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.
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Example of Intermediate Disturbance
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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
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Primary Succession
- succession on newly exposed, soil-free areas
e.g. glacial retreat ( community gradients)
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Primary Succession at Glacier Bay
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e.g. volcanic eruption
1981
1999
Primary succession around Mount St. Helens
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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.
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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
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Another factor that influences biodiversity AREA
Species-Area Relationship
Species richness increases with the size of the
habitat.