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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 ofeach 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 samefour tree species(same species richness),but they differ inspecies evenness(relative abundance ofeach 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 Field Station
GREEN OAKS LAB
Two forest habitats were sampled:
Spoil Banks: completelyclear-cut for strip-mining purposes in 1940-1941- however, some non-local trees planted during60-year recovery period H’ = 2.191
Old Growth: relatively undisturbed, butselectively logged in the 50’s and 60’s(avoided cutting White Oaks)
H’ = 2.499
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
Mussel
Acorn barnacle Gooseneck barnacle
Sessile invertebrates
Starfish Chiton
Whelk
Mobile invertebrates
• Intense competition for space among sessile invertebrates; one is dominant competitor
• One mobile invertebrate species keeps dominant competitor in check and maintainsbiodiversity: 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
(Expt’l)
Keystone
Keystone
Recent Example: Sea Otters
Like the rocky intertidal zone, kelp forests are communitiesof extremely high biodiversity.
Sea otters feed on sea urchins, which, in turn, feed on kelp.
However, orcas haverecently turned tofeeding on sea ottersalong west coast.
As a result, sea urchinshave increased and kelpforests have declined.
Disturbance
Events that damage communities, removeorganisms from them, and alter resourceavailability (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
Primary Succession
Secondary Succession
The change in species richness and species composition of a community over time,usually after a disturbance of some kind.
Primary Succession- succession on newly exposed, soil-free areas
e.g. glacial retreat ( community gradients)
Primary Succession at Glacier Bay
Primary succession around Mount St. Helens
1981
1999
e.g. volcanic eruption
Biodiversity canincrease with forest age.
Secondary Succession
-succession in disturbed areas, where at least soil remains (usually due to clear-cutting or fire)
e.g. old field succession
Spoil banks
Old growth
Latitudinal GradientsSpecies richness increasesalong polar-equatorial gradients.
WHY?
Evolutionary History-tropics are older thantemperate zones
Climate-solar energy, water,temperature all higherin tropics
Bird Species
Tree Species
Another factor that influencesbiodiversity: LATITUDE
Species richness increases with the size of the habitat.
“Species-Area Relationship”
Another factor that influencesbiodiversity: AREA