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