Community Ecology

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7

Changes in soil nitrogen content during succession at Glacier Bay

Successional stage

Pioneer

Dryas

Alder

Spruce

Soil nitrogen (g/m2)

0

10

20

30

40

50

60

Slide 44

Human Disturbance

Humans have the greatest impact on biological communities worldwide. Human disturbance to communities usually reduces species diversity.

Humans also prevent some naturally occurring disturbances, which can be important to community structure.

Slide 45

Disturbance of the ocean floor by trawling

Slide 46

Biogeographic factors affect community biodiversity

Latitude and area are two key factors that affect a community’s species diversity.

Species richness generally declines along an equatorial-polar gradient and is especially great in the tropics.

Two key factors in equatorial-polar gradients of species richness are probably evolutionary history and climate.

The greater age of tropical environments may account for the greater species richness.

Slide 47

Climate is likely the primary cause of the latitudinal gradient in biodiversity.

Two main climatic factors correlated with biodiversity are solar energy and water availability. They can be considered together by measuring a community’s rate of evapotranspiration.

Evapotranspiration is evaporation of water from soil plus transpiration of water from plants.

Slide 48

Area Effects

The species-area curve quantifies the idea that, all other factors being equal, a larger geographic area has more species.

A species-area curve of North American breeding birds supports this idea.

Slide 49

Island Equilibrium Model

Species richness on islands depends on island size, distance from the mainland, immigration, and extinction.

The equilibrium model of island biogeography maintains that species richness on an ecological island levels off at a dynamic equilibrium point.

Studies of species richness on the Galápagos Islands support the prediction that species richness increases with island size.

Slide 50

The equilibrium model of island biogeography

Number of species on island

Equilibrium number

(a) Immigration and extinction rates

Rate of immigration or extinction

Extinction

Immigration

Rate of immigration or extinction