35.5Disturbances are common in communities

Objectives

• Describe how disturbances can have positive and negative effects.

• Compare primary and secondary succession.

• Explain how human activities can affect species diversity.

Vocabulary

• ecological succession

• primary succession

• secondary succession

• introduced species

Disturbances to Communities

• continually changing– fires, floods, droughts, etc.

• not always negative• sometimes human-caused

Ecological Succession

• The series of predictable changes that occurs in a community over time is called succession.

• To remember: Think of succession as it relates to monarchies (governments based on kings and queens).

Primary Succession

• primary succession: the series of changes that occur in an area where no ecosystem previously existed

• Examples: new island, area of rock uncovered after ice melts

Pioneer Species

• pioneer species: the first species to populate an area

• To remember: The Wright brothers were pioneers of aviation.

Pioneer Species

• These are often lichen and mosses carried by wind and water.

• little needed to survive• They break rock as they grow.• In death, they provide nutrients to develop

soil.

So what happens next?

• Plant seeds land in this soil.• Over time, soil grows richer.• Eventually the community becomes stable

unless disrupted.• Stability can take centuries!

Secondary Succession

• secondary succession: the series of changes that occur after a disturbance in an existing ecosystem

• causes: fires, hurricanes, tornadoes, farming, logging, mining

• faster than primary succession

Human Activities and Species Diversity

• humans have greatest impact on communities

• 60% of Earth’s land used by humans• negative effects on diversity

Clearing the Land

• forests cut down• plains used for farming• diverse forest or grassland used to grow

single crop

Introduced Species

• organisms that humans move from native location to a new area

• intentional and accidental• can disrupt ecosystem for native organisms