Changes in a community

Ecological succession

Unit 3 Topic 6

Ecological succession:• Natural, gradual changes in the types of

species that live in an area; can be primary or secondary

• The gradual replacement of one plant community by another through natural processes over time

Primary Succession

• Begins in a place without any soil

– Cooled lava flows

– Bare rock exposed from: massive flooding & mudslides, glacial retreats

Primary Succession

• Starts with the arrival of living things such as lichens that do not need soil to survive (Called PIONEER SPECIES)

• Soil starts to form as lichens, microbes & the forces of weather/erosion help break down rocks into smaller pieces

• Lichens die & decompose, adding small amounts of organic matter (more soil)

• Simple plants like mosses and ferns can grow in the new soil

• The simple plants die, adding more organic material

Primary Succession

• The soil layer thickens, and grasses, wildflowers, and other herbaceous plants begin to take over

• Now insects, birds and small mammals can inhabit the field.• These plants die, and they add more nutrients to the soil

Primary Succession

• Shrubs and tree seedlings can now survive• The habitat can support a wider diversity of small animal species.

Primary Succession

• Trees with high disperal abilities can colonize.• Those that are fast growing and early maturing will dominate.• Now larger mammals and birds can be supported by the young

forest.

Primary Succession

• Trees that are shade tolerant start to dominate the community.• The forest transitions into a “climax” forest, in which, large, slow

growing and long-living species dominate.• Mature forests sustain a diverse community of animal fauna, often

different than that of the field and shrublands communities.

Climax Community

• A stable group of plants and animals that is the end result of the succession process

• Does not always mean big trees– Grasses in prairies– Cacti in deserts

Summary of changes in biodiversity over time

Generally, biodiversity increases over time.

Succession of species

lichens & mosses grasses

treesbushes & small trees

pioneer species

climax forest

compete well in high sunlight

shade tolerant speciesstable community

What causes succession?

• Tolerance– early species are “weedy”– tolerant of harsh conditions

• Facilitation & Inhibition– early species facilitate habitat changes

• build soil• increase nutrient and water retention of soil over time• change soil pH • change light levels

– allows other species to out-compete

Changes in abiotic factors over time

Abiotic factor

How it changes

Why it changes Benefits to succession

Soil quantity

IncreasesMore detritus increases humus

Provides more area for plants to anchor their roots in

Soil quality

Improves Gains organic matter

Provides more nutrients for plant growth

Soil water retention

Increases Increased organic matter retains more water; shading decreases evaporation

More water is available for plants

Changes in abiotic factors over time

Abiotic factor

How it changes

Why it changes

Benefits to/ impacts on succession

DecreasesLight availability

Shading increases over time

Favors shade-tolerant species and inhibits shade-intolerants

Temperature

Decreases

Shading increases over time

Favors heat-intolerant species and inhibits plants that need higher temps.

Nitrogen or phosphorus Increases

Increased N fixation, Increased decomposition

Favors plants with higher N or P requirements

Secondary succession• Occurs after a disturbance that leaves the soil intact

– Fire– Massive storm– Forest clear-cutting– Death of a large mature tree

• Also occurs in abandoned agricultural fields• Soil is intact• Often there is a seed bank• Starts with annual plants (wildflowers, herbs)• Lots of organic matter for nutrient recycling• Can help to maintain regional biodiversity

Disturbances as natural cycle

• Disturbances are often necessary for community development & survival

- release nutrients- increases biodiversity

- release nutrients- increases biodiversity

- increases habitats- rejuvenates community

- increases habitats- rejuvenates community

Fire climax species

Jack PineJack Pine

adaptations to survive and reproduce in areas that experience frequent fires

adaptations to survive and reproduce in areas that experience frequent fires

Scenarios

• A massive landslide occurs leaving completely bare bedrock behind.

• What is the immediate effect on the ecosystem?

• What are the ongoing long-term effects on succession?

• A forest is completely uprooted by a tornado.

• What is the immediate effect on the ecosystem?

• What are the ongoing long-term effects on succession?

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