Disturbance and Succession

Disturbance and Succession• Equilibrium theory: ecosystems are stable

environments in which the biotic interactions among species determine the structure of the communities present.

• Ecological succession: transition between biotic communities to some theoretical “climax” state. – Primary- no previous biotic community– Secondary- previously occupied by a community– Aquatic- transition from pond or lake to terrestrial

community

• Disturbances are non-equilibrium events.

Primary Succession• Mosses invade an area

and provide a place for soil to accumulate.

• Larger plants germinate in the new soil layer resulting in additional soil formation.

• Eventually shrubs and trees will invade the area.

Secondary Succession

Aquatic Succession

Disturbance

• Reduces populations.• Loss or modification of habitat.• Creates opportunities for other species.• Drives increases in biodiversity by initiating

succession of communities.• Greater biodiversity increases an

ecosystem resilience to disturbance.

The Forth Principle of Ecosystem Sustainability

• Ecosystems show resilience during a disturbance; they bounce back from adversity.

Resilience Mechanisms:Forest Fire Example

• Nutrient release to soil.• Regrowth by remnant roots and seeds.• Invasions from neighboring undisturbed

ecosystems (biodiversity benefits).• Rapid restoration of energy flow and

nutrient cycling.

Fifth Principle of Ecosystem Sustainability

• Ecosystems depend on biodiversity.– Stabilizes populations– Increases resilience

• Humans benefit.– Unknown resources– Esthetic and morals– Ecosystem health

Assignment #5Witness Secondary Succession

• Secondary succession can be easily demonstrated if you begin this week and check the site periodically over the next month.

• Find a location where plants are currently growing, either with a low plant biodiversity (monoculture) or high plant biodiversity (naturalized field).

• Stake out a small area (one foot square) and count the number of different plant species in your plot and note which are dominant, rare or intermediate in number.

• Next scrape all the plants from the area. The bare soil should be surrounded by plants that have not been removed. Every few days the area should be watered if needed.

• Periodically (at least once per week) record any changes to your area. Within a short period of time plants should return to the bare area, demonstrating the first sequence in secondary succession.

• How many species are growing? Compare your results with those of a classmate with either a lower or higher initial plant biodiversity? Write a paragraph on the comparison and include what you think explains differences.

Human Impacts• Increased population growth:

– Increasing our biotic potential.– Decreased our environmental resistance.

• Contributed to the degradation of ecosystems:– Introduce species– Eliminate natural predators– Alter abiotic factors– Misunderstand the role of fire– Reduce biodiversity

• Have we exceeded the carrying capacity?

Implications For Humans• Protecting and managing the natural environment to

maintain the goods and services vital to human economy and survival.

• Establishing a balance between our own species and the rest of the biosphere.

• Adaptive Ecosystem Management: an approach that accepts uncertainty of our knowledge base and flexible to change as new information is gathered.

Exam #1 Results

3

13 15

26

11

0

5

10

15

20

25

30

Num

bers

of S

tude

nts

A rangeB rangeC rangeDF

Letter Grade Range

Average = 76.5% = C+/B-

F ≤ 52%; D 53%-64%; C range 65%-76%; B range 77%-88%; A range 89%-100%