Transcript Disturbance and Succession - Penn State York Home Page

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 #5
Witness 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
Numbers of Students
30
Average = 76.5% = C+/B-
26
25
20
13
15
15
11
10
3
5
0
F
D
C range
B range
A range
Letter Grade Range
F ≤ 52%; D 53%-64%; C range 65%-76%; B range 77%-88%; A range 89%-100%