Transcript Population Processes

Community Ecology
Chapter 9
Succession
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Temporal patterns in communities
Replacement of species by others within
particular habitat (colonization and
extinction)
Non-seasonal, continuous, directional
Degradative succession
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Decomposers breaking down organic
matter
Leads to disappearance of everything,
species included
Autotropic succession
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Does not lead to degradation
Habitat continually occupied by living
organisms
Two types of autotropic
succession
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Allogenic succession
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Autogenic succession
Allogenic succession
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Serial replacement of species driven by
changing external geophysical processes
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Examples:
1) silt deposition changing aquatic habitat to
terrestrial habitat
 2) increasing salinity of Great Salt Lake
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Autogenic succession
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Change of species driven by biological
processes changing conditions and/or
resources
Example: organisms living, then dying, on
bare rock
Autogenic succession can occur
under 2 different conditions
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In an area that
previously did not
support any community
Primary succession
Example: terrestrial
habitat devoid of soil
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In an area that previously
supported a community, but
now does not
Secondary succession
Example: terrestrial habitat
where vegetation was
destroyed, but soil remained
Primary succession
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Volcanic eruptions
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Glaciers
Secondary
succession
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Floods
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Fires
Rate of succession
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Primary - slow - may take 1000s of years
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Secondary - faster - fraction of the time to
reach same stage
Autogenic succession begins…
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First community comprised of
rselected species - pioneer species
r-selected species
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Good colonizers
Tolerant of harsh conditions
Reproduce quickly in unpredictable
environs
Example: lichens
Pioneer species
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Carry out life processes and begin to
modify habitat
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Extract resources from bare rock
Break up/fragment rock with roots
Collect wind-blown dust, particles
Waste products accumulate
Die and decompose
Soil development begins
Continuing change
Colonizers joined by other species suited
for modified habitat
 Eventually replace colonizers
 Better competitors in modified habitat
 Less r-selected, more K-selected
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More change
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Communities gradually become dominated
by K-selected species
Good competitors, able to coexist with
others for long periods of time
Stability
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Communities may become stabilized
Reach equilibrium
Little or no change in species composition,
abundance over long periods of time
Climax community
End stage of succession
Will climax stage be reached?
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Rarely is climax stage reached quickly
Slow succession most common, climax stage
almost never achieved
Community usually affected by some major
disturbance (e.g., fire) before climax stage is
reached
Resets succession, forces it to start again from
some earlier stage
Terrestrial succession
Predictability of Succession
Deterministic- process with a fixed outcome
Community restoration
via succession?
Relay Floristics
Relay Floristics
Lake or pond succession
Disturbances - A Force
Structuring Communities
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Non-biotic disturbances
can ‘structure” a
community
Kill off individuals and
open up “gaps” the same
way a predator might
But gaps may result from
removal of many
species/individuals, not
few as in most predators
Disturbances
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Relatively discreet event
in time that causes
abrupt change in
ecosystem, community,
or population structure
Changes resource
availability, substrate
availability, or the
physical environment
Disturbances
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Intensity, size,
frequency
Small disturbances of
low intensity are much
more frequent than
large disturbances of
high intensity
Disturbances
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Fire
Wind
Water
Animals
Earthquakes, volcanoes
Disease
Humans
Gap Colonizers
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Gaps opened by
disturbances are
colonized by nearby,
remaining organisms
Size of gap
determines how it is
filled
Gap Colonizers
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Small gaps may be
taken over by single
individual, or “leaning”
of neighbor
Conditions may be
suitable for many
species, but first-come,
first-served (lottery)
Gap Colonizers
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Conditions present
may only favor one
species
Gap remains until
colonized by that
species
Gap Colonizers
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Large gaps may be
colonized by many
individuals/species,
and proceed through
a successional
process
Secondary?
Succession in a Gap
Succession in a Gap
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Fairly predictable
change in species
Additional
disturbance knocks
community back to an
earlier stage in
succession
Multiple gaps
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If single disturbance
produces multiple
gaps, these proceed
through succession in
phase with one
another
May impact structure
of entire community
Multiple gaps
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If multiple gaps
develop over time
from different
disturbances,
succession occurs out
of phase
Little impact on
overall community
structure
Frequency of Disturbance
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Intermediate disturbance hypothesis species diversity should be highest at
intermediate levels of disturbance
Species
richness
Disturbance frequency
Frequency of Disturbance example
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Intertidal algae communities on boulders (climax
in 2-3 years)
Small boulders - 42% chance of monthly
disturbance
Medium boulders - 9% chance of monthly
disturbance
Large boulders - 0.1% chance of monthly
disturbance
Medium boulders had highest species richness
(lower on small and large boulders)
Disturbance vs. Diversity?
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“No relationship
between disturbance
frequency and
diversity” is the most
common observation
Only 16% of studies
show “expected”
pattern