Lecture 18: Succession Continued
Degradative Succession:
• Dead organic matter (plant matter, animal
matter)
• Short time scale
• Ends when resource is completely
mineralized & metabolized
• e.g. sequence of insects/bacteria to invade a
dead animal…progression of larvae is very
predictable in time
Autogenic Succession
• Results from changes brought about by
organisms themselves
• When organisms change the environment
can harm or benefit other species or
themselves
• Time scale: related to lifespan of organisms
involved
Allogenic Succession
• Succession that results from factors external to
the community
• Can be massive disturbance or seasonal
changes or changes in environmental factors
• Time scale: related to time scale of disturbance
• e.g. phytoplankton communities change
seasonally
(nutrients/temperature/light intensity)
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Autogenic/Allogenic Processes
• Not mutually exclusive
• Different time scales
• Community structure at a given time may be
more a response to one or the other
• e.g. climate change (slow) then autogenic
change more important
• e.g. frequent disturbance, allogenic succession
What affects a species’ position
in a succession?
• Life History Features
• Nature of changes
caused by succession
Life History
Features
Generally:
• Good Colonizers
= Bad Survivors
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Biological Effects
• Facilitation
• Inhibition
• Tolerance
Biggest difference: variation in the way early
colonists are killed off…
Facilitation
• Early stages aid in the development of later
stages
• Contribute to nutrient & water levels of soil
• Modify microenvironment of soil surface
• Marine systems: enhance quality of settling &
establishment sites
• Most important in primary succession
• Early colonists are poor competitors
Inhibition
The presence of 1 species prevents another from
becoming established
Method:
• eating it, reducing resources below acceptable levels,
direct conflict
Climax species deal in inhibition
Can be affected by precedence:
• Bryozoans can beat tunicates & sponges if establish 1st
Later species can only replace early species when they
die (local disturbances – physical/predator)
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Tolerance
• Species are equally capable of invading
• Thus, affected by competitive ability
• Early stages: poor competitors, short life
spans, established quickly
• Late stages: superior competitors, slow
growth
• Competition is key factor
The Climax community
• “End-point”: more nutrients tied up in organic
material, more material into detritus than
consumers (more supportive tissues), soil nutrient
& mineral uptake faster & stored longer
• Traditional view: only 1 climax within a region
• Related to climatic conditions
• Non-compliance (soils, topography, fire, animals)
= interrupted stages in move towards climax
Monoclimax vs. Polyclimax
• Climax communities hard to identify
(esp. because N.Am. still recovering from glaciers)
• Many types of vegetation can be climaxes
• Depends on local conditions
Pattern-climax Theory (Whittaker 1953):
• Composition of community depends on local
environmental conditions
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What determines climax?
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Soil nutrients
Moisture
Slope
Exposure
Fire
Grazing
Etc…
Transient/Cyclic Climaxes
Transient:
• Low persistence
(community regularly destroyed)
• e.g. seasonal ponds
• e.g. dead animals (no climax reached)
Cyclic Climaxes
• Cyclic:
• Usually driven by life history characteristics
of dominant species
• e.g. wind or frost heaving (leads to bare
earth)
• e.g. Earth hummocks
• If cycle persists, it is as much a climax as a
steady-state
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So, what now?
• Climax communities are not stagnant
• Changes in structure as results of death,
birth, growth
• Therefore variations in density, distribution
etc.
Continuum Index
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Curtis & McIntosh (1951)
Forest in Wisconsin
Gradient of moisture conditions
Increasing values:
lead to sugar maple climax
but also represent local climaxes
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Modelling
• Change in community = transition
• Transition probabilities
• Markov process: probability of occurrence
of a particular state is dependent only on the
present state of the system & not the path to
the present state
• e.g. forest: counted number of incipient
saplings
Figure 28.13
Ambrosia sp.
Larrea sp.
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Problems with these models
• Simplistic
Transition probabilities are affected by:
• Initial biotic factors
• Order of species arrival
• Etc.
Does not EXPLAIN just PREDICTS
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