Succession and Stability
Seals
Chapter 20
• Reduced snowfall in ice field sources
• Warmer temp. during winter
• Reduction in cloud cover and rain in
summer
Outline
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Introduction
Primary Succession
Secondary Succession
Disturbance
– Ecosystem Recovery
• Mechanisms of Succession
• Community and Ecosystem Stability
Introduction
• Succession: Gradual change in plant and
animal communities in an area following
disturbance.
– Primary succession on newly exposed
geological substrates.
– Secondary succession following disturbance
that does not destroy soil.
• Climax Community: Late successional
community that remains stable until
disrupted by disturbance.
Primary Succession at Glacier
Bay
• Ecologist studied changes in plant
diversity during succession.
– Total number of plant species increased with
plot age.
– Species richness increased rapidly in early
years of succession and more slowly during
later stages.
• Not all groups increased in density throughout
succession.
Margerie Glacier
Primary Succession at Glacier
Bay
Secondary Succession in
Temperate Forests
• Oosting found number of woody plant species
increased during secondary succession at
Piedmont Plateau.
– Johnston and Odum found increase in bird diversity
across successional sequence closely paralleled
increase in woody plant diversity observed by
Oosting.
• Secondary succession occurs where a plant
community has been disrupted. A common
example is when abandoned farmland is allowed
to revert to the wild. In deserted farm fields, the
pioneer plants are often grasses, shrubs, weeds,
and tree saplings. Eventually, taller trees move
in and alter the environment by creating moist
and shaded conditions. Ultimately, a relatively
stable climax community will be attained.
Buell- Small Succession Study
Succession in Stream
Communities
• Fisher studied rapid succession in
Sycamore Creek, AZ.
– Evaporation nearly equals precipitation - flows
generally low and intermittent.
• Subject to flash floods.
– Observed rapid changes in diversity and
composition of algae and invertebrates.
• Invertebrates found refuge because many adults in
aerial stage.
– Re-colonized after flooding.
Ecosystem Changes During
Succession
• Chapin documented substantial changes
in ecosystem structure during succession
at Glacier Bay.
– Total soil depth and depth of all major soil
horizons show significant increase from
pioneer community.
• In addition, organic content, moisture, and N
concentrations all increased.
– Physical and biological systems are inseparable.
Ecosystem Changes During
Succession
Four Million Years of Ecosystem
Change
• Chronosequences (seq. of ages
represented at study sites) such as that
found at Glacier Bay are limited.
– Hawaiian Islands have formed over hot spots
on the Pacific tectonic plate, forming an island
chain varying greatly in age.
• Hedin et.al. found differing patterns of nutrient
distribution across the chronosequence.
Recovery of Nutrient Retention
Following Disturbance
• Bormann and Likens found felling trees in
Hubbard Brook substantially increased
nutrient losses.
– Herbicide used to suppress regrowth.
• When application stopped, succession proceeded,
nutrient losses decreased,and primary production
increased.
Mechanisms of Succession
– Facilitation
– Tolerance
– Inhibition
Facilitation
• Proposes many species may attempt to
colonize newly available space.
– Only certain species will establish.
• Colonizers “Pioneer Species” modify environment
so it becomes less suitable for themselves and
more suitable for species of later successional
stages.
Tolerance
• Initial stages of colonization are not limited
to pioneer species.
– Early successional species do not facilitate
later successional species.
Inhibition
• Early occupants of an area modify the
environment in a way that makes it less
suitable for both early and late
successional species.
– Early arrivals inhibit colonization by later
arrivals.
– Assures late successional species dominate
an area because they live a long time and
resist damage by physical and biological
factors.
Mechanisms of Succession
Successional Mechanisms in
Rocky Intertidal Zone
• Scientist investigated
mechanisms behind
succession of algae and
barnacles in intertidal
boulder fields.
– If the inhibition model is in
effect, early successional
species should be more
vulnerable to mortality.
• Results showed early
successional species had
lowest survivorship and were
more vulnerable to herbivores.
Community and Ecosystem
Stability
• Stability: Absence of change.
• Resistance: Ability to maintain structure
and function in face of potential
disturbance.
• Resilience: Ability to recover from
disturbance.
Park Grass Experiment
• Hertfordshire, England
– Studied effects of fertilizer treatments.
• Continued for 150 years.
• Scientist investigated ecosystem stability.
– Used community composition variability as
measure of stability.
– Represented composition as proportion of
community consisting of each plant form.
Park Grass Experiment
• scientist showed that although community
stability is present, populations of
individual species can change
substantially.
– Stability depends on resolution an area is
investigated at.
Disclimax
• Disclimax or "disturbance climax"
describes a community that is held at an
earlier successional stage by repeated but
unpredictable disturbances that prevent
succession from reaching the climax
community that might be expected for the
climate of the area.
• The original prairies of Illinois are examples of
disclimax communities.
• The early successional grass and perennial
plants are fire tolerant because of their
underground roots and stems.
• Repeated fires destroy shrubs, young trees, and
other plants that would change the environment
and result in further successional changes that
would eventually result in the establishment of a
deciduous forest.
• Agricultural practices are essentially an
artificial form of maintaining disclimax.
• Crops like corn and soybeans as well as
the common weeds found in agricultural
fields have the characteristics of pioneer
species and require repeated soil
disturbance.