BIOL 4120: Principles of Ecology
Lecture 17: Community
Ecology
Dafeng Hui
Room: Harned Hall 320
Phone: 963-5777
Email: dhui@tnstate.edu
Outline (Chapter 18)
Community dynamics
18.1 Community structure changes through time
18.2 Primary succession occurs on newly exposed
substrates
18.3 Secondary succession occurs after disturbances
18.4 Study of succession has a rich history
18.5 Succession is associated with autogenic changes
18.6 Species diversity changes during succession
18.7 Succession involves heterotrophic species
18.8 Systematic changes occur as a result of allogenic
changes
18.9 Concept of community revisited
18.1 Community structure changes
through time
Successional
changes over 30
years in a western
Pennsylvania field.
Cropland or
Grazed grassland
 grasses,
goldenrod, weedy
herbaceous plants
 shrubs
(blackberry,
hawthorn) fire
cherry, pine,
aspen  forest of
maple, oak, cherry
or pine.
Succession
Definition: The process of gradual and seemingly
directional change in the structural of the community
through time fro field to forest
Temporal change in community structure
Sere (from the word series): sequence of communities
from grass to shrub to forest historically
Seral stage: a point of continuum of vegetation
through time
can be short or long (1 or 2 yrs to several decades)
Succession happens in both terrestrial and aquatic
environments
William Sousa
Process of succession in a
rocky intertidal algal
community in southern
California
Use concrete blocks for
algae to colonize
Panel b shows succession
Early successional species
(pioneer species):
High growth rate, small
size, high degree of
dispersal, high rates of
population growth
Late successional species:
Species dominance change along time
Low rate of dispersal,
slower growth rate, larger
and live longer.
Hubbard Brook Experimental
Forest, New Hampshire
Process of succession after
forest clearing
Prior to clearing, beech and
sugar maple seedling dominate
understory
Following clearing, pin cherry,
yellow birch etc.; will be
replaced by of beech and maple
later
Primary succession (example
before) and
secondary succession (this
example)
Species dominance change along time
18.2 Primary succession occurs on
newly exposed substrates
Primary succession begins on sites that never have supported a
community, such as rock outcrops and cliffs, sand dunes, and
newly exposed glacial till.
Primary
succession
on a coastal
sand dune
colonized
by beach
grass
Later on,
shrub, then
trees (pines
and oak)
Glacier Bay fjord complex in southeastern Alaska. Ice retreats,
primary succession occurs
18.2 Secondary succession occurs
after disturbances
Terrestrial environment:
Old field succession in the Piedmont region of North Carolina by
Dwight Billings in the late 1930s
Abandoned farmland
Decline in pine and
increase in hardwood
(oak and hickory)
18.3 Rich history of succession
study
Friderick Clements (1916, 1936): Monoclimax hypothesis
view community as a highly integrated superorganism,
the process of succession represents gradual and progressive
development of community to ultimate or climax stage (similar
as development of an individual organism)
F. Egler (1954):Initial floristic composition
succession at any site depends on which species gets
there first. No species is competitively superior to another.
Once the original dies, the site becomes available to others.
Joesph Connell (1977): three models (facilitation model,
inhibittion model, and tolerance model)
Overall: adaptations and life history traits of individual species
influence species interactions and ultimately species distribution
and abundance under changing environmental condisiotns.
18.4 Succession is associated with autogenic
changes in environmental conditions
Environmental changes can be grouped into two classes:
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Autogenic (direct result of organisms within community)
• Created by community itself
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Light in a forest
Allogenic (a feature of physical environment)
• Created by physical environment
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Elevation on a mountain
Stress in a salt marsh
Succession: changes in community structure through time;
specifically, changes in species dominance)
When plant succession, it creates autogenic environmental change
in a place
Both primary and secondary succession, colonization alters
environmental conditions
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When plant succession, it creates autogenic environmental change
in a place. For example, light environment (vertical distribution)
Light availability decline from canopy to ground levels
Initial colonization, the light at ground level is high, seedlings are
able to establish themselves.
As plants grow, their leaves intercept sunlight, reducing light to
short plants
The reduction in light enables fast-growing plants to out-compete
the other species and dominate the site
Sun-adapter, share-intolerant plants exhibit high rates of
photosynthesis and growth under high-light conditions. Under low
light, they can not survive
Shade-tolerant species exhibit much low photosynthesis rate and
growth under high-light conditions, but are able to continue
photosynthesis and growth, and survival under low-light (tradeoff)
In the early stage, shade-intolerant species dominate because of
their high growth rate. They grow and shade the slower growing,
shade-tolerant species. As time progresses and light level decline
below the canopy, seedlings of the shade-intolerant species can’t
grow and survive in the shaded conditions. At this time, although
shade-intolerant species dominate the canopy, no new individual
are being recruited. In contrast, shade-tolerant species will
germinate and grow, and replace the old, dead shade-intolerant
spp.
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Example of succession
• 1st
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• 2nd
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Shade intolerant
species
Eventually dominate
only in canopy
Shade tolerant species
invade
• Shade intolerant die out
due to no seedlings
• Shade tolerant take
over
18.6 Species diversity change during
succession
In addition to shifts in species dominance, patterns of
plant species diversity change over the course of
succession.
Comparison of species diversity at different sites within
an area that are at different stages of succession
Chronosequences (or chronoseres)
These groups of sites
For example, farmland abandoned at different times
Changes in plant diversity during secondary succession of
an oak-pine forest in Brookhaven, New York.
Species richness increases into the late herbaceous stage, declines
into the shrub stage, then increases in the early forest stages, then
decreases thereafter.
Normal growth rate
Double growth rate
Hypothetical succession involving five plant species
Species diversity increases initially as new species colonize the site. As
autogeneically changing environments and competition result in the
displacement of early successional species, diversity declines
Pattern of succession under three different
disturbance frequencies
Wrong?
Intermediate disturbance hypothesis: community with
intermediate disturbance has large species diversity
18.7 Succession involves heterotrophic
species
Not only autotrophic component
of community (plant
succession) show succession,
changes in heterotrophic
component of the community
also occur.
Decomposition
Tree fall  bark beetle, wood
boring beetle  fungi 
bacteria  predator insects
(centipedes, mites,
pseudoscorpions, beetles) 
fungi  moss and lichens 
seedlings
As plant succession advances, changes in structure and composition of
the vegetation result in changes in the animals life that depends on
vegetation as habitats.
18.8 Systematic changes in community structure
occur as a result of allogeneic environmental change
at a variety of timescales
Shifting patterns of community structure in response
to autogenic environmental changes often occur
at time scales relating to the establishment and
growth of the vegetation.
Purely abiotic environmental change can produce
patterns of succession over time scales ranging
from days to millennia.
Paleoecology: study of distribution and abundance of ancient
organisms and their relationship to the environment
18.10 The concept of community revisited
Two views of community: organismal view and individualistic
view
Reality lies somewhere in the middle
Organismal community is a spatial concept: variety of plant and
animal species interacting and influencing the overall
structure
Continuum view is a population concept, focusing on the
response of the component species to the underlying features
of the environment.
An example (demonstrate two views)
Patterns of co-occurrence for 4 plant species on a landscape along a
gradient of altitude
End
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Changes in diversity in
one part of community
can affect another part
• Greater diversity of vertical
layers of forest
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More bird species
• Variation in diversity during
forest succession
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Affects mammal species