Chapter 17 Processes Shaping Communities

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Chapter 17Processes Shaping Communities
Lecture Date 3-10-04 Lecturer Jonathan Maul
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Previously you learned about community structure.
Community structure can change through 1)
SPACE 2) TIME
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Change in communities through space ZONATION
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The classic example of zonation is a salt marsh.
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Zonation on a rocky shore along the North Atlantic
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Zonation on a sandy beach along the Mid-Atlantic
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2 Types of Transition Zones

1. Edge where 2 or more veg. communities meet.
2. Ecotone where 2 or more veg. communities
   intergrade.

This will be covered in detail in Chapter 19.
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EDGE (a) ECOTONES (b,c,d)
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Classification of communities is Scale Dependent
I (Section 13.9)
1) Classifying is a human action to facilitate
description and transfer of information. 2) It
is very important to identify your scale when
classifying communities. 3) Smaller Scale gtgt
Precision Larger Scale ltlt Precision
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Classification of communities is Scale Dependent
- II
4) In the next 3 graphical examples you will see
vegetative communities classified at 3 different
scales. 5) Each example is actually a smaller
part of the next.
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Small Scale
(Fig. 13.4)
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Med. Scale
(Fig. 13.7)
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Large Scale
(Fig. 13.8)
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Classification of communities is Scale Dependent
- III
6) Small scale of a single hillside, one can
actually discuss individual tree species. 7)
Middle scale of several peaks appears to be a
range of communities depending on moisture and
elevation. 8) Large scale of Great Smokey Mtns.
is described as Oak-Chestnut forest.
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Previously you learned about community structure
Community structure can change through 1)
SPACE 2) TIME
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Change in communities through time SUCCESSION

• Succession refers to a given
• point in space.
• Succession is common to all
• environments.

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The process goes from early successional to late
successional
x
x
x
x
x
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Characteristics of Early Successional Species
1) High growth rate. 2) Small size. 3) Wide
dispersal. 4) Fast population growth rate.
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Characteristics of Late Successional Species
1) Low rates of dispersal and
colonization. 2) Slow growth rates. 3) Larger
size. 4) Longer lives.
Figure 21.3 (d)- cut out colored part of figure,
put circle around late succ. Section.
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Figure 13.10 Here is an example showing early
and late successional algae species
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Successional sequence tends to be very
predictable or repeatable

• This suggests to Ecologists a common mechanism
  controlling the process of succession.

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Rate at which succession occurs is dependent on
the community in question.
e.g.) algae communities have a fast rate 1-3
yrs.
Notice time scale on x axis.
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Rate at which succession occurs is dependent on
the community in question.
e.g.) forest communities have a slow rate 5-80
yrs.
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Types of Succession
1) Primary Succession occurs at a site
previously unoccupied by a community. Examples a
) Lava flows. b) Artificial substrate like
algae exp. c) Newly formed bodies of
water. d) Newly exposed land. 2) Secondary
Succession occurs on previously occupied sites
following disturbance. Examples a) Forest
clearcuts. b) Weather damaged sites.
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Disturbance
Discrete event in time that disrupts
communities and creates opportunities for new
individuals. 1) Can be small single tree in
forest (40m radius). 2) Can be large miles of
clearcutting, fire, or disease. 3) Forest
disturbance alters factors such as light, soil,
temperature, nutrients, soil moisture, and
relative humidity.
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One 3-Dimensional view of disturbance
1. Frequency
2. Severity
3. Extent
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Intermediate Disturbance Hypothesis
1) Proposed by J.H. Connell in 1978. 2) Suggests
that species diversity is controlled by the
frequency and intensity of disturbance. 3)
Essentially, he proposed that species diversity
is maximal at intermediate levels of disturbance.
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Relationship between diversity and an
intermediate level of disturbance
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Climax Community - I
1) Succession appears to approach an endpoint
which is called the climax community. 2)
Characteristics of a theoretical climax community
include a) Tolerance of ambient conditions.
b) Equilibrium between production
respiration. c) Equil. between energy captured
and released. d) Equil. between nutrient
uptake and return. e) Many species. f)
Complex food chain.
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Climax Community - II
3) Problem Imagine no human impacts and all
communities at their climax. Would this mean
that all early successional species go
extinct??? 4) Answer No, the climax community
is not at equilibrium. Changes are continuously
occurring throughout the climax community. For
example see the following figure --gt
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Graphical depiction of homogenous climax
community w/ Holes consisting of early
successional plant species.
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Climax Community - III
5) The holes in the climax community of the
previous slide are called Gaps. 6)
Characteristics of forest Gaps include a) ?
Light Intensity. b) ? Soil Availability. c)
? Temperature. d) ? Nutrients. e) ? Soil
Moisture. f) ? Relative Humidity.
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Succession affects the distribution and abundance
of animals (Section 13.13)
1) Replacements can be observed in animal species
at various stages of succession.
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NOTE Meadow Mouse is replaced by White-footed
Mouse and Black-throated Green Warbler is
replaced by Veery in older forests.
Figure 21.19
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Succession affects the distribution and abundance
of animals
2) Overall species diversity changes with
successional stage as well.
Notice drop in diversity at the young forest
stage.
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Community over Geologic Time

• So far, we have discussed temporal changes on
  the order of months (algae communities) to
  decades (forest communities).
• Geologic time is another component that must be
  addressed.

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Succession over Geologic Time
(Fig. 13.16)
Postglacial migration of 4 tree genera. White
lines indicate extent of present ranges.
SPRUCE
WHITE PINE
OAK
MAPLE
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Influences on Community Structure
1) Direct Interactions 1) Competition 2)
Mutualism 3) Predation 4) Parasitism 2)
Indirect Interactions 1) Interaction between
species 1 and 3 below
consumes
consumes
Species 1
Species 2
Species 3
Indirect interaction of sp.1 on sp. 3
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Influences on Community Structure
1) Direct Interactions 2) Indirect
Interactions Can operate through niche overlap
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The Fundamental Niche Constrains Community
Structure
1) Certain range of conditions are necessary for
a species to survive. 2) Can be shown by the
bell-shaped curves for 1 environmental gradient.
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Some Basics on Niches
extra info
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Section 22.3 Changes in Niche Dimensions
1) Fundamental Niche Hutchinsonian niche in the
absence of competitors. 2) Realized Niche
Subset of fundamental niche used when competitors
are present because competitors make some
resources unavailable.
Resource
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In 1958 Robert MacArthur studied Niches with 5
Warblers
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How?? Through different adaptations for
acquiring resources.