The Nature of the Community

1
The Nature of the Community

• Chapter 20

2
Community

• Any assemblage of populations of living organisms
  in a prescribed area or habitat
• Trees at Lake Louise
• Rodents in Long-Leaf Pine
• Ants in disturbed areas

3
Two models of the Community

• Rivet model
• Obligate association or obligate exclusion of
  species
• Redundancy model
• No interaction between species
• Two models represent the extremes of possibilities

4
The Community View of Frederic E. Clements

• Clements saw the community as a superorganism in
  which the functions of various species are
  connected like the parts of the body.
• Clementss view included the following ideas
• that component species had coevolved so as to
  enhance their interdependent functioning
• that communities were discrete entities with
  recognizable boundaries

5
The Community View of Henry A. Gleason

• Gleason saw the community as a fortuitous
  association of species whose adaptations and
  requirements enable them to live together under
  the particular conditions of a particular place.
• Gleasons view included the following ideas
• that component species occurred together largely
  by coincidence
• that there was no distinct boundary where one
  community meets another

6
Diverse Concepts of Community

• The holistic concept of Clements and others
  recognizes that we can only understand each
  species in terms of its contributions to the
  dynamics of the entire system.
• The individualistic concept of Gleason and others
  recognizes that community structure and function
  simply express the interactions of individual
  species, and do not reflect any organization
  above the species level.

7
Community Concepts - A Middle Ground?

• An intermediate or mixed view of communities also
  exists, which
• accepts the individualistic view that most
  interactions are antagonistic and that
  communities are haphazard assemblages of species
• accepts the holistic premise that some attributes
  of communities arise from interactions among
  species, reinforced through coevolution

8
(No Transcript)
9
(No Transcript)
10
A Natural Unit of Ecological Organization?

• The holistic view of communities predicts a
  closed community
• the distributions of species are coincident
• the boundaries between communities (ecotones) are
  distinct
• The individualistic view of communities predicts
  an open community
• the distributions of species are independent
• the boundaries between communities are diffuse

11
Theoretical view of open vs. closed community
12
Open vs. Closed communities
13
When do communities have distinct boundaries?

• The concept of closed communities predicts
  discrete boundaries between communities
• such boundaries should be expected under two
  circumstances in nature
• when there is an abrupt transition in the
  physical environment
• when one species or life form dominates strongly,
  such that the edge of its range determines the
  limits of many other species

14
Ecotones

• Ecotones represent boundaries between closed
  communities
• such boundaries occur when there are sharp
  discontinuities in the physical environment
• the interface between terrestrial and aquatic
  communities
• the boundary between soil types with contrasting
  properties (such as the boundary between
  serpentine and nonserpentine soils)

15
Plants contribute to conditions maintaining
ecotones.

• Transitions between broad-leaved and
  needle-leaved forests become more pronounced
  because of conditions created by the plants
  themselves
• increased soil acidity and greater accumulation
  of undecayed litter distinguishes the
  needle-leaved forest
• Fire may sharpen the boundary between prairies
  and forests in the Midwestern U.S.
• perennial grasses resist fire damage, but fires
  cannot penetrate deeply into forests

16
Sharp ecotone between aquatic and terrestrial
habitats
17
Change in soil elements
18
Ecotone between grassland and forest
19
Change in species
20
The Continuum Concept 1

• Ecotones are less likely to form along gradients
  of gradual environmental change
• the deciduous forest region of eastern North
  America does not fit the concept of the closed
  community
• few species have closely overlapping geographic
  ranges, tending to be independently distributed
• sharp ecotones are not found
• As ecologists sought to understand the ecology of
  the eastern forests, they turned to the continuum
  concept.

21
The Continuum Concept 2

• The continuum concept embodies several key
  concepts
• plants and animals replace one another
  continuously along environmental gradients
• species have different geographic ranges,
  suggesting independent evolutionary backgrounds
  and ecological relationships
• because few species have broadly overlapping
  ranges, the assemblage of species found in any
  particular place does not represent a closed
  community

22
Range of trees found in Kentucky
23
Gradient Analysis

• A gradient analysis is usually undertaken by
  measuring the abundances of species and physical
  conditions at a number of locations within a
  landscape
• the abundances of species are then plotted as a
  function of the value of any physical condition
• Studies by R.H. Whittaker in the Great Smoky
  Mountains revealed few cases of distinct ecotones
  between associations of species
• species were distributed more or less
  independently over ranges of ecological
  conditions, with few cases of consistent
  association between species

24
Gradient analysis of tree communities in Smoky
Mtns
25
Gradient analysis of tree communities
26
(No Transcript)
27
(No Transcript)
28
Distributional Relations of Species in Communities

• Species should show similar geographic patterns
  as communities
• Floristic provinces are large vegetational
  communities
• Boundaries are called tension zones
• Usually also the limit of many species
  distributions
• Can use measure of similarity to compare 2
  communities (ranges from 0 to 1)

29
(No Transcript)
30
(No Transcript)
31
(No Transcript)
32
(No Transcript)
33
(No Transcript)
34
Indicator Species

• Indicator species stand for a given community
• Need to meet certain assumptions
• Includes Umbrella, Flagship, Keystone
• Should have worldwide applications
• Tiger beetles

35
(No Transcript)