General Ecology: Lecture 13
Competition (Part 2)
I.
II.
Concept of a niche
A.
Definitions (based on Hutchinson’s)
1.
Theodore Giesel (“On Beyond Zebra”)
2.
Hutchinson: For each environmental variable it faces in its world, an organism has
a range of values for which it can grow and multiply
B.
Problems with defining niches for real species
1.
Infinite number of variables that define a space of n-dimensions
a)
The most we can depict visually are 3 dimensions as a volume [Fig. 14.15]
2.
Assume variables can be ordered and measured for limits
a)
Think how this relates to, and differs from, Shelford’s Law of tolerance
3.
Can change through time (i.e. seasonal shifts in temperature tolerance)
C.
Usefulness of concept
1.
For a pair of important variables, can more fully define as a contour diagram to
determine “prime” vs. “marginal” conditions, thus helping to explain the
distribution and abundance of a particular organism.
a)
Example: Feeding niche of the blue-gray gnatcatcher [Fig. 14.17]
2.
Can determine regions of niche overlap for different species for variables
determined to be important [Fig. 14.19]
a)
Note that when more than one variable is considered, overlap of niches is
apparently reduced [Fig. 14.21]
D.
Fundamental vs. realized niche [Fig. 14.16]
1.
Fundamental niche:
a)
Defined theoretically: The n-dimensional “hypervolume” that defines all
the environmental space in which the organism could live.
b)
In the physical world: all locations that satisfy the organism’s requirements
2.
Realized niche
a)
Understand how this differs from a fundamental niche!
From the perspective of niches, how do species deal with competition?
A.
Gause’s hypothesis/competitive exclusion principle (i.e. Species 1 or Species 2 wins)
1.
Gause’s hypothesis: “As a result of competition, two similar species scarcely ever
occupy similar niches, but displace each other in such a manner that each takes
possession of certainly particular kinds of food and modes of life in which it has an
advantage over its competitor.”
a)
Refer back to laboratory experiments on Paramecium and the differences in
results depending upon the similarities of the two species competing.
2.
Hardin: Competitive exclusion principle: “Complete competitors cannot coexist.”
a)
Which of the Lotka-Volterra phase-plane diagrams predicts competitive
exclusion?
B.
Resource partitioning [Fig. 14.9]
1.
Defined:
a)
When alone, a species may use a wide range of a particular type of resource.
For example, a bird species may feed on fish of a wide range of sizes.
Whereas, when resources are shared, organisms might only utilize a subset
of those resources.
2.
Examples
a)
Example 1: Soil resources and plants [Fig. 14.10]
b)
Example 2: Terns on Christmas Island [see PowerPoint, not in text]
C.
Niche compression
1.
Competitors force a once wider-ranging species to retreat to optimal patches of
habitat.
2.
As in resource partitioning, there is a decrease in the proportion of the fundamental
niche that is utilized.
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3.
Different from resource partitioning because it deals with the partitioning of habitat
(i.e. space) rather than resources
a)
Deals with the physical space occupied
b)
Note that the two could be closely related depending upon the distribution of
resources.
D.
Ecological release
1.
This is essentially the opposite of niche compression. In this scenario, a species will
expand to fill unoccupied habitats
a)
Example: Species arrives on an uncolonized island.
2.
Text example (for both niche compression and ecological release): Dove species in
New Guinea (see text, p. 261)
E.
Niche shift
1.
A shift from one realized niche to another within the fundamental niche.
a)
Example: sunfish in artificial ponds with different vegetation zones (see
text, p. 261-262 for complete explanation)
2.
NOTE: the terms “niche compression”, “niche shift” and “ecological release” all
imply a niche in spatial terms, rather than in terms of resources…
F.
Character displacement (evolutionary time scale)
a)
If competition persists between two species, their characteristics will diverge
due to natural selection

Classic example: Darwin’s finches in the Galapagos [Fig. 14.23].
Be sure you understand this example!

Note that this is different that resource partitioning because there is
actual a shift in the beak size itself. This is a result of natural
selection for a once rare genotype.
III. The abundance of coral reef fish: How is it possible for so many similar species to coexist?
A.
Explanations consistent with the competitive exclusion principle
1.
Resource partitioning
2.
Niche compression
3.
Niche shift
a)
Essentially, all of these suggest that each species is extremely specialized
4.
Multiple niche dimensions must be considered (see. Fig. 14.21)
a)
What might some of these “dimensions” be?
B.
Non-equilibrium conditions
1.
Factor 1: Mobile nature of larvae
a)
Small differences in reproductive timing and location could lead to much
variability as to which larvae make it to each reef.
b)
Interannual differences in current patterns assure that one species isn’t
consistently successful at reaching a particular reef over time.
2.
Factor 2: Disturbances in the environment (storms, hurricanes)
a)
cause mortality and prevent superior competitors from reaching levels that
exclude inferior competitors
3.
Factor 3: Predation!
a)
Predators may target the species that are most abundant, so that less
abundant species are able to increase. (We’ll talk more about this in an
upcoming lecture.)
C.
Can we relate this back to the Lotka-Volterra competition equations?
1.
Species never reach their zero-growth isoclines (so where on graph are they?)
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Study questions
1.
Niches
a.
What is a niche? Compare the Hutchinson vs. Seussian version.
b.
What is some of the confusion surrounding the definition of a niche?
c.
What are some of the practical problems in defining a niche in Hutchinson’s way?
d.
How does the concept of a niche relate to Shelford’s law of tolerance?
2.
What point about niches is being illustrated in figure 14.21?
3.
What is the difference between the fundamental niche and the realized niche?
4.
Be able to state the competitive exclusion principle in your own words. Which Lotka-Volterra
phase-plane diagrams depict competitive exclusion?
5.
Define each of the following terms in terms of the species’ use of the niche in general, changes in
geographical distribution, and whether there is an actual change in behavioral or morphological
characteristics of the species involved. Also, provide an example of each from the lecture or text.
Don’t just list the example, but actually use it to explain the principles involved in each process!
Bottom line: You should know the differences between these and be able to recognize
examples (either ones you have already seen, or completely novel examples…)
a.
Resource partitioning
b.
Niche compression
c.
Niche shift
d.
Character displacement
6.
How is niche compression similar to resource partitioning? How does it differ? Under what
conditions would the two appear to be the same thing?
7.
What is ecological release, and how does it compare to niche compression?
8.
Character displacement is often confused with resource partitioning. What is the difference?
Could one be the result of the other? Explain.
9.
Provide several hypotheses to explain why several groups of similar species of fish are able to
coexist on coral reefs. Include hypotheses consistent with competitive exclusion as well as nonequilibrium hypotheses.
10.
How can we relate non-equilibrium hypotheses back to the Lotka-Volterra model? Explain…
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