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General Ecology: Lecture 8
October 27, 2003
I.
II.
How can we explain the apparent coexistence of a large number of species in field
communities?
Specific example: Diversity in coral reef fishes
A.
Hypotheses related to juveniles/adults that have settled on the reef
1.
Niche partitioning/high level of specialization prevents competitive exclusion.
a)
This hypothesis appears to be supported on the level of Families (i.e.
explains why there are so many families).

Examples: feeding strategies in different reef fish.
b)
This explanation is more difficult to support for closely related species.
Some examples:

Damselfish species: Longfin, Dusky, Threespot, Yellowtail

Numerous similarities/subtle differences

Angelfish species: Gray, French, Queen

Numerous similarities in feeding, habitat and general
ecology.

Subtle differences used by those supporting the nichepartitioning hypothesis

Reproductive behavior somewhat different for Queen
angels.

NOTE: See also Macarthur’s example of resource
partitioning in Warblers, Fig. 12.15. This is the
“classic example” of resource partitioning.
2.
Predation prevents competitive exclusion.
a)
A generalist predator might preferentially eat members of more common
species… (follow through this argument…)
3.
Disturbances occur frequently enough that superior competitors are never
able to dominate (=non-equilibrium hypothesis).
a)
Key here is that competition doesn’t exist for a long enough time period to
wipe out the “inferior” competitor
b)
Shown that “moderate” levels of disturbance promote highest levels of
diversity.
c)
Extreme disturbance, such as hurricanes may decrease diversity.
4.
Resources are actually non-limiting.
B.
Hypotheses related to larval ecology
1.
Temporal and spatial variability in larval recruitment of different species to
the reef keeps a few species from dominating the reef.
a)
Various factors contribute to this variability (be able to list several)
2.
Temporal and spatial variability in larval recruitment of different species to
the reef keeps a few species from dominating the reef.
a)
Larvae of a potentially dominant species do not settle in numbers high
enough to competitively exclude other species.
b)
Because the population doesn’t remain local, the numbers can’t be built up
over generations.
How can it be demonstrated that competition is actually occuring?
A.
We tend to assume it is occuring when similar species occupy the same area…
B.
Criteria for demonstrating that competition: weak to strong evidence [Table 12.1]
III.
Evolution of competitive ability
A.
r-selection vs. K-selection
1.
r-selection:
a)
When predominant?
b)
Competive ability?
2.
K-selection
a)
When predominant?
b)
Competitive ability?
c)
K value vs. α
B.
Grime’s Theory of plant strategies: Plant strategies shaped by stress (physical-chemical)
and intensity of disturbance.
1.
Low stress/low disturbance:
2.
High stress, low disturbance
3.
Low stress/high disturbance
C.
Character displacement
1.
Defined
2.
Classic example: Darwin’s finches in the Galapagos [Fig. 12.23]
D.
Diffuse competition
1.
Occurs among multiple species
2.
Difficult to measure, because interactions between any pair are weak.
Study questions
1.
Be able to describe/explain several alternate hypotheses as to why closely-related fish are able to
co-exist on coral reefs. Six different hypotheses were presented in class. You should understand
how these work, not just memorize the list!
a.
For example, you should be able to explain how a predator might prevent competitive
exclusion, not just say that it can
b.
Another example: Provide arguments, pro and con, for the niche-partitioning hypothesis
c.
And…how would disturbance prevent a superior competitor from wiping out the inferior
competitor?
d.
Etc… Come up with these types of questions on your own for the six different hypotheses
to help you think critically about them.
2.
List/describe the six criteria, from weak to strong, that demonstrate competition.
3.
What is the difference between r and K selection? What conditions promote r-selection? What
conditions promote K-selection? Which of the these tend to be selected to be better competitors?
4.
List several differences between r vs. K selected species (see your Introductory or Principles of
Biology course!)
5.
For two K-selected species, can the one with the lower K win in competition? If so, how?
6.
According to Grime’s Theory, what two factors are key in affecting plant growth/reproduction
trategies? Describe the three major strategies of plants and under what conditions each type will
be selected for.
7.
Is the “competitive” strategy really describing K-selection? If not, how does it differ?
8.
What is character displacement? What type of evidence is used to demonstrate character
displacement? Give an example of character displacement.