Competition
Chapter 13
1
Outline
 Resource

Competition
Modes
 Niches
 Mathematic

Lotka-Volterra
 Competition

and Laboratory Models
and Niches
Character Displacement
2
Competition

The effects of
competition in nature
can be observed
through observation &
experimentation.

Many organisms
compete for limited
resources and may
defend territories to
protect their access to
these resources.
3
Modes of Competition
 Interference

Competition:
Involves direct aggressive interactions
between individuals.
4
Modes of Competition
 Intraspecific:

Competition with members of the same
species.
5
Modes of Competition
 Interspecific:

Competition between individuals of different
species.
• Reduces the fitness of both species.

Reduction in fitness may not be equal.
6
Intraspecific Competition Among
Herbaceous Plants

Competition in a
forest can be as
intense as
competition on a coral
reef.

Takes place mostly
underground.
7
Intraspecific Competition
Among Herbaceous Plants

Plant growth rates
and weights have
been found to
increase in low
density populations.

Competition for
resources is more
intense at higher
population densities.
• Usually leads to
mortality among
competing plants Self-Thinning
8
Intraspecific Competition
Among Herbaceous Plants

The self-thinning
process can be
represented by
plotting the average
weight of individuals
vs. density.

Slope averages -3/2.
• -3/2 thinning rule.
9
Intraspecific Competition
Among Planthoppers

Denno and Roderick
attributed prevalence of
competition among
Homoptera to their habit
of aggregating, rapid
growth, and the mobile
nature of their food
supply.

Demonstrated intraspecific
competition within
populations of planthopper
Prokelesisia marginata.
• Probably result of limited
resources.
10
Interference Competition Among
Terrestrial Isopods

Survival was lower at
higher population
densities.
 Food was not limited.
 Cannibalism may have
been a factor.
 Interference
competition can occur
in the absence of
obvious resource
limitation.
11
Niches
 Niche:
Summarizes environmental factors
that influence growth, survival, and
reproduction of a species.

An organism’s role within the environment.
12
Niches
 Gause:
Principle of Competitive
Exclusion

Two species with identical niches cannot
coexist indefinitely.
• One will be a better competitor and thus have
higher fitness and eventually exclude the other.
13
Niches
 Hutchinson

defined niche as:
n-dimensional hyper-volume
• n equals the number of environmental factors
important to survival and reproduction of a species.


Fundamental niche – hypervolume or the
entire area where a species could survive in
the absence of biotic interactions.
Realized niche includes interactions such as
competition that may restrict environments
where a species may live.
14
Feeding Niches of Galapagos
Finches

Grant found
differences in beak
size among ground
finches translates
directly into diet.

Size of seeds eaten
can be estimated by
measuring beak
depths.
15
Feeding Niches of Galapagos
Finches

Individuals with
deepest beaks fed
on hardest seeds.
16
Feeding Niches of Galapagos
Finches

After 1977 drought,
the remaining seeds
were very hard. Thus,
mortality was most
heavy in birds with
smaller beaks.
17
The Habitat Niche of a Salt Marsh
Grass

Spartina angelica is a
species produced by
allopolyploidy – a
process of speciation
initiated by the
hybridization of two
species.



S. maritima (European)
S. alterniflora (North
American)
The hybrids doubled their
chromosome number and
were then able to
reproduce as a new
species.
18
The Habitat Niche of a Salt Marsh
Grass

This species has an
increased tolerance
for water-saturated
soil and can inhabit
lower zones in a salt
marsh.
19
Mathematical and Laboratory
Models
 Models
provide a theoretical foundation for
studying interspecific competition in
nature.
20
Mathematical and Laboratory
Models
 Metz



summarized models:
Abstractions and simplifications, not
facsimiles of nature.
Man-made construct; partly empirical and
partly deductive.
Used to provide insights into natural
phenomena.
21
Modeling Competition
 The
effect of competition on the growth of
a population can be predicted by
mathematical models.
22
Lotka Volterra
 Lotka-Volterra
competition models are
based on the logistic equation of
population growth – the s-shaped curve.
23
Lotka Volterra
 Four
possible outcomes of the LotkaVolterra competition equations:




Species 2 eliminated.
Species 1 eliminated.
Either species 1 or species 2 eliminated,
depending on starting conditions.
Both species coexist.
24
Lotka Volterra
 In
general, LV predicts coexistence of two
species when, for both species,
interspecific competition is weaker than
intraspecific competition.
25
Paramecia Lab Experiments
 Gause
demonstrated resource limitation
with Paramecium caudatum and
Paramecium aurelia in presence of two
different concentrations of food - Bacillus
pyocyaneus.
26
Paramecia Lab Experiments

When grown alone,
carrying capacity was
determined by
intraspecific
competition.
 When grown together,
P. caudatum quickly
declined.

Reduced resource
supplies increased
competition.
27
Flour Beetle Experiments
 Tribolium
beetles infest stored grain
products.

Park studied interspecific competition
between T. confusum and T. castaneum
under varied environmental conditions.
28
Flour Beetle Experiments

Growing the two species
together suggested
interspecific competition
restricts the realized
niches of both species to
fewer environmental
conditions.



Hot-wet conditions favor T.
castaneum.
Cool-dry conditions favor T.
confusum.
Unpredictable results with
intermediate conditions.
29
Competition and Niches
 Competition
can restrict species to their
realized niches.

But if competitive interactions are strong and
pervasive enough, they may produce an
evolutionary response in the competitor
population.
• Changes fundamental niche.
30
Niche Overlap and
Competition Between Barnacles

Connell examined the relationship between two kinds of
barnacles – Balanus balanoides and Chthamalus
stellatus.

Chthamalus is found in the upper intertidal.
• Limited by competition with Balanus.

Balanus is found in the mid & lower intertidal.
• Limited by exposure to air in upper intertidal.
31
Niche Overlap and
Competition Between Barnacles

The fundamental
niche of Balanus is
equal to its realized
niche.
 The fundamental
niche of Chthamalus
is much broader than
it’s realized niche.
 Both are limited by
predation in the
lowest part of the
intertidal.
32
Competition and Niches of
Small Rodents
 Brown
studied competition among rodents
in Chihuahuan Desert.
 Three factors make this experiment stand
out.



Large scale
Experimental trials well replicated
Long term
33
Competition and Niches of Small
Rodents
 Brown
predicted if competition among
rodents is mainly for food, then small
granivorous rodent populations would
increase in response to removal of larger
granivorous rodents.


Insectivorous rodents would show little or no
response.
Results supported hypothesis.
34
Competition and Niches of
Small Rodents
35
Character Displacement
 Individuals
that compete less should have
higher fitness than those that compete
more.
 Because degree of competition is
assumed to depend upon degree of niche
overlap, interspecific competition has been
predicted to lead to directional selection
for reduced niche overlap.

Evolutionary divergence in niches.
36
Character Displacement
 The
process of evolution toward niche
divergence in the face of competition is
called character displacement.
37
Character Displacement
 Example:
Galapagos finches, G. fortis and
G. fuliginosa.

Allopatric (living apart) on some islands.
• Both species have similar beak sizes.

Sympatric (living together) on another island.
• No overlap in beak sizes.
• Different feeding niches – reduces competition.
38
Character Displacement
39
Character Displacement
 Taper



and Case: Necessary criteria:
Morphological differences between sympatric
species are statistically greater than
differences between allopatric populations.
Differences between sympatric and allopatric
populations have genetic basis.
Differences between sympatric and allopatric
populations evolved in place, and are not
derived from different founder groups already
differing in the character.
40
Taper and Case: Characteristics



Variation in the character must have a known
effect on use of resources.
Must be demonstrated competition for the
resource and competition must be directly
correlated with character similarity.
Differences in character cannot be explained
by differences in resources available to each
of the populations.
41