Transcript SPECIES INTERACTIONS

POPULATION
INTERACTIONS
READINGS:
FREEMAN, 2005
Pages 1214-1220 and 1227-1229
POPULATION
INTERACTIONS
• Populations do not exist alone in nature.
They are found in the presence of many
potential competitors, predators and
mutualists.
• The presence or absence of another
species can have a profound or little
impact on the abundance of the other
species.
FIVE IMPORTANT
INTERACTIONS BETWEEN
TWO SPECIES
•
•
•
•
•
COMMENSALISM (+/0)
MUTUALISM (+/+)
COMPETITION (-/-)
PARASITISM (+/-)
PREDATION (+/-)
The symbols +, - and 0 refer to the effect of one
species on another when both are living together.
Population Interactions Influence
Abundance
• When populations of different species
interact, the effects on one on the other
may be positive (+), negative (-) or
neutral (0).
• By comparing populations living alone
and together, several types of
interactions can be identified.
COMMENSALISM
• When populations of commensal species are
together, one population is benefited but the
other is not significantly affected.
• The effect of the interaction on population
growth and individual survival is:
LIVING ALONE
A
B
LIVING TOGETHER
A
B
COMMENALISM
0
0
+
0
(The COMMENSAL (A) does better when the host is
present. The HOST (B) is not affected by the
interaction.)
COMMENSALISM
• The cattle egret and
cattle or other
grazing African
ungulate species.
• The egret benefits
from catching
insects that cattle
“scare-up” while
grazing.
• Cattle unaffected.
COMMENSALISM
• E. coli (Escherichia coli) is
a common bacteria found
living in the guts of
mammals, including
humans, where it gets all
it needs to thrive.
• In most circumstances,
humans are not harmed
by its presence and no
benefit has been
discovered.
COMMENSALISM
• Bromeliads are a
group of flowering
plants that attach to
trees (epiphytes).
They gain access to
sunlight and catch
water.
• The trees are not
harmed or
benefited.
THE 3 MOST STUDIED INTERACTIONS
LIVING ALONE
A
B
LIVING TOGETHER
A
B
MUTUALISM
+
+
[Both populations are found in greatest abundance when together.]
COMPETITION
0 0
[When both populations live together, abundance of each is lower.]
PREDATION
+
+
[Prey (A) are in greatest abundance when predators are absent. Predators
(B) are in greatest abundance when prey are present.]
MUTUALISM
• Populations interact to the benefit of both.
• Mutualism may be obligate (necessary for
survival of one or both species) or facultative
(advantageous to one or both species).
• The basis for agricultural domestication of
plants and animals by humans.
• Common in nature, but the effect on
population dynamics is difficult to
demonstrate and often complex.
MUTUALISM
• Although free nitrogen is about
80% of the atmosphere, plants
are unable to use it until it is
“fixed” into ammonia and
converted to nitrates by bacteria.
• A common example of this
mutualism between plants and
nitrogen fixing bacteria is found in
lawns containing white clover.
Next time you are looking for a
four leaf clover, thank nitrogen
fixing bacteria. You need the
nitrogen that they fix.
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MUTUALISM
• One of the most
commonly observed
mutualism is the
pollination of flowering
plants by an insect or
humming bird.
• The pollinator benefits
from the interaction by
receiving nectar.
• The plant gets its pollen
transferred from one
plant to another.
MUTUALISM
• The lichen is a mutualistic
association between a
species of algae and a
species of fungus.
• The fungus retains water
and takes up minerals.
• The algae provides
carbohydrates and other
organic nutrients as the
result of photosynthesis.
The Rhizobium/Soybean
Connection
• The mutualism between
Rhizobium and soybeans is an
important source of nitrogen
fixation in Illinois farm fields.
• Rhizobium, a bacterial genus,
can convert atmospheric nitrogen
(N2) into ammonia (NH3). Thus,
making this essential nutrient
available to these legumes.
• In turn legumes, such as
soybeans and clover, supply
Rhizobium with carbohydrates
and other nutrients for growth and
reproduction.
Rhizobium
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Soybean Field
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LEGUMES/NITROGEN
FIXING BACTERIA
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• Nitrogen fixing bacteria
enter the root hairs of
legumes in the seedling
stage. The bacteria
causes the plant to
produce nodules.
• The host plant in return
supplies carbohydrates,
amino acids and other
nutrients that sustain
their bacterial partners
(bacteriods).
NITROGEN FIXATION
(Nature’s Ways)
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MUTUALISM
• Some species of ants and
treehoppers form an
interesting mutualism that
resembles tending (care
giving).
• The ants provide
protection for the
treehoppers.
• In turn, the treehoppers
provide honeydew for the
ants.
Experiment Demonstrates
Mutualism
• Hypothesis: Ants protect treehoppers from
spiders.
• Method: Remove ants at random from host
plants that contain treehoppers.
• Results: The number of young treehoppers
per plant is higher on plants with ants than on
plants without ants.
• Conclusion: Treehoppers produce honeydew
that attract ants seeking food. Ants protect
treehoppers from predation by spiders.
Question: Is the relationship between ants and treehoppers mutualistic?
Freeman’s Figure 53-16 part 1
Hypothesis: Ants harvest food from treehoppers and protect treehoppers from jumping
spiders.
Null hypothesis: Ants harvest food from treehoppers but are not beneficial to treehopper
survival.
Experimental setup:
Plants with ants
Plants with
ants removed
Study plot, 1000 m2
Prediction: More young treehoppers will be found when ants are present than when ants
are absent.
Prediction of null hypothesis: There will be no difference in the number of young
treehoppers on the plants.
Figure 53-16 part 1 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
Prediction: More young treehoppers will be found when ants are present than when
ants are absent.
Freeman’s Figure 53-16 part 2
Prediction of null hypothesis: There will be no difference in the number of young
treehoppers on the plants.
Average number of young
treehoppers per plant
Results (Year 1):
100
80
Plants with ants
60
40
20
Plants without ants
10
0
20
25
July
30
5
10
15
August
Conclusion: Treehoppers benefit from the interaction with ants, which protect
treehoppers from predation by jumping spiders.
Figure 53-16 part 2 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
Other experiments are required
to determine type of interaction
The outcome of the interaction is dependent on
predator (spider) abundance and cost of producing
honeydew to treehoppers:
• When spiders are abundant and cost of producing
honeydew is moderate, both ants and treehoppers
benefit (+/+).
• When spiders are scarce and cost of producing
honeydew is moderate, ants benefit and treehoppers
are unaffected (+/0).
• When spiders are rare and cost of producing honey
dew is high, ants benefit, but treehoppers decline (+/).
OBLIGATE MUTUALISTS
• The fig wasp and fig and
yucca moth and yucca are
obligate mutualists.The
insects are sole pollinators of
the plants. The insects lay
eggs in the flowers of the
plants. Larvae feed off of
some of the developing
seeds.
• Neither species can persist
without the other.
THE 3 MOST STUDIED INTERACTIONS
LIVING ALONE
A
B
LIVING TOGETHER
A
B
MUTUALISM
+
+
[Both populations are found in greatest abundance when together.]
COMPETITION
0 0
[When both populations live together, abundance of each is lower.]
PREDATION
+
+
[Prey (A) are in greatest abundance when predators are absent. Predators (B) are
in greatest abundance when prey are present.]
COMPETITION
• Mutual use of a limited resource by
populations of two or more species.
• Each individual adversely affect another in
the quest for food (nutrients), living space,
mates, or other common needs.
• When individuals harm one another is
attempting to gain a resource.
• Abundance of both is greater when alone,
than when together.
COMPETITION
• May be:
interspecific, or
intraspecific
• Due to:
exploitation, or
interference
• Result in:
mutual extinction, or
exclusion of one, or
coexistence
Categories of Competition
• When competition is between individuals of:
---- same species (intraspecific)
---- different species (interspecific)
• When a resource is in short supply that used
by one it is not available to the other
(exploitation).
• When an action or substance produced by
one is directly harmful to the other
(interference).
Outcomes of Competition
• 1. One wins; other loses …..
(competitive exclusion)
• 2. Neither wins ……..
(coexistence)
• 3. Both lose ……..
(mutual extinction)
Only 1 and 2 above are of ecological or
evolutionary significance
Exploitation and Intraspecific
Competition
Reindeer on St Mathews Island
7000
6000
6000
5000
Number
• Resource depletion may
result in too many
individuals in the
population. Thus, the
population crashes.
• Reindeer on Saint
Matthews Island died off as
the result of depletion of
lichens (food).
4000
3000
2000
1350
1000
0
1940
42
29
1945
1950
1955
Year
1960
1965
1970
Exploitation and Intraspecific
Competition
• A seed company
advises gardeners to
“spread seeds thinly in
a furrow, after plants
grow then thin to 8
inches apart”. Why?
• Plants too far apart or
too close together will
only produce a few
seeds. Why?
Interference and Intraspecific
Competition
• Territorial behavior has
evolved in many species
as a response to
intraspecific competition.
• Male red wing blackbirds
stake out a territory in
defense of nests and
mates.
Interference and Intraspecific
Competition
• The red grouse
males stake out
territories that are
defended against
other males.
• The size of a
territory determines
red grouse density.
• This is called
territorial behavior.
Why Do Red Grouse
Populations Cycle?
• Hypothesis: Changes in aggression influence
number of young males that can establish
territories.
• Method: Old males with established territories
received testosterone transplants, which
increases aggression, in four separate locals.
These populations were compared with 4
control populations (no testosterone
implants). Population densities in the 8 areas
were compared.
Why Do Red Grouse
Populations Cycle?
• Results: 1.The density of adults in the 3 experimental
populations declined and in the other population density
stopped increasing. Control population densities increased.
2. The decline in density of males was greater than found
in the control areas. 3. The ratio of young to old males
decreased more in experimental populations than controls.
4. The density of females was lower in experimental
populations than in controls.
• Conclusion: Changes in aggressiveness and territorial
behavior of male red grouse can effect population
dynamics. This study confirms others showing that
territorial size is inversely related to male breeding density
(larger territories- lower breeding male density).
Exploitation and Interspecific
Competition
• A classic example of
competitive exclusion
between species is
found in the
experimental results of
Gause (see page 1216
in Freeman).
• Bios 101 students have
performed experiments
where both species
coexist.
Competitive exclusion in two species of Paramecium
Number of individuals
Freeman 53.3a
400
300
Paramecium
aurelia
200
100
0
Paramecium
caudatum
0
5
10
15
Time (days)
Figure 53-3a Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
20
25
Interference and Interspecific
Competition
• Chthamalus (top)
populations are
overgrown in the lower
intertidal zone by
Balanus (bottom).
• This classic study of
competitive exclusion
is described in detail
by Freeman.
Barnacle species are distributed in distinct zones.
FreemanChthamalus
Figure
53-6a
in upper
intertidal zone
Mean tide level
Balanus in lower
intertidal zone
Figure 53-6a Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
Testing the hypothesis that competition occurs
Question: Why is the distribution of adult Chthamalus restricted to the
upper intertidal zone?
Figure 53-6b part 1
Hypothesis: Adult Chthamalus are competitively excluded from the lower
intertidal zone.
Alternative hypothesis: Adult Chthamalus do not thrive in the physical
conditions of the lower intertidal zone.
Experimental setup:
Upper
intertidal
zone
Lower intertidal zone
1. Transplant rocks containing
young Chthamalus to lower
intertidal zone.
2. Let Balanus colonize the
rocks.
3. Remove Balanus from half
of each rock. Monitor
survival of Chthamalus on
both sides.
Chthamalus
Balanus
Prediction: Chthamalus will survive better in the absence of Balanus.
Prediction of alternative hypothesis: Chthamalus survival will be low
and the same in the presence or absence of Balanus.
Figure 53-6b part 1 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
Prediction: Chthamalus will survive better in the absence of Balanus.
Freeman Figure 53-6b part 2
Prediction of alternative hypothesis: Chthamalus survival will be low and the same in
the presence or absence of Balanus.
Results:
Percent survival
80
60
Chthamalus survival is higher
when Balanus is absent
40
20
0
Competitor
absent
Competitor
present
Conclusion: Balanus is competitively excluding Chthamalus from the lower intertidal
zone.
Figure 53-6b part 2 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.
POPULATION
INTERACTIONS
READINGS:
FREEMAN, 2005
Pages 1214-1220 and 1227-1229