1. POPULATION INTERACTIONS
READINGS: FREEMAN, 2005 Pages and

2. 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.

3. 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.

4. 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.

5. 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 LIVING TOGETHER
A B A B
COMMENALISM
(The COMMENSAL (A) does better when the host is present. The HOST (B) is not affected by the interaction.)

6. 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.

7. 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.

8. 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.

9. THE 3 MOST STUDIED INTERACTIONS
LIVING ALONE LIVING TOGETHER
A B A B
MUTUALISM
[Both populations are found in greatest abundance when together.]
COMPETITION
[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.]

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.
Soybean Field

15. LEGUMES/NITROGEN FIXING BACTERIA
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).

16. NITROGEN FIXATION (Nature’s Ways)

17. 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.

18. 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.

19. Freeman’s Figure 53-16 part 1
Question: Is the relationship between ants and treehoppers mutualistic?
Freeman’s Figure 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 part 1 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.

20. Freeman’s Figure 53-16 part 2
Prediction: More young treehoppers will be found when ants are present than when ants are absent.
Freeman’s Figure part 2
Prediction of null hypothesis: There will be no difference in the number of young treehoppers on the plants.
Results (Year 1):
100 80
Plants with ants 60
Average number of young treehoppers per plant 40 20
Plants without ants 10 20 25 30 5 10 15
July
August
Conclusion: Treehoppers benefit from the interaction with ants, which protect treehoppers from predation by jumping spiders.
Figure part 2 Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.

21. 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 (+/-).

22. 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.

23. THE 3 MOST STUDIED INTERACTIONS
LIVING ALONE LIVING TOGETHER
A B A B
MUTUALISM
[Both populations are found in greatest abundance when together.]
COMPETITION
[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.]

24. 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.

25. COMPETITION May be: interspecific, or intraspecific Due to:
exploitation, or
interference
Result in:
mutual extinction, or
exclusion of one, or
coexistence

26. 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).

27. 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

28. Exploitation and Intraspecific Competition
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).

29. 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?

30. 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.

31. 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.

32. 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.

33. 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 The decline in density of males was greater than found in the control areas 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).

34. 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.

35. Competitive exclusion in two species of Paramecium
Freeman 53.3a
400
300
Paramecium aurelia
Number of individuals
200
100
Paramecium caudatum 5 10 15 20 25
Time (days)
Figure 53-3a Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.

36. 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.

37. Barnacle species are distributed in distinct zones.
Freeman Figure 53-6a
Chthamalus in upper intertidal zone
Mean tide level
Balanus in lower intertidal zone
Figure 53-6a Biological Science 2/e ©2005 Pearson Prentice Hall, Inc.

38. 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:
1. Transplant rocks containing young Chthamalus to lower intertidal zone.
Upper intertidal zone
2. Let Balanus colonize the rocks.
Lower intertidal zone
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.

39. 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: 80 60
Chthamalus survival is higher when Balanus is absent
Percent survival 40 20
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.

40. POPULATION INTERACTIONS
READINGS: FREEMAN, 2005 Pages and