Inter-specific relationships Inter-specific relationships are interactions among organisms of different species. Typically, these interactions are classified

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  • Inter-specific relationshipsInter-specific relationships are interactions among organisms of different species. Typically, these interactions are classified based on whether they are beneficial to one or both of the species involved or whether they are detrimental to one of the species involved.


    The symbols +, - and 0 refer to the effect of one species on another when both are living together.Types of exploitation

  • COMMENSALISMWhen 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 BCOMMENALISM 0 0 + 0(The COMMENSAL (A) does better when the host is present. The HOST (B) is not affected by the interaction.)

  • COMMENSALISMThe cattle egret and cattle or other grazing African ungulate species.The egret benefits from catching insects that cattle scare-up while grazing.Cattle unaffected.

  • COMMENSALISME. 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.

  • COMMENSALISMBromeliads 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.

  • MUTUALISMPopulations 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.

  • MUTUALISMAlthough 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.

  • MUTUALISMOne 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.

  • MUTUALISMThe 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.

  • OBLIGATE MUTUALISTSThe 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.

  • COMPETITIONMutual 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.

  • COMPETITIONMay be: interspecific, or intraspecificDue to: exploitation, or interferenceResult in: mutual extinction, or exclusion of one, or coexistence

  • Categories of CompetitionWhen 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 Competition1. 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 CompetitionResource 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).

  • Exploitation and Intraspecific CompetitionA 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 CompetitionTerritorial 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 CompetitionThe 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 CompetitionA 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.

  • Freeman 53.3aCompetitive exclusion in two species of Paramecium0510152025Time (days)3002001000Paramecium aureliaParamecium caudatumNumber of individuals400Figure 53-3a Biological Science 2/e 2005 Pearson Prentice Hall, Inc.

  • Interference and Interspecific CompetitionChthamalus (top) populations are overgrown in the lower intertidal zone by Balanus (bottom).This classic study of competitive exclusion is described in detail by Freeman.

  • Freeman Figure 53-6aBarnacle species are distributed in distinct zones.Chthamalus in upper intertidal zoneBalanus in lower intertidal zoneMean tide levelFigure 53-6a Biological Science 2/e 2005 Pearson Prentice Hall, Inc.

  • Figure 53-6b part 1Question: Why is the distribution of adult Chthamalus restricted to the upper intertidal zone?Experimental setup: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.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.ChthamalusTesting the hypothesis that competition occursUpper intertidal zoneLower intertidal zoneBalanusPrediction: 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.

  • Freeman Figure 53-6b part 2Conclusion: Balanus is competitively excluding Chthamalus from the lower intertidal zone.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.Results:Percent survivalCompetitor absentCompetitor presentChthamalus survival is higher when Balanus is absent806040200Figure 53-6b part 2 Biological Science 2/e 2005 Pearson Prentice Hall, Inc.