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Eco-interactions between organisms P.

Interactions between Organisms


n interaction between two organisms may be ben______, ad______ to either side or n______.

Since organisms are continually evolving, it should not be surprising to note that different types of interaction occurring in nature are sometime difficult to be put into distinct categories.

Type of Interaction



Nature of the interaction




Interaction is favourable to both and is obligatory


Commensal (A) benefits whereas the host (B) is not affected


Predator kills and consumes the prey (B)


Parasite exploits the host (B) which is affected adversely


Organism competing and adversely affecting each other --- (A) and (B)

Summary of the various kinds of interactions that may occur between two organisms

I) Mutualism

When both populations ben____ from an association that is ob______, we call this mutualism. Why is it obligatory ? The answer is that the partners has become phy____________ interdependent on one another.

  1. Autotrophs -- Heterotrophs interaction

a) Symbiotic algae. Many aquatic invertebrates possess symbiotic algae, include the r___-forming corals. In the case of invertebrate hosts, perhaps the relationship looks a little more like ‘internal farming’, e.g. Certain marine molluscs feed on seaweed but incorporate the chloroplasts into the cells lining their gut. Here the chloroplasts may live and photosynthesise for some days while releasing carbohydrates to the mollusc.

b) Mycorrhiza. Roots of most healthy plants are usually associated with fungus called my_______. This association is so important that many higher plants fail to grow in the absence of mycorrhiza.
Characteristically in_______ nutrients are supplied to the higher plants enabling them to live in low nutrient status soil, and 'in return' the fungus is supported by the plant which is the photosynthesising partner.

c) lichens. Lichens are composed of a f_____ and an a___. But the relationship is more than the sum of the parts. Lichens show struct____ modifications, and phy_______ activities (such as the formation of so-called lichen acids) that are found in neither component when isolated.

The fungus, which may make up 95 per cent of the lichen dry mass, obtains or____ compounds from the photosynthesis of the alga. The alga receives more nut_____, car___ dioxide, it can occupy a wider range of habitats, and can better withstand des________.
Q. Name the autotrophs in the above interactions. In what ways are they benefited from the association?

  1. Explain the experimental evidences that support that there is nutritional interdependence between the fungus and the Scots pine

Lichen: structure and physiology

Reference Reading 1: Crops without chemicals
With the world's population increasing at an annual rate of about 2 per cent, improvements in our total crop production and distribution are essential if we are to avoid a catastrophe of widespread starvation.

In the 1960s, the Green Revolution provided the undernourished world with cereal strains of much improved yield, but these made heavy demands on the fertility of the soil. So far the only answer to this problem has been the use of expensive nitrogenous fertilizers. The fertilizers themselves have produced problems when their application on farmlands has led to the eutrophication of lakes and waterways.

This is one of the reasons why agricultural botanists have been directing their attention for some time towards the prospect of developing cereal crops which can directly assimilate atmospheric nitrogen and so reduce the demand for nitrogenous fertilizers. In nature, nitrogen fixation is an ability confined to a few genera of bacteria and blue-green algae/bacteria. Some associated with host plants while others are free-living.

Perhaps the best-known nitrogen-fixer is the bacterium Rhizobium which is found in the root nodules of leguminous
Legume crops have long been used in rotation with non-legumes to improve the fertility of the soil.
Rhizobium bacteria enter the legume roots and invade cortical cells of the root which are stimulated into meristematic activity.
The root nodules which develop may appear pink because of the synthesis of leghaemoglobin by the legume root tissue. This is the only form of haemoglobin known in the plant kingdom; like animal haemoglobin, it has the ability to bind oxygen.
In nodules which do not produce leghaemoglobin, it has been shown that the Rhizobium bacteria do not fix nitrogen.

This knowledge about nitrogen fixation is now being put to good use in the search for ways of producing nitrogen-fixing cereal food crops. Much effort has been expended in trying to persuade Rhizobium from legumes to invade the roots of wheat and other cereal plants, but with little success.

Even if nitrogen-fixing cereal crops can be produced, however, there may be serious drawbacks. Nitrogen fixation consumes a great deal of energy (up to 24 molecules of ATP for each molecule of nitrogen fixed) and the presence of nitrogenase in many cereals would turn out to be something less than the economic miracle anticipated.

a Give the meaning of the following terms:

1 eutrophication

2 facultative anaerobes

3 obligate mutualist.

b Describe two ways in which nitrogen fixed by Rhizobium bacteria in leguminous plants may enter the soil.

c 1. Suggest a function of the leghaemoglobin produced in the root nodules of legumes.

2 Explain how this benefits the Rhizobium bacteria.

d. Give a reason for the comment that ‘the presence of nitrogenase in many cereals could turn out to be less than the economic miracle anticipated’.

Reference reading 2: The Endosymbiosis theory --- Origin of eukaryotic cells

The endosymbiosis theory postulates that

  • The mitochondria of eukaryotes evolved from aerobic bacteria living within their host cell.

  • The chloroplasts of eukaryotes evolved from endosymbiotic cyanobacteria.

The Evidence

  • Both mitochondria and chloroplasts can arise only from preexisting mitochondria and chloroplasts. They cannot be formed in a cell that lacks them because nuclear genes encode only some of the proteins of which they are made.

  • Both mitochondria and chloroplasts have their own genome and it resembles that of bacteria not that of the nuclear genome.

    • Both genomes consist of a single circular molecule of DNA.

    • There are no histones associated with the DNA.
  • Both mitochondria and chloroplasts have their own protein-synthesizing machinery, and it more closely resembles that of bacteria than that found in the cytoplasm of eukaryotes.

    • The first amino acid of their transcripts is always fMet as it is in bacteria (not methionine [Met] that is the first amino acid in eukaryotic proteins).

    • A number of antibiotics (e.g., streptomycin) that act by blocking protein synthesis in bacteria also block protein synthesis within mitochondria and chloroplasts. They do not interfere with protein synthesis in the cytoplasm of the eukaryotes.

    • Conversely, inhibitors (e.g., diphtheria toxin) of protein synthesis by eukaryotic ribosomes do not — sensibly enough — have any effect on bacterial protein synthesis nor on protein synthesis within mitochondria and chloroplasts.

Q. Suggest the possible advantages for such an association between an autotroph and a heterotroph.
a) Advantages to the heterotrophs

b) Advantages to the autotrophs

  1. Autotroph -- Autotroph interactions

Anabaena is a member of the blue-green bacteria, Anabaena is found as an endosymbiont in the floating fern Azolla. The association allows Azolla to live in very nut_____-poor waters.
Q. Suggest how Anabaena
can help azolla to strive in nut_____-poor waters.

  1. Heterotroph -- Heterotroph interactions.

c________-digesting Bacteria and Protozoans in the guts of rumi_____ and ter____. The gut mutualists receive a constant supply of f___ and, in the case of ruminants, a continuously high tem______ which is ideal for rapid metabolic activity. The ruminants and termites benefit because they can absorb the products of digestion of cellulose.

The gut microorganisms may also produce B-vit_____, and convert inorganic nitrogen into a____ acids which are absorbed by the animal. Supplementing protein in the poor diet of the basically vegetarian animals

Non-obligatory Associations

There are many other interactions that are mutually beneficial but is n___-obligatory and looser than mutualistic ones.

a) Pollination and Dispersal
The benefit to the plants is obvious and the animals may feed on nectar or a fleshy fruit, and so on. It must be said that in some cases the modification of the plants is so extr_____ that only a single species may be attracted or may fulfill the role as pollinator. In these circumstances, the relationship has crossed the boundary to mutualism. For example, figs can only be pollinated by certain wasps.

  1. Mullerian mimicry

During the course of evolution, prey species have evolved a number of strategies to reduce the likelihood of their being eaten. One of these methods is to become unpalatable either by making toxins or by feeding on plants and concentrating their toxins.

Un_________ or toxic animals then employ wa______ coloration to advertise their distastefulness: they use red, white, black or yellow. It is to the advantage of the prey (and to the predators) if unpalatable species sh___ the same warning colours. Thus, unrelated species may bear a strong superficial resem______ to each other. This is known as Mullerian mimicry.

III) Commensalism

It is an interaction in which one species benefits and the other is unaff______. Examples can be identified into four broad groups in which commensalism may be based on:

a) Cleaning Oxpeckers, removes ticks and fleas from the skin of cattle, zebra, giraffes, and others, and the Egyptian plover, cleans the teeth of the Nile crocodile.

b) Protection and camouflage. For instance, hermit crabs, may carry on its shell anemones, which seem to protect it from octopuses.
c) Support

Trees are often covered with individuals of other plants, which grow on their trunks and branches. These ‘epiphytes’ gain support & grow nearer the light then they would have done on the ground. Examples of e_______ are lichens, some algae, mosses, ferns & orchids, it seems that the trees are not greatly affected by their presence.

  1. Transport

Where one species may use another to, in effect, hitch a ride! The remora, Remora remora. is well known. It clings to the underside of sharks using its specially modified dorsal fin.

IV) Predation

In predation the association is unequal: one population benefits, the other suffers.

  1. The evolutionary race between predator and prey

To be a successful predator, an animal must be adapted to loc____, cap____, ing___ and dig___ its prey, while the preys also ev____ a number of corresponding adaptations to prevent themselves from being eaten. There is, in fact, a constant evol_________ race between predator and prey.

Predator vs Prey

Predator are adapted to capture and kill

Prey are often equipped to avoid being captured and killed

Prey location vs Predator evasion

good senses of smell, hearing, for____-facing eyes with stereoscopic vision

good senses of sm___, he____, lat____-placed eyes with very wide field of v____

Capture vs Escape

Well-camouflaged, stealthy and speedy,

prey often find safe h_____ places, cam______, mimicry, has the speed to r__ away

some hunt in group

gain protection by living in l____ groups and communicating danger to each other

Ingestion vs Resistance & deterrent

Spec______ mouth parts, strong and sharp t____, poi_____ injection for killing prey

H___ shell, poisonous or foul-tas____ chemicals. Plants may be protected by t____ chemicals such as cyanogenic glucosides, silica bodies, sp___

  1. Chemical defence in plants

Plants incapable of locomotion cannot avoid being eaten except by making itself unattractive or distasteful to predators. They may be phys____ tough, covered with spines and thorns, or chem_____ harbour chemicals that deter potential predators. These chemical are extremely interesting because they are often bio______ active in animals.

Herbivory/ plant eating is analogous to parasitism. Since herbivory does not necessarily lead to the death of the 'prey'. Nevertheless, plants have evolved an enormous array of antipredator devices. Laurel, some clovers, and bracken all produce cyanide if chewed or otherwise damaged. The cy____ (-CN) group, is attached to a sugar to form a glycoside. Also in the cells, but kept separate, is a glycosidase enzyme. If the cell is damaged these two substances come into contact and cyanide is released.

However, there is a price to pay for these selective forces. In order to overcome obstacles presented by the prey, predators may become increasingly spe______ as they become more adapted. In other words the range of possible prey may become restricted. Many insects are monophagous, that is depend on one food species; some are oligophagous and maybe feed on just one family of plants. Polyphagous insects can feed on a variety of plants.

  1. Which group of insects, monophagus, oligophagous or polyphagous do you think pose greater threats to crops? Why?

Reference reading: The defence mechanism of Bracken (a fern)
Bracken defends itself in the following ways. It produces cyanide like clover and other harmful chemicals which make it distasteful; these include phenolics, lignins, tannins, carcinogens and a sheep blindness factor. It also produces a chemical which mimics insect moulting hormones, an ecdysone. Furthermore, bracken also has a protective association with ants. The ants feed on sugar produced at extrafloral nectaries and, in return, they drive off many potential herbivores. Despite this armory, over 20 species of phytophagous (plant-eating) insects have been found on bracken in UK. This illustrates the general point that selective forces on the prey are balanced by those on the predator.

Reference reading : monarch butterfly, cardiac poisons and Batesian mimicry:

One example of allelopathy involving animals is provided by the monarch butterfly. Its caterpillar stage feeds on the milkweed plant which produces chemicals that act as strong cardiac (heart) poisons in vertebrates.
This, or its unpleasant taste, presumably discourages grazing herbivores from eating the plant. However, the caterpillars can tolerate the poison, and can store it and carry it into the adult butterfly stage.

The butterfly, in turn, gains protection from predatory birds. The bright colouration and striking markings of the butterfly therefore serve as a warning to potential preda­tors and act as a protective device. An incidental consequence of this is that some butterflies mimic the markings of the monarch, thus also gaining protection. This is known as Batesian mimicry.

Batesian (non-toxic) mimics are disadvantageous to the species they mimic (the distasteful model) because potential predators may encounter harmless mimics and thus take longer to learn to avoid the model.
Generally speaking, many plants subjected to grazing contain toxic chemicals to which only a few herbivore species are tolerant.

V) Parasitism

A parasite is an organism that lives on or in its h___ and that nourishes itself at the exp____ of the host without rapidly destroying it as a predator does its prey, but often inflicting some degree of injury affecting its welfare'. What distinguishes parasitism is that the prey does not die im_________. Parasitism is like a weak form of predation and analogous, as we have seen, to herbivory.

Animal parasites come principally from the groups Protozoa, Platyhelminthes and Nematoda. Outside these groups we might cite leeches, ticks, fleas, mosquitoes and lice.

A key problem for all parasites is to establish the association in the first place. Commonly parasites have very complex life-c involving several h___.

The nature of the host-parasite interaction in animals necessitates a variety of meta___ and phys_______ adaptations. It may also induce reactions to the parasite by the host which in turn the parasite must overcome.
Almost all bacterial, viral and fungal diseases are par____ and even angiosperm plants can become parasites; plants like dodder are wholly parasitic on other plants. Finally, parasitism is not confined to higher organisms. Bacteria may be parasitised by viruses, called bacteriophages.

Some structural, physiological and reproductive specializations of parasites

Types of specializations


  • Highly specialised mouth____ as in fluid feeders. Development of ‘suckers’ in some parasitic green plants.

  • Bor___ devices to effect entry into a host.

  • Att_____ organs such as hooks or suckers

  • Resistant outer cov____.

  • Degeneracy of s____ organs associated with the constancy of the parasite's environment. Absence or deg______ of feeding and loco_____ organs - characteristic of g__ parasites.


  • Enz___ production to digest host tissue external to parasite.

  • Anti________ in blood feeders

  • Chemo-sen________ in order to reach the optimum location in the host's body.

  • Production of anti-enz_____ in gut parasites.
  • Ability to respire adequately in an________ conditions


  • Hermaphrodite condition thus aiding possible self-fertilisation

  • Enormous numbers of repro_____ bodies i.e. eggs, cysts and spores.

  • Highly Res_____ reproductive bodies when external to the host e.g. eggs with protective shells.

  • Employment of specialised reproductive phases in life cycle. e.g. asexual reproduction phases in larval forms -- Malaria plasmodium

  • Use of sec_______ hosts as vectors.

VI) Competition

Competition for res______ may be int___specific or int___specific. In competition, since both species suffer, avoidance mechanisms tend to evolve, leading to niche spec________.

It is important as a pop_____ regulating factor, it is also a very potent evo________ force.

Example 1 : Specialization / Avoidance through competition

Barnacles are animals (Arthropoda) covered by a shell. They live fastened to coastal rocks in the area which is exposed at low tide, called the intertidal zone.
Once attached to a rock barnacles are fixed to the spot until they die. They feed on zooplankton when they are covered by the sea.

In Scotland, one species of barnacle, Balanus balanoides, occurs in the low part of the intertidal zone. Another species, Chthamalus stellatus, which is of a similar size to Balanus, occurs higher up on the rocks.

Young Chthamalus often attach themselves in the lower shore, but no adults are found there if Balanus is present in numbers. If there is no Balanus, Chthamalus can easily live in the lower shore. On the other hand, even if there are no Chthamalus, Balanus cannot establish itself in the upper parts of the shore.

a. List the differences in environment between the upper shore and the lower shore.
b. What possible factors are limiting the upshore limit of Balanus ?
c. Why do you think Chthamalus is restricted to the upper shore?
d. It this is an example of competition, what do you think the two species are competing for ?
e. Design an experiment to determine whether the two species are competing for the resource you named in ‘d’.

f. Suggest the signficance of the two species of Barnacles specializing on slightly different microhabitats.

Example 2: The Avoidance of Competition and Adaptive radiation

Individuals involved in interspecific competition are likely to suffer through lower food intake, less reproduction and a greater risk of disease. This is true even for the 'winner' of the competitive interaction. The loser may face starvation, loss of shelter or nesting sites, and death from other causes. It might, therefore, be expected that natural s_______ occurs in competing individuals leading to adaptations to av___ competition. Such an adaptation is called character displacement, in which similar species living together become more dif______ than usual, so they can divide the available resources without competition.

Adaptive radiation by one or both species may then occur over a period of time with the result that they come to occupy separate n____, thus min_______ the extent of competition. Thus competition can be a very potent force of evolution.

Q. State 3 ways in which competition is important ecologically or in evolution of new species:

VII) Concluding remarks

We have seen how different populations may interact in a variety of ways. We have also seen how some forms of interaction may create sel____ pressures: the predator-prey relation is a classic example. The prey species develops a defensive chemical or a modified behaviour and the predator responds accordingly. We also saw that this may lead to a progressive spec_______ by the predator leading to a dependence on a few or even one prey species.

Whether we are talking of predator-prey or host-parasite relationships or symbionts, there comes a point when to understand the evolution of one we need to understand the story of the other. When evolutionary interactions are interdependent like this we call it co-evolution.

Assignment : Look for a further example of co-evolution resulting from the interaction between two organisms, collect pictures, write short description and discuss points of biological interest in the relationship, be ready to present your assignment in 2 weeks time.. (Don’t write more than one A4 pages!)


Suggested Answers to Study Item:

Crops without chemicals
a. 1. Eutrophication is the enrichment of water with plant nutrients, particularly with nitrates and phosphates. The nitrates comes mainly from fertilizers & phosphate mainly from detergent & sewage. Eutrophication results in excessive growth of algae, water weeds and anaerobic condition when bacteria absorb oxygen when decomposing the dead algae.
2. Those that can live without oxygen if needs arises, but normally live in microhabitats that contain oxygen.
3. Organisms which must be in partnership with another species in order to survive and reproduce; a partnership from which both partners benefit.

b. When plants die, diffusion of organic and inorganic nitrogenous compounds from roots. Seed dispersal; plant eaten by herbivore and egested.

c. i) It seems to prevents large concentration of oxygen from building up in the inner cortical cells.

ii) protect nitrogenase against attack by oxygen.

d. N-fixation is a very energy-demanding process, the constant need to supply glucose to bacteria may reduce crop production.
In addition, it would also be essential to transfer the leghaemoglobin genes to the cereals so that the Rhizobium in the cereal roots would be protected against oxygen.

Competition between two species of Barnacles


Upper Shore

Lower Shores

Longer exposure to air, humidity lower

Action of drying effect of wind more pronounced

Less exposure to air

Temperature fluctuation greater

Salinity fluctuation larger

Conditions more stable

Less frequently covered by the sea

receive less splashes from sea water

Danger of predations from sea-living organisms
Food brought in from sea water

b. Desiccation / Exposure to air / extremes of temperature, salinity fluctuation.

c. Expansion of niche into lower shore restricted by competition from Balanus.

d. Attachment to rock surface.

e. Find stones with both species of barnacles.

Place them in the lower tide level where both species can thrive,

leave half of the stones untouched.

For the other half, chip away the Balanus that are in contact with the Chtlamalus.

Observe some time later if better growth of Chtlamalus occurs where Balanus were removed.

f. This specialization allows the two species to co-exist in a habitat, avoiding competition, exploiting slightly different niches.


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