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Part XXII: The Interdependence of Living Things : Special Relationships between Animals and Plants Index

Introduction
Parasitic Relationships : Plants
Parasitic Relationships : Animals
Chapter Summary (useful for revision)
Questions relating to this chapter

Topic Chapters Index

 

Lichen growing in Antarctica  © Shirley Burchill

Lichen growing in Antarctica

 

Fact File No.91

In the human gut there are millions of microorganisms - bacteria, fungi and protozoa which live by feeding on the non-digested food which reaches the colon and the rectum. Also, human skin is covered with microorganisms which feed on dead skin cells and sweat. Many of these micro-organisms can be classed as being mutualistic since some produce chemicals, such as vitamins, which we absorb.

 

Highland cattle are ruminants © Shirley Burchill

Highland cattle are ruminants

 

0xpeckers on the back of a rhinoceros © Shirley Burchill

0xpeckers on the back of a rhinoceros

 

A clown fish between the tentacles of a sea anemone © Shirley Burchill

A clown fish between the tentacles of a sea anemone

THE INTERDEPENDENCE OF LIVING THINGS

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Mutualistic Relationships

 

Lichens

One of the best known mutualistic relationships is found in lichens. Lichens are found almost everywhere; from the arctic tundra, where they provide food for the reindeer in the winter, to the equatorial forests. They are often the first plants to grow on bare rock and they are able to survive the hot sun in exposed areas.

 

Drawing of a LS section through a lichen  © Shirley Burchill

Section through a lichen to show the position of the algal cells and the fungal hyphae.

The lichen is made up of two organisms; a fungus and an alga. The algal cells live inside the hyphae of the fungus. The algal cells photosynthesize and give sugars and oxygen to the fungus. In return the fungus provides protection, water and salts for the small algal cells. Also, the fungus is able to grow on bare rock and other areas where other plants cannot. This is because the fungus is able to take a firm hold where most plant roots are unable to penetrate.

Neither the fungus nor the alga would be able to survive in these hostile areas on its own. Together, however, they can compete successfully with other plants for light and space.

 

Ruminants and Micro-organisms

In the animal world a good example of mutualism is that between a ruminant and the many millions of micro-organisms which live in its rumen. You will remember that a ruminant, such as a giraffe, has a sac called a rumen situated just before the stomach in its gut.

When a ruminant swallows its food, which is vegetation, the food goes into the rumen. Here the plant material is digested by the microorganisms. The micro-organisms make a special chemical which can break down the plant cells but the ruminant is not able to make this chemical.

The micro-organisms digest the plant material for both themselves and the ruminant. In return the ruminant provides them with a constant supply of vegetation as well as a safe and a warm place in which to live.

 

More Mutualistic Relationships

Plants of the pea family, leguminous plants, have hundreds of little round structures called nodules on their roots. The roots secrete a chemical substance which stimulates certain bacteria to grow and divide. These bacteria which live inside the root nodules can take nitrogen from the air and make nitrates. These nitrates are needed by the plant for growth. In return the bacteria are protected in the root nodules and gain certain nutrients from the plant which are necessary for their own growth.

Not all mutualistic relationships involve one organism living inside another organism. Dairy ants and aphids have a mutualistic relationship. The ants protect the aphids from possible predators. In return, the aphids provide the ants with honeydew.

The Egyptian plover takes insects from the backs of buffaloes, giraffes and rhinos. The plover has also been observed taking leeches from the open mouths of crocodiles! In this association the plover receives a supply of food and the other animal rids itself of unwelcome pests.

 

YouTube © BBC Uploaded on Feb 12, 2007
David Attenborough looks at how some creatures have formed mutually beneficial partnerships - if you scratch my back, I'll scratch yours.

 

Some flowers have a mutualistic association with a specific insect. One species of orchid found in Madagascar, keeps its nectar at the bottom of a tube which is between 20cm to 35cm long. It is totally dependent on a species of hawkmoth for pollination. This hawkmoth has a proboscis which is 22,5cm long.

With some associations it is not so easy to decide whether the other partner is gaining something or being harmed in the relationship. Sometimes the partner may be totally unaffected by the relationship.

One example is to be found between the sea anemone and the clown fish. The clown fish is able to live amongst the tentacles of the sea anemone, even though the anemone's stinging cells are fatal to other kinds of fish. It is easy to see that the clown fish benefits from protection and, perhaps, from scraps of food which the sea anemone does not take into its body. It is not so clear, however, what the sea anemone gains from the relationship.

Another example is a small crab (Lissocarcinus) which lives on the surface of the sea cucumber. The crab is well camouflaged in this position since it is the same colour as the sea cucumber. Also, it benefits from some of the plankton which escapes the filtering mouthparts of the host. It would appear that the sea cucumber remains unaffected by the presence of the crab on its body.

 

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