Special relationships between organisms
Monacia Williams, Contributor

Here we are together again for another lesson. Did you try the experiment with the bread? If you did, do you now have a better understanding of how microorganisms work? Did you notice the changes in the colour of the bread as the days went by? Interesting wasn’t it?

Today we are going to look at some special relationships that exist between organisms within their habitats. These special relationships are given the name symbiosis. They exist between plants and other plants, between plants and animals and between animals and other animals.

Symbiosis occurs in three forms:

• commensalism
• mutualism
• parasitism

Commensalism

This is a relationship that exists between two organisms in which only one of the organisms derives benefit from the relationship. However, the other organism is not harmed or affected in any way.

Examples of organisms sharing this type of relationship are quite common. As you travel to and visit different places, look around you. Do you notice that many large trees usually have other small, green plants growing on them? These small plants are usually found high on the branches of the trees. Do you also notice that they have no connection with the soil? These are epiphytes. Most orchids are epiphytes, so are many Bromeliads, which are members of the same family to which the pineapple belongs.

Another example that is common to the West Indian islands is the relationship between cattle and the egret. The egret is the long-necked, white bird that you are likely to see in a field with cows. As the cows walk, they disturb the grass and, in so doing, insects in the grass will be dislodged, making them accessible to the egrets. In some parts of Africa, the relationship between the egret and the cattle changes from commensalism to mutualism.

Mutualism

In this type of relationship, both organisms derive benefit. Note that in the relationship between the egret and the cow, the egret serves as a warning for the cows when a predator is approaching, so that the cows are able to make good their escape.

Another example of mutualism is one that you have met before. Do you remember when we did the nitrogen cycle, one type of nitrogen fixation was carried out by nitrogen fixing bacteria in the nodules (small swellings) on the roots of leguminous plants? Well, this is an example of mutualism. The plants benefit because they are provided with a ready supply of nitrates, and the bacteria benefit because they have a place to live and are provided with an available supply of energy.

Parasitism

In this type of relationship, one organism is totally dependent on another for food and shelter. This organism is known as the parasite and the organism on which the parasite depends is known as its host.

The parasite derives all the benefits. The host will suffer some harm, but is not usually killed. The parasite can live either on the outside of the host – ectoparasite – or within the host – endoparasite.

Examples include:

• love bush – dodder on trees.
• ticks on dogs
• mealy bugs on plants
• scale insects on plants
• round worms and tapeworms in humans

Another interesting relationship that falls outside of symbiosis is that of predator and prey. Predators are carnivores that catch and eat other animals for food. The animals that are hunted and killed are the prey.

The relationship between the predator and the prey can be described as a ‘cat and mouse’ one. The prey is always trying to find ways to prevent itself from being eaten, while the predator is constantly finding ways to capture the prey. As the prey develops features such as camouflage and mimicry, the predator develops better eyesight and speed. In this way, predator/prey ratios have remained constant throughout the ages!

Now, did you ever imagine that the environment was so interesting? Ask your teacher to take you to an area of the seashore that does not have much human traffic and have fun exploring that environment. Take along a garbage bag or two and pick up the garbage while you explore!

See you next week when we begin Section B - life processes.

This male student (second left) of Waterford High School points to the high marks he got on a test as his friends look on, along Passagefort Drive in Portmore, recently. - Anthony Minott/Freelance Photographer

Monacia Williams teaches at Glenmuir High School.