Relationship Between Organisms in the Same Environment – JSS2 Basic Science Lesson Note (First Term)

An ecological relationship is the way organisms in the same ecosystem interact with and depend on one another. There are six types: two oppositional (predation and competition) and four symbiotic (mutualism, commensalism, amensalism and parasitism). Each type is described by whether each organism gains, is harmed or is not affected.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define an ecological relationship and name the six types;
2. explain predation and name the predator and the prey in an example;
3. describe competition, including intraspecies and interspecies competition;
4. explain mutualism, commensalism, amensalism and parasitism and say who benefits and who is harmed in each;
5. give at least one example of each relationship.

What is an ecological relationship?

An ecological relationship is the interaction between living things that share an ecosystem.

• Connection: all organisms in an ecosystem are linked to one another.
• Web of relations: each population interacts with other populations, and ecological relationships describe how they are connected.
• Adaptation: the relationship an organism has depends on how it has adapted to the pressures of its environment over time (evolution).

What are the types of ecological relationship?

There are six types of ecological relationship: predation, competition, mutualism, commensalism, amensalism and parasitism.

• Oppositional relationships: predation and competition.
• Symbiotic relationships: mutualism, commensalism, amensalism and parasitism.

What is predation?

Predation is a relationship in which one organism hunts and eats another organism.

• Predator: the organism that hunts. It is usually a carnivore that hunts, kills and eats other animals.
• Prey: the organism that is hunted.
• Energy transfer: energy that came from the sun passes from the prey to the predator when the prey is eaten.
• Type of relationship: positive-negative, because the predator gains and the prey is harmed.

What are examples of predation?

• Snake and mouse: the snake is the predator and the mouse is the prey.
• Striped marlin and sardines: the marlin lives in the Pacific Ocean and preys on sardines.
• Birds and earthworms: various birds eat earthworms.
• Snake and hawk: a predator can also be prey. A snake may become a meal for a hawk.

How does predation control population size?

Predation controls the number of animals in a population.

1. When predators are few, the number of prey rises.
2. With more prey to eat, predators reproduce more and may change their hunting habits.
3. As predators increase, the number of prey falls.
4. Food becomes scarce for the predators, and many of them may die.

What is cannibalism?

Cannibalism is a form of predation in which an animal eats members of its own species.

• Cause: it can happen when food is scarce.
• Example: frogs are known for cannibalism.

What is an herbivore-plant relationship?

An herbivore-plant relationship is predation in which the prey is a plant.

• Galapagos tortoise: it feeds on the cactus plants that grow on the Galapagos Islands.
• Koala: it has a special digestive system that breaks down tough eucalyptus leaves and is not harmed by their poison.
• Squirrel: it is an herbivore (the “predator”) and the nuts it eats are the plant (the “prey”).

What are food chains and food webs?

A food web is a set of connected food chains showing who eats whom in an ecosystem.

• Food chain: a single line showing how food and energy pass from one organism to the next.
• Food web: many food chains joined together, showing how connected predators and prey are.
• Effect of loss: when one species disappears, its predators lose a food source and its prey is no longer eaten by it.
• Knock-on effect: changes in these populations affect other populations.

What is an energy pyramid?

An energy pyramid shows how energy flows through an ecosystem along a food chain, from producers to the animals that eat them.

• Energy: the energy for all living things comes from the sun.
• Producers: plants that capture light energy and turn it into chemical energy (stored in sugar). They form the base of the pyramid.
• Consumers: organisms that cannot make their own food.
• Herbivores: consumers that eat plants (producers). Examples: goats and cows.
• Carnivores: consumers that eat animals. Example: a lion eats a zebra.
• Omnivores: consumers that eat both animals and plants. Example: badgers.
• Scavengers: consumers that eat dead animals. Example: vultures.
• Decomposers: organisms that break down dead remains and waste products. Examples: fungi, mould and bacteria.
• Energy at each level: less energy is available at each higher level of the pyramid.

What is competition?

Competition is a relationship in which organisms struggle for the same resources, such as food, water, space or mates.

• Type of relationship: negative-negative, because both organisms are harmed by the struggle.
• Kinds: intraspecies competition and interspecies competition.

What is intraspecies competition?

Intraspecies competition is competition between organisms of the same species.

• Elk: two male elk fight for a female mate.
• Elephants: males fight so that the dominant elephant breeds with the female.
• Dolphins: they swim and play together, but compete for a meal when it is time to eat.

What is interspecies competition?

Interspecies competition is competition between organisms of different species.

• Example: a lizard and a frog compete for the same food, such as a small insect.
• When it happens: it occurs when two species share an ecological niche and need the same resource.

What is the competitive exclusion principle?

The competitive exclusion principle states that two species that need the same limited resources cannot live together in the same habitat for long.

• Winner: direct competition between different species usually produces a winner.
• Loser: the losing species dies out or moves to another ecosystem that can support it.

What is mutualism?

Mutualism is a relationship in which both organisms benefit from each other. It is a positive-positive relationship.

• Bee and flower: the bee collects nectar (used to make honey). It spreads pollen so that the flower can reproduce.
• Oxpecker and impala: the oxpecker lands on the impala and eats the ticks on it. The bird gets a meal and the impala is rid of ticks.
• Monkeys: they pick fleas from one another, which is like a treat and helps both.
• Spider crab and algae: algae live on the crab’s back. The algae get a home and the crab is camouflaged from predators.
• Clownfish and sea anemone: the anemone protects the clownfish and the clownfish protects the anemone. The clownfish also gets a protected home territory.
• Humans and gut bacteria: bacteria in the intestines get a good place to live. They help us break down food and get vitamins.

What is commensalism?

Commensalism is a relationship in which one organism benefits while the other is not affected. It is a positive-neutral relationship.

• Pilot fish and shark: the pilot fish follows beneath the shark and eats scraps of its kill. The shark is not affected.
• Bird and tree: the bird builds a nest in the tree and gets shelter. The tree gains nothing.
• Transparent shrimp and reef: the shrimp hides in the reef (camouflage). The coral is not affected.
• Barnacles and whale: barnacles are crustaceans that glue themselves to rocks, ships and whales. On a whale they get a home and reach food faster, and the whale is not affected.

What is amensalism?

Amensalism is a relationship in which one organism is harmed while the other is not affected. It is a negative-neutral relationship.

• Tree and small plants: a large tree shades the small plants below it, which grow poorly. The tree is not affected.
• Cattle and grass: cattle trampling on grass damage it. The cattle gain nothing from the damage.

What is parasitism?

Parasitism is a relationship in which one organism (the parasite) benefits while the other (the host) is harmed. It is a positive-negative relationship.

• Parasite: the organism that benefits. It usually lives on or inside the host.
• Host: the organism that is harmed.
• Malaria: the malaria parasite lives in humans and is passed on by the bite of an infected mosquito.
• Ticks and fleas: they live off many large mammals.
• Head lice: they feed on blood from the human head.
• Pine bark beetle: beetles lay eggs in pine trees. The young eat the inner layers of the tree and harm it.
• Adaptation: natural selection favours parasites that are best at finding hosts and feeding on them.

How do the six relationships compare?

Each relationship is described by the effect on the two organisms, as the table shows.

Evaluation

1. What is an ecological relationship?
2. Name the two oppositional relationships and the four symbiotic relationships.
3. What is the difference between a predator and its prey? Give one example.
4. Explain the difference between intraspecies and interspecies competition.
5. State who benefits and who is harmed or unaffected in mutualism, commensalism, amensalism and parasitism.

Assignment

Give one example each of mutualism, commensalism and parasitism from the lesson. For each example, name the organisms involved and say whether each one benefits, is harmed or is not affected.