San Yu Adventist School
San Yu Adventist School
P6 Science Notes
Interactions Ch 2 – Environment Pt 1
Name: ______
Part 1: Living In Communities
- Organisms Form Their Own Populations
Definition: An organism is a living thing. A plant such as a balsam or a fern is an organism. An Animal such as an ant or a fish is also an organism.
• If we keep only a parrot at home, it lives with us without any of its own kind around. But in nature, organisms seldom live alone. They usually live in groups together with their own kind. If you see a sparrow on a tree, then you will be able to find more sparrows nearby.
• When organisms of the same kind live together and reproduce in a particular place, they form a group of their own kind called a population.
Definition: A population in a particular place is the total number of organisms of the same kind (or species) living and reproducing in that place.
E.g. The population of grasshoppers in a field include all the big and small adult grasshoppers as well as their nymphs that are found in the field.
E.g. The population of water-lily plants in a pond include all the adult and young water-lily plants that are found in the pond.
• The population of any kind of organism changes over time.
That is, it may increase or decrease in number over time.
It mainly depends on three factors.
- Reproduction of the organisms in the population
- Death of the organisms in the population
- Migration
If the rate of birth is higher than the rate of death, the population size will increase. Otherwise it will decrease.
Migration is also a factor that may sometimes affect the population. Due to major changes in living condition, organisms do move from one place to another.
E.g. When more food and water become available in a particular place, more organisms of the same kind may move in from other places to join the existing population. The population thus increases.
E.g. But in the reverse condition, some of the organisms may move to other places to look for food and water. As a result, the population decreases.
However, major changes in living condition of a particular place and migration do not happen regularly.
2. A Habitat - Where A Population Lives
Definition: The place where a population of an organism lives is called its habitat.
A population of monkeys lives in a jungle. The jungle is its habitat.
E.g. A population of penguins lives on an island near the South Pole of the Earth. The island is its habitat.
• We can usually find populations of several kinds of organisms in the same habitat.
E.g. We can find different kinds of water plants, fish, frogs, toads, and insects in a pond.
E.g. We can find different kinds of flowering plants, spiders, earthworms, snails, butterflies, bees, ants and some other insects in a garden.
• A habitat provides the organisms living there with everything they need to live. It can be any place that provides air, sunlight, warmth, shelters, food and water, supporting the lives of all organisms living there.
E.g. A habitat can be a stream, pond, garden, swamp, beach, rotting log, and even single tree.
• Because different habitats offer different living conditions, the populations of organisms found in one habitat are usually different from those found in another.
E.g. What populations of organisms do you expect to find in the following habitats?
(a) A field habitat
(b) A stream habitat
3. A Community - Populations Live Together
• The populations of organisms living in a habitat depend on one another in many ways. We say that they are interdependent.
a)Plants are the most important source of food in a habitat because they produce food. Some populations of animals depend directly on plants for food.
E.g. Grasshoppers and rabbits feed on plants.
b)Some populations of animals depend on plant eaters for food. We can also say that the populations of such animal eaters depend indirectly on plants for food.
E.g. Birds feed on some plant eaters such as grasshoppers while snakes feed on other plant eaters such as rabbits.
c)Many populations of animals such as birds, insects, monkeys and koala bears depend on plants for both shelter and protection from their enemies.
d)Fruit bearing plants depend on animals for pollination and dispersal of seeds.
E.g. Butterflies feed on the nectar of flowers. At the same time, they also help to carry pollen grains from one flower to another flower of the same kind.
E.g. Birds feed on fruits. When they eat, they swallow the seeds as well. The seeds are dispersed when they are passed out in the droppings of the birds later.
e)Animals also pass out waste that makes the soil more fertile. This helps the plants to grow healthily.
• All the interdependent populations living together in a habitat form a community. These populations include all kinds of plants and animals that depend on one another for survival.
• The size of a population can affect other populations living in the same habitat.
Some organisms feed on others. This means that there are both food producers and food consumers in a habitat.
When the populations of the food producers decrease, the populations of the food consumers will also decrease, because less food is available to them.
E.g. The population of seals living on an island depends on fish for food. If the populations of fish around the island decrease because of the change of weather, the seals will have less food. Many of the seals will die of starvation. As a result, the population of seals in the island community will decrease.
On the other hand, when the populations of food consumers decrease, the populations of food producers will increase. This is because there are less food consumers feeding on them.
E.g. If a large number of birds living in a wood die because of a certain disease, there are much fewer birds feeding on the insects. As a result, the populations & insects in the wood community will increase.
• Sometimes because of human activities, a complete habitat and its community can be destroyed.
Man often clears forests to get wood. Because we chop down a large number of trees in such human activities, organisms that live in trees will lose their shelter. Animas that feed on leaves or fruits will lose their food source. As a result, many of them will die due to starvation. The remaining ones will have to find a new habitat because they have lost their homes and there is little food left for them.
Man also burns down forests to claim lands for farming or other developments. A large number of organisms including plants and animals are burnt to death. Many of those that survive the fire will also die of hunger.
• We shall now study three different types of habitats and communities of organisms usually found in them.
* A Field Community
- A field seems like just an open space with only one kind of grass growing on it. What other organisms can we find in a field?
If we observe closely, we can find that there are in fact populations of several different plants living there.
E.g. Some of the common plants are mimosa, goose grass and vernonia.
There are populations of animals in the field community too.
E.g. Grasshoppers, spiders and snails are some of the animals that live among the plants in the field.
E.g. Earthworms, ants and woodlice are others that live in the soil.
• Sometimes we can also find animals such as birds, dogs and cats around. However, these animals do not belong to the field community because they do not live there. They are just visitors. They may enter the field for different reasons:
E.g. A bird may and on the field to feed on the insects.
E.g. A cat may come to the field because it sees that birds are around. Cats like to hunt for small animals such as birds, chicks, lizards and frogs.
E.g. A dog may visit the field to pass motion as it has been trained by its human master to do so.
*A Pond Community
A pond is an aquatic habitat, which means a water habitat. The organisms found in a pond thus live in water. In a pond community, we can find populations of plans and animals too.
• In Primary Four Science, we have studied water plants which are also called aquatic plants. A number of those aquatic plants that we have learnt can be found in a pond habitat. Let us quickly revise them here.
1)Floating plants
They are aquatic plants that float on water surface with their roots hanging down in the water.
E.g. Duckweeds and water hyacinths.
2) Submerged plants
They are aquatic plants that grow completely under water.
E.g. Hydrillas, and tape grass.
3) Partially submerged plants
They are aquatic plants that are anchored to the bottom of the pond, but with their upper parts emerging above the water.
E.g. Water lilies and arrowheads.
- Aquatic plants are very important to animals that live in the pond.
They provide food and oxygen to the animals.
They also provide shade and shelter to the animals and their eggs.
- Populations of animals in the pond community occupy different parts of the pond. We may group them as followings:
1) Animals found on the water surface of the pond.
E.g. Pondskaters and water boatmen.
2)Animals that live in the water or at the bottom of the pond.
E.g. Fish, tadpoles, backswimmers, damselfly larvae, midge larvae and water snails.
3)Some larger animals can also be found near the pond.Sometimes they too enter the pond.
E.g. Birds, dragonflies and frogs.
4)We can also find a large group of various tiny organisms living in the water. They are so small that we can only see them with a microscope. Many other aquatic animals in the pond community rely on these microorganisms for food.
*A Single Plant Community
Many small organisms live and reproduce on a single plant. They may spend their entire lives on the same plant. Therefore a single plant can also be a habitat by itself.
E.g. What are the organisms that form a community on a banana tree? We can usually find populations of caterpillars, snails, ants and slugs on the leaves of the banana tree.
E.g. There are other animals which are visitors to the banana tree community. These include lizards, spiders and birds. They do not live on the banana tree, but sometimes they may visit the banana tree for food or shade.
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