Remove One Kind of Animal and the Whole Place Changes
Learn to separate the living and non-living parts of a surrounding, tell habitat from population and community, see why every organism matters, and sort relationships into competition, predation, parasitism and mutualism.
What happens to a place if just one kind of animal disappears?
Far more than you would expect. Remove the frogs from a field and the insects they ate multiply, the snakes that ate the frogs go hungry, and the crops suffer — from a single absence.
Every organism is tied to others, which is why none of them is spare. This page covers everything in the CBSE Class 8 Science chapter's first part: biotic and abiotic components, habitat, population and community, why each organism matters, and the kinds of interaction.
Every organism is tied to others, which is why none of them is spare. This page covers everything in the CBSE Class 8 Science chapter's first part: biotic and abiotic components, habitat, population and community, why each organism matters, and the kinds of interaction.
What are the biotic and abiotic components of a surrounding?
Everything around an organism is its environment, and it divides into two.
- Biotic components — all the living things: plants, animals, birds, insects, fungi and microorganisms.
- Abiotic components — all the non-living things: air, water, soil, sunlight, temperature, humidity, minerals and rocks.
In a pond, the biotic parts are the fish, frogs, algae, water plants and insects; the abiotic parts are the water, dissolved oxygen, sunlight, mud and temperature.
Organisms sense and respond to their surroundings, and the responses are easy to observe:
- A sunflower turns its head towards the Sun
- A touch-me-not (chhui-mui) plant folds its leaves the instant it is touched
- Roots grow downward and shoots grow upward, responding to gravity and light
- Earthworms come to the surface after rain and retreat from light
- Cockroaches scatter when a light is switched on
- Animals migrate or hibernate as the season changes
- Our pupils narrow in bright light
The two sets are not independent, and that is the point of pairing them. The abiotic conditions decide which organisms can live somewhere — a cactus survives desert heat and little water, a lotus needs standing water — so changing sunlight, water or temperature changes the living community that a place can support.
- Biotic components — all the living things: plants, animals, birds, insects, fungi and microorganisms.
- Abiotic components — all the non-living things: air, water, soil, sunlight, temperature, humidity, minerals and rocks.
In a pond, the biotic parts are the fish, frogs, algae, water plants and insects; the abiotic parts are the water, dissolved oxygen, sunlight, mud and temperature.
Organisms sense and respond to their surroundings, and the responses are easy to observe:
- A sunflower turns its head towards the Sun
- A touch-me-not (chhui-mui) plant folds its leaves the instant it is touched
- Roots grow downward and shoots grow upward, responding to gravity and light
- Earthworms come to the surface after rain and retreat from light
- Cockroaches scatter when a light is switched on
- Animals migrate or hibernate as the season changes
- Our pupils narrow in bright light
The two sets are not independent, and that is the point of pairing them. The abiotic conditions decide which organisms can live somewhere — a cactus survives desert heat and little water, a lotus needs standing water — so changing sunlight, water or temperature changes the living community that a place can support.
What is the difference between habitat, population and community?
They describe three different scales, from a place to a group to everything living there.
- A habitat is the place where an organism lives, providing its food, water, shelter and the conditions it needs. A pond is the habitat of a fish; soil is the habitat of an earthworm; a tree is the habitat of a squirrel.
- A population is all the individuals of one kind of organism living in the same area at the same time. All the frogs in a pond form one population; all the mango trees in an orchard form another.
- A community is all the populations living together and interacting in an area — the frogs, fish, insects, algae and water plants of that pond, taken together.
And putting it all together:
- An ecosystem is the community together with its abiotic surroundings, interacting as a unit.
So a pond ecosystem is the fish, frogs, algae and insects plus the water, mud, sunlight and dissolved gases. A forest, a grassland, a desert and even an aquarium are ecosystems.
Worked through a single example — a pond:
- Habitat — the pond itself
- Population — all the catfish in it
- Community — all its living things together
- Ecosystem — that community plus the water, light and mud
The word that separates population from community is one kind. A population is a single species; a community is many — which is why "the population of a pond" is an incomplete phrase until you say population of what.
- A habitat is the place where an organism lives, providing its food, water, shelter and the conditions it needs. A pond is the habitat of a fish; soil is the habitat of an earthworm; a tree is the habitat of a squirrel.
- A population is all the individuals of one kind of organism living in the same area at the same time. All the frogs in a pond form one population; all the mango trees in an orchard form another.
- A community is all the populations living together and interacting in an area — the frogs, fish, insects, algae and water plants of that pond, taken together.
And putting it all together:
- An ecosystem is the community together with its abiotic surroundings, interacting as a unit.
So a pond ecosystem is the fish, frogs, algae and insects plus the water, mud, sunlight and dissolved gases. A forest, a grassland, a desert and even an aquarium are ecosystems.
Worked through a single example — a pond:
- Habitat — the pond itself
- Population — all the catfish in it
- Community — all its living things together
- Ecosystem — that community plus the water, light and mud
The word that separates population from community is one kind. A population is a single species; a community is many — which is why "the population of a pond" is an incomplete phrase until you say population of what.
Why does every organism in a community matter?
Because each is connected to others as food, as a controller of numbers, or as a provider of some service — so removing one sets off changes that spread.
Predicting what would happen if one kind disappeared:
- If all the frogs vanished from a field — the insects they ate would multiply and damage crops, while the snakes and birds that ate frogs would lose food and decline.
- If all the earthworms vanished from the soil — the soil would not be turned over or aerated, water would drain poorly, dead matter would break down more slowly and the soil would lose fertility.
- If all the bees and butterflies vanished — many flowering plants would not be pollinated, so fruit and seed production would collapse, affecting every animal that eats them.
- If all the decomposers vanished — dead plants and animals would pile up unchanged, and the nutrients locked inside would never return to the soil for plants to reuse.
- If the grass were removed from a grassland — the deer would starve, and so would the tigers that eat deer.
- If all the tigers were removed — the deer would multiply unchecked, overgraze the grass, and eventually starve when the grass gave out.
Snakes make the point in a form Indian farmers know well: killing snakes allows rats to multiply, and the rats then destroy far more stored grain than the snakes ever threatened.
The pattern in every case is the same, and it is what the question is really testing. A change never stops at the organism removed — it passes along the connections, often in the direction people do not expect, which is why even a predator nobody likes turns out to be doing necessary work.
Predicting what would happen if one kind disappeared:
- If all the frogs vanished from a field — the insects they ate would multiply and damage crops, while the snakes and birds that ate frogs would lose food and decline.
- If all the earthworms vanished from the soil — the soil would not be turned over or aerated, water would drain poorly, dead matter would break down more slowly and the soil would lose fertility.
- If all the bees and butterflies vanished — many flowering plants would not be pollinated, so fruit and seed production would collapse, affecting every animal that eats them.
- If all the decomposers vanished — dead plants and animals would pile up unchanged, and the nutrients locked inside would never return to the soil for plants to reuse.
- If the grass were removed from a grassland — the deer would starve, and so would the tigers that eat deer.
- If all the tigers were removed — the deer would multiply unchecked, overgraze the grass, and eventually starve when the grass gave out.
Snakes make the point in a form Indian farmers know well: killing snakes allows rats to multiply, and the rats then destroy far more stored grain than the snakes ever threatened.
The pattern in every case is the same, and it is what the question is really testing. A change never stops at the organism removed — it passes along the connections, often in the direction people do not expect, which is why even a predator nobody likes turns out to be doing necessary work.
What are competition, predation, parasitism and mutualism?
Four kinds of relationship, sorted by who benefits.
Competition — two or more organisms need the same limited resource, and both are hindered.
- Plants in a field competing for sunlight, water and soil minerals
- Two male birds competing for the same nesting site
- Weeds competing with crops, which is why fields are weeded
Predation — one organism, the predator, kills and eats another, the prey. The predator gains, the prey dies.
- A lion hunting a deer; a frog catching an insect
- A snake eating a rat; an eagle catching a fish
- Predation keeps prey numbers in check, so it benefits the community as a whole
Parasitism — one organism, the parasite, lives on or inside another, the host, and gains at the host's expense. The host is harmed but usually not killed outright.
- A tapeworm or roundworm inside the human intestine
- Lice on the scalp, ticks on a dog, leeches on cattle
- Cuscuta (amarbel) growing on a shrub and taking its food
Mutualism — both organisms benefit.
- A bee gets nectar from a flower while carrying pollen between plants
- Rhizobium bacteria in the root nodules of a legume get shelter and food while fixing nitrogen for the plant
- Lichen, in which an alga makes food and a fungus provides shelter and water
- Birds that eat ticks off cattle, gaining food while relieving the cattle
The pair often confused is predation and parasitism, and the difference is speed and outcome. A predator kills its prey quickly and moves on; a parasite keeps its host alive for a long time, because a dead host is no use to it — which is why a tapeworm weakens a person over months rather than killing them at once.
Competition — two or more organisms need the same limited resource, and both are hindered.
- Plants in a field competing for sunlight, water and soil minerals
- Two male birds competing for the same nesting site
- Weeds competing with crops, which is why fields are weeded
Predation — one organism, the predator, kills and eats another, the prey. The predator gains, the prey dies.
- A lion hunting a deer; a frog catching an insect
- A snake eating a rat; an eagle catching a fish
- Predation keeps prey numbers in check, so it benefits the community as a whole
Parasitism — one organism, the parasite, lives on or inside another, the host, and gains at the host's expense. The host is harmed but usually not killed outright.
- A tapeworm or roundworm inside the human intestine
- Lice on the scalp, ticks on a dog, leeches on cattle
- Cuscuta (amarbel) growing on a shrub and taking its food
Mutualism — both organisms benefit.
- A bee gets nectar from a flower while carrying pollen between plants
- Rhizobium bacteria in the root nodules of a legume get shelter and food while fixing nitrogen for the plant
- Lichen, in which an alga makes food and a fungus provides shelter and water
- Birds that eat ticks off cattle, gaining food while relieving the cattle
The pair often confused is predation and parasitism, and the difference is speed and outcome. A predator kills its prey quickly and moves on; a parasite keeps its host alive for a long time, because a dead host is no use to it — which is why a tapeworm weakens a person over months rather than killing them at once.
Exam tip
Exam tip: saying who benefits in every interaction
Interaction questions are marked on who gains and who loses, so state it.
Write it as a pair: mutualism — both benefit; parasitism — the parasite benefits and the host is harmed; predation — the predator benefits and the prey is killed; competition — both are hindered. Then give the example.
When naming an example, name both organisms: Rhizobium and the leguminous plant, not just "bacteria".
Keep the three scales distinct — habitat is a place, population is one kind of organism, community is all of them, and ecosystem adds the abiotic surroundings.
For a "what if it disappeared" question, trace at least two further effects, one in each direction along the food connection.
And remember the parasite keeps its host alive, which is the line that separates it from a predator.
Write it as a pair: mutualism — both benefit; parasitism — the parasite benefits and the host is harmed; predation — the predator benefits and the prey is killed; competition — both are hindered. Then give the example.
When naming an example, name both organisms: Rhizobium and the leguminous plant, not just "bacteria".
Keep the three scales distinct — habitat is a place, population is one kind of organism, community is all of them, and ecosystem adds the abiotic surroundings.
For a "what if it disappeared" question, trace at least two further effects, one in each direction along the food connection.
And remember the parasite keeps its host alive, which is the line that separates it from a predator.
Did you know
Why does killing snakes end up costing a farmer more grain?
Because the snakes were holding down the rats.
A snake in a field eats rats, and rats eat stored and standing grain. Remove the snakes and nothing is preying on the rats, so their numbers climb — and a growing rat population destroys far more grain than the snakes were ever suspected of costing.
It is a clean illustration of the chapter's central point. The value of an organism is not in how useful it looks, but in the connections it maintains — and those only become visible once they are broken.
A snake in a field eats rats, and rats eat stored and standing grain. Remove the snakes and nothing is preying on the rats, so their numbers climb — and a growing rat population destroys far more grain than the snakes were ever suspected of costing.
It is a clean illustration of the chapter's central point. The value of an organism is not in how useful it looks, but in the connections it maintains — and those only become visible once they are broken.
Key takeaways
Ecosystems and interactions: quick revision
- Biotic components are the living things; abiotic are air, water, soil, sunlight and temperature — and the abiotic conditions decide which organisms can live there.
- Organisms sense and respond: sunflowers turn to the Sun, touch-me-not folds its leaves, earthworms avoid light.
- A habitat is a place, a population is all individuals of one kind, a community is all populations together, and an ecosystem is the community plus its abiotic surroundings.
- Removing one organism spreads changes along the connections — losing frogs means more insects and fewer snakes; losing decomposers means nutrients never return to the soil.
- Competition hinders both, predation kills the prey, parasitism harms the host without killing it, and mutualism benefits both.
- Examples: weeds and crops (competition), snake and rat (predation), tapeworm and human (parasitism), bee and flower or Rhizobium and legume (mutualism).
You will remember all of this far better after answering five questions on it than after reading it twice.
- Organisms sense and respond: sunflowers turn to the Sun, touch-me-not folds its leaves, earthworms avoid light.
- A habitat is a place, a population is all individuals of one kind, a community is all populations together, and an ecosystem is the community plus its abiotic surroundings.
- Removing one organism spreads changes along the connections — losing frogs means more insects and fewer snakes; losing decomposers means nutrients never return to the soil.
- Competition hinders both, predation kills the prey, parasitism harms the host without killing it, and mutualism benefits both.
- Examples: weeds and crops (competition), snake and rat (predation), tapeworm and human (parasitism), bee and flower or Rhizobium and legume (mutualism).
You will remember all of this far better after answering five questions on it than after reading it twice.