Ecosystems and change over time (grade 7) · lesson

How species interact

💡 Species can help each other, harm each other, or compete, and each kind of relationship has a name.

In mutualism, both species benefit. Bees get nectar, and flowers get their pollen carried.

In parasitism, one species benefits and the other is harmed. A tick feeds on a dog's blood.

In predation, one animal eats another. In competition, both need the same limited resource. In commensalism, one benefits and the other is not affected.

🧠 Let's work it out

For each species, ask: does it gain, lose, or stay the same?
Gain and gain is mutualism.
Gain and harm is parasitism or predation.
Gain and no effect is commensalism.

What do you think comes next?

mutualism: both species benefit
parasitism: one benefits and the host is harmed
commensalism: one benefits and the other is not affected

🔗 Khan Academy: Interactions in ecosystems · Khan Academy ↗

Food webs

💡 In a food web, a change to one species affects the species that eat it and the species it eats.

Producers, like plants, make their own food from sunlight. Consumers eat other living things. Decomposers break down dead things.

A food web shows who eats whom. Many food chains overlap.

If a predator is removed, its prey usually grow in number at first, since fewer are eaten.

🧠 Let's work it out

Follow the arrows.
Grass is eaten by rabbits.
Foxes eat rabbits.
Remove the foxes, and nothing else eats the rabbits.

What do you think comes next?

producer: a living thing that makes its own food from sunlight
consumer: a living thing that eats others for food
decomposer: a living thing that breaks down dead matter

Energy flow

💡 Energy flows from the sun through a food chain, and only about 10% passes to each next level.

Energy enters a food chain from the sun. Plants capture it. Then it passes to each animal that eats.

Each living thing uses most of its energy to move, grow, and stay warm. Much of it leaves as heat. Only about 10% passes to the next level.

That is why there is less energy, and usually fewer animals, at each higher level.

🧠 Let's work it out

Take 10% at each step.
Grass has 10,000 units.
A rabbit gets 10% of that: 1,000.
A fox gets 10% of 1,000: 100.

What do you think comes next?

food chain: a path showing how energy passes from one living thing to the next

🔗 Khan Academy: Matter and energy in ecosystems · Khan Academy ↗

Matter cycles

💡 Matter is reused: decomposers break down dead things and return nutrients and carbon to be used again.

Unlike energy, matter is not lost. It cycles round and round.

A tree takes in carbon dioxide and uses the carbon to build leaves. When a leaf falls, fungi and bacteria break it down.

The decomposers return carbon dioxide to the air and nutrients to the soil. Plants use them again. A rotting log slowly turns into soil this way.

🧠 Let's work it out

Follow one carbon atom.
Air → tree leaf → fallen leaf → fungus → back to the air.
It ends where it started, ready to be used again.

What do you think comes next?

nutrient: a substance living things need to grow

Changes to ecosystems

💡 Changes like new roads or new species can shrink populations, and good designs protect the habitat living things need.

A road can split a forest into smaller pieces. Animals that need a large area lose habitat, and some are hit by cars.

An invasive species is one brought in from somewhere else. It can eat the food that native animals need, so native populations shrink.

People can design solutions. Trees along a river shade the water and hold the soil, which keeps fish habitat healthy.

🧠 Let's work it out

To judge a plan, ask what it does to the habitat.
Does it keep water clean and cool?
Does it let animals move where they need to go?

What do you think comes next?

habitat: the place where a living thing finds what it needs
invasive species: a species brought into a new place where it causes harm