Biology: ecosystems and population dynamics (grade 9) · lesson

Populations and their limits

💡 A population grows until limited resources hold it near the carrying capacity.

A population changes by births minus deaths (plus or minus animals moving in or out). When it levels off for years, births and deaths are balanced. That level is the carrying capacity.

Limiting factors can be biotic (living), like predators and disease, or abiotic (nonliving), like temperature and oxygen. Some get worse with crowding, such as disease that spreads between animals. Others, like a flood or a freeze, strike no matter how crowded.

To count fish you can't see, tag some and release them. Later, the share of tagged fish in a new catch should match the share in the whole pond.

🧠 Let's work it out

Tag 40.
Later catch 50, and 10 are tagged: that is 1/5.
If 40 tagged fish are 1/5 of the pond, the pond holds 40 × 5 = 200.

What do you think comes next?

carrying capacity: the largest population an area can support over time
abiotic: nonliving
biotic: living

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Energy flows up a food chain

💡 Only about 10% of the energy at one level passes to the next, so there is less energy at each higher level.

Energy enters most food chains as sunlight captured by producers, like grass. It passes to the animal that eats them, then to the animal that eats that one.

Most energy at each level is used for living or lost as heat. Only about 10% is passed up. That is why there are fewer top predators than plants.

🧠 How to think about it

Producers 20,000 kJ → primary consumers 10% = 2,000 kJ → secondary consumers 10% = 200 kJ. Take 10% once for each step up.

producer: an organism that makes its own food, usually by photosynthesis
consumer: an organism that eats other organisms

Matter is recycled

💡 Atoms are reused: decomposers return nutrients, and carbon moves between air and living things.

Fungi and bacteria are decomposers. They break down dead plants and animals and return nutrients to the soil, so producers can use them again.

Carbon dioxide enters the air through respiration and burning fuels. It leaves the air through photosynthesis, and carbon is locked away when shells and rock form.

🧠 Let's work it out

For each process, ask which way the carbon moves.
Breaking down fuel or food releases it to the air.
Building sugar or shells takes it out.

What do you think comes next?

decomposer: an organism that breaks down dead matter

Changes ripple through food webs

💡 A change at one level of a food chain spreads up and down to the other levels.

In a food chain, each level eats the one below. If one level shrinks, the level it eats is eaten less and grows, and so on down the chain.

Big fish eat small fish, small fish eat zooplankton, and zooplankton eat algae. Remove big fish and small fish increase. They eat more zooplankton, so zooplankton drop. With fewer grazers, algae increase.

🧠 How to think about it

Work one level at a time, and write up or down at each step. Each level usually moves opposite to the level just above it.

Stability and recovery

💡 Ecosystems with more species are more stable, and damaged land recovers in stages called succession.

When an ecosystem has many species, some are likely to survive a drought or disease. That keeps the system working. With only a few species, one disaster can wipe out most of the plant cover.

After a fire that leaves soil, secondary succession starts: grasses and weeds first, then shrubs, then trees. Primary succession starts on bare rock with no soil and takes much longer.

🧠 Let's work it out

Ask what survived.
If the soil is still there, fast-growing plants return first.
If even the soil is gone, life must start over on rock.

What do you think comes next?

biodiversity: the variety of species in an area
succession: the gradual regrowth of an ecosystem in stages