Picture a small lake in cottage country on a summer morning. Mist rises off the water, a loon dives for a perch, frogs call from the reeds, and the rocks along the shore are slowly being cracked apart by tree roots. Every one of those things is connected. The mist is water moving into the air. The perch needs oxygen dissolved in the water. The trees need minerals from the soil and rock. If you change one part, say the water warms by a few degrees or a farm upstream adds fertilizer, the other parts change too. Ecology is the study of how living things interact with each other and with their surroundings, and this unit is about those connections: how they keep ecosystems working, and what happens when people push them out of balance.
Earth's four spheres
Scientists often split Earth into four spheres: big parts of the planet that each contain a different kind of material. They are not separate shells you could peel apart. They overlap and constantly trade matter and energy with each other.
- Atmosphere: the layer of gases around Earth. Dry air is about 78% nitrogen gas, 21% oxygen gas and about 1% argon, with tiny amounts of other gases such as carbon dioxide (about 0.04%, which is over 420 parts per million). Water vapour is also present, and its amount changes with the weather.
- Hydrosphere: all of Earth's water, in oceans, lakes, rivers, groundwater, ice, snow and clouds. About 97% of it is salt water in the oceans. Only about 3% is fresh, and most of that is frozen in ice sheets and glaciers or stored underground.
- Lithosphere: the solid, rocky outer layer of Earth (the crust and the very top of the mantle), including the rocks, minerals and soil we stand on.
- Biosphere: every living thing on Earth plus the places where life exists. It overlaps the other three spheres: birds fly in the atmosphere, fish swim in the hydrosphere, and worms and roots live in the soil of the lithosphere.
| Spheres interacting | Everyday example | What moves or changes |
|---|---|---|
| Hydrosphere → atmosphere | Water evaporates from Lake Ontario on a hot day | Water moves into the air as water vapour |
| Biosphere → lithosphere | Tree roots grow into cracks and split rock | Rock breaks into smaller pieces, helping form soil |
| Biosphere ↔ atmosphere | In sunlight, leaves take in carbon dioxide and give off oxygen; animals (and plants, through respiration) take in oxygen and give off carbon dioxide | Gases are exchanged between living things and the air |
| Hydrosphere → lithosphere | Spring rain runs off a field into a creek | Water carries soil and dissolved minerals downhill |
| Atmosphere → hydrosphere | Wind whips up waves on Georgian Bay | Energy and oxygen are mixed into the water |
| Lithosphere → atmosphere | A volcano erupts | Gases and ash are added to the air |
What an ecosystem is
An ecosystem is all the living things in an area together with the non-living parts of their environment, all interacting. An ecosystem can be as big as the boreal forest that stretches across northern Canada or as small as a rotting log. The living parts are called biotic factors (from the Greek bios, life): plants, animals, fungi, bacteria and other organisms. The non-living parts are called abiotic factors (a- means "not"): sunlight, temperature, water, air, soil, minerals and pH. A species' habitat is the place where it lives and finds what it needs, and its niche is its role in the ecosystem: what it eats, what eats it, and how it affects its surroundings.
- Plants such as sugar maples and cattails
- Animals such as white-tailed deer and loons
- Fungi such as mushrooms and moulds
- Bacteria and other microorganisms in the soil and water
- Interactions like predation, competition and pollination
- Sunlight and day length
- Temperature and precipitation
- Water and the oxygen dissolved in it
- Soil type and minerals such as nitrates
- Acidity (pH) of soil or water
- Organism: one individual living thing, such as one snowshoe hare.
- Population: all the members of one species living in the same area at the same time, such as all the snowshoe hares in a park.
- Community: all the populations of different species living in the same area, such as the hares, lynx, spruce trees, mosses and fungi of that park.
- Ecosystem: the community plus the abiotic factors it interacts with (soil, water, sunlight, climate).
- Biosphere: all the ecosystems on Earth together.