Human Impact on the Environment
Ecology and the Environment · 8 question types
Sewage is the waste water from homes, businesses and industry, containing human waste, food scraps, detergents, and sometimes industrial chemicals. When raw or poorly-treated sewage enters a water body, it causes a similar oxygen-depletion problem to fertiliser-driven eutrophication.
How sewage damages aquatic ecosystems
- Sewage contains lots of organic matter that bacteria can digest.
- Aerobic bacteria multiply rapidly, breaking down the organic matter.
- As they respire, they use up the dissolved oxygen in the water.
- Oxygen-sensitive species (mayflies, stoneflies, trout) die or move away.
- Only species that can tolerate low oxygen survive (sludge worms, blood worms, rat-tailed maggots, certain bacteria).
- Biodiversity collapses.
Sewage also contains pathogenic bacteria and viruses that can cause disease in humans who drink the water, swim in it, or eat shellfish from it.
Bioindicators: using species to measure pollution
You can tell how polluted a water body is by looking at which species are living in it. Different species have different tolerances for oxygen and pollution:
| Species found | What it tells you about the water |
|---|---|
| Mayfly nymphs, stonefly nymphs, freshwater shrimps | High dissolved oxygen, very low pollution (clean water) |
| Caddisfly larvae, watercress, riffle beetles | Moderate oxygen, low pollution |
| Bloodworms, water lice, leeches | Low oxygen, moderate pollution |
| Sludgeworms (Tubifex), rat-tailed maggots | Very low oxygen, severe pollution |
| No life at all | Severe pollution; oxygen completely depleted |
Sampling a stream with a kick net and identifying what is there is a standard way to assess water quality. Bioindicator species are useful because they reflect conditions over weeks or months, not just a single moment (which a chemical test would).
Lichens as air-quality bioindicators
The same idea works for air. Different lichens (small symbiotic organisms growing on rocks and tree bark) have different tolerances for sulfur dioxide:
| Lichen type | What it indicates |
|---|---|
| Bushy and shrubby lichens | Clean air |
| Leafy lichens | Slightly polluted air |
| Crusty lichens only | More polluted air |
| No lichens at all (just algae) | Very polluted air, lots of SO₂ |
A walk through a city centre and a walk through unpolluted countryside will quickly show this pattern.