What is ecology?
Ecology is the study of how living things (organisms) live with each other and with their surroundings. The word comes from Greek: oikos means home and logos means study. So it is "the study of homes".
Words to know:
- Habitat: the place where an organism lives, for example a pond or a tree hole.
- Population: all the organisms of one species in one place.
- Community: all the populations living together.
- Ecosystem: the community plus its non-living surroundings.
How do ecologists study nature?
- Observation: watch and note without disturbing.
- Field sampling: count organisms in a fixed square (a quadrat) or along a line (a transect), then work out the density.
- Experiment: change one factor and keep the rest same, in a lab or in a field plot.
- Monitoring: measure the same thing again and again for years, for example water quality of a lake.
- Modelling: use maths or a computer to predict what will happen.
Classification of ecological factors
An ecological factor is anything in the surroundings that affects an organism. We sort them in three groups:
- Abiotic (non-living): light, temperature, water and humidity, soil, wind, salt, oxygen, fire.
- Biotic (living): food, enemies (predators), competitors, parasites, partners such as pollinators.
- Anthropogenic (made by people): pollution, farming, cutting trees, building dams.
Factors do not act alone. Light changes temperature, temperature changes how much water is in the air, and so on. A factor can also change a lot with time: day and night, summer and winter.
Ecological laws: tolerance and the minimum
Law of tolerance (Shelford). For every factor, a species has a lowest limit, a highest limit and a best middle. Near the best point the organism grows well (optimum). Close to the limits it is stressed (pessimum). Outside the limits it dies.
Law of the minimum (Liebig). Growth is controlled not by the total of all resources, but by the one that is scarcest. This is the limiting factor. A field with plenty of water and light but no nitrogen grows poorly until nitrogen is added.
Two more ideas the 3D shows:
- The limiting factor can change. After you fix one, the next shortest stave limits you.
- Factors interact: a plant that gets less water can tolerate less heat.
Steno- and eurybiont species
Species differ in how wide their tolerance range is.
- Stenobiont species have a narrow range for one or more factors. "Steno" means narrow. Example: trout need cold water (stenothermal) and high oxygen.
- Eurybiont species have a wide range. "Eury" means wide. Example: carp, crows, rats and dandelions live in many places.
We add a word for the factor: stenothermal / eurythermal (temperature), stenohaline / euryhaline (salt), stenophagous / euryphagous (food). A species can be steno for one factor and eury for another.
Stenobionts are good bio-indicators: lichens vanish in polluted air, so seeing them tells us the air is clean.
Try it at home
Put cress seeds on wet cotton in four dishes: one in sunlight, one in shade, one dry, one in the fridge. After a week compare. You tested light, water and temperature, one at a time.
Key formulas and definitions
- Density = number of organisms ÷ area sampled (for example plants per m²)
- Law of tolerance: lowest limit < optimum < highest limit
- Law of the minimum: growth is set by the scarcest factor
- Steno = narrow range, Eury = wide range
- Factors: abiotic + biotic + anthropogenic
Worked examples
1. Sort into abiotic and biotic: sunlight, earthworm, soil pH, mango tree, rainfall, fungus.
Abiotic: sunlight, soil pH, rainfall (no life). Biotic: earthworm, mango tree, fungus (living).
2. A farmer has plenty of water, sunlight and nitrogen, but the soil has very little phosphorus. Crop growth is poor. Which law explains this and what should he do?
Law of the minimum. Phosphorus is the scarcest factor, so it limits growth. Adding phosphorus fertiliser lifts the limit, until the next scarcest factor takes over.
3. Species A lives between 12 and 18 °C. Species B lives between 2 and 38 °C. Which is eurythermal? What is the tolerance range of each?
Species B is eurythermal (wide range). A: 18 − 12 = 6 °C wide, B: 38 − 2 = 36 °C wide.
4. In a quadrat of 0.5 m × 0.5 m, a student counts 7 dandelion plants. What is the density per square metre?
Area = 0.5 × 0.5 = 0.25 m². Density = 7 ÷ 0.25 = 28 plants per m².
Common mistakes
- Thinking "abiotic" means "dead". Abiotic means never alive: sun, water, rocks. A dead leaf is biotic matter.
- Believing more of a good thing is always better. Too much water or heat also takes you to the pessimum zone.
- Mixing up the two laws. Tolerance is about the range of ONE factor. Minimum is about WHICH resource runs out first.
- Calling every wide-range species "strong". Eury only means it tolerates many conditions, not that it wins everywhere.