Ecology & Food Chains
Ecology and the Environment · 8 question types
Every organism is affected by two kinds of factor in its environment:
- Biotic factors are the living parts of the environment: other organisms.
- Abiotic factors are the non-living parts of the environment: physical and chemical conditions.
Whether a particular species can survive in a given habitat depends on both kinds. A polar bear cannot live in a tropical rainforest, not just because it has no penguins to eat, but because the temperature is too high.
Common abiotic factors
| Factor | Why it matters |
|---|---|
| Light intensity | Plants need light for photosynthesis. Low light limits plant growth, which limits the food available to everything else |
| Temperature | Affects the rate of every enzyme-controlled reaction. Outside an organism's tolerance range, the enzymes denature and cells stop working |
| Water availability | All life needs water for cellular reactions and (in animals) for blood and tissue fluid. Deserts have very little life; rainforests have very dense life |
| Oxygen availability | Aerobic organisms need oxygen for respiration. Important in water (where oxygen is much less abundant than in air) |
| Soil pH | Plants vary in the pH they tolerate. Acid-loving plants (heathers, blueberries) cannot grow in alkaline soil and vice versa |
| Mineral ion availability | Plants need nitrate, magnesium, phosphate and others to build proteins, chlorophyll and DNA |
| Wind speed | Strong winds dry out leaves, knock down small plants, and stop pollen reaching stigmas. Influences plant species at exposed sites |
| Salinity | Salt content of water or soil. Most organisms cannot tolerate sea-water salinity; specialised ones live only in salty habitats |
Naming an abiotic variable to control
Question: Give one or two abiotic variables that should be controlled in the investigation described, or name an abiotic factor that could affect where a plant species grows (1–2 marks).
Asked in 8 of the 23 papers, usually tacked onto a practical from another topic, so what is being tested is whether you know that abiotic means non-living. Name a physical or chemical condition: temperature is on almost every list, and light intensity, pH, carbon dioxide or oxygen concentration, water or humidity, mineral ions, salinity and wind speed all appear. The volume or mass of a material also counts (the volume of buffer, the mass of compost, the volume of milk). Do not name the variable the investigation is changing: when fertiliser was the independent variable, "fertiliser" was ignored. If you are asked to describe how to control one, the marks come in pairs, the variable and the means: temperature with a water bath, pH with a buffer.
Common biotic factors
| Factor | What it does |
|---|---|
| Availability of food | More food → more individuals can survive and reproduce |
| Predation | Predators control the population sizes of their prey species |
| Competition | Members of the same or different species compete for the same resources (food, water, space, mates) |
| Disease | Pathogens can rapidly reduce a population |
| Mutualism | Some species help each other (e.g. bees pollinating flowers in exchange for nectar) |
Case study: red squirrels vs grey squirrels
A famous case study of biotic factors. Native red squirrels lived across Britain for thousands of years. In the 1800s, grey squirrels were introduced from North America. Today red squirrels are confined to small parts of Scotland and a few islands, while greys dominate.
Three biotic factors explain the decline of the red squirrel:
- Competition for food and habitat. Grey squirrels are larger and can eat a wider range of food (including unripe acorns, which reds cannot digest). When the two species share an area, greys outcompete reds for the available food.
- Disease. Grey squirrels carry a virus called squirrelpox. Greys are immune to it; reds are not. Reds catching the virus from grey squirrels usually die within two weeks.
- Habitat preference. Greys do better in mixed broadleaf woodlands (which dominate southern Britain); reds do slightly better in coniferous forests (more common in the Scottish Highlands). This is why reds have hung on in those areas.
This is not an example of natural selection making one species better; it is the introduction of an invasive competitor changing the biotic factors faced by the natives.