Biotic and Abiotic Factors — Ecology & Food Chains | IGCSE Biology

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 12% of your exam marks.

stable

High

Stable12%

Food chains, energy transfer, and ecological definitions are regularly tested, often as short-answer questions.

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

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.