Abiotic & Biotic Factors (Edexcel IGCSE Science (Double Award)): Revision Note
Exam code: 4SD0
Abiotic & Biotic Factors
An abiotic factor is a non-living factor within an environment
e.g. temperature, light intensity and wind speed
How abiotic factors affect population size and distribution
Abiotic factor | How abiotic factor affects community | Example |
|---|---|---|
Light intensity | Light is needed for photosynthesis. If light levels are low, plants grow more slowly and produce less food for herbivores. This can affect the entire food chain | In dense forests, only shade-tolerant plants can survive on the darker forest floor, while sun-loving plants grow in open areas or at the canopy level. |
Temperature | Temperature affects the rate of enzyme-controlled reactions in organisms. If it’s too cold or too hot, enzymes don’t work efficiently, which reduces survival and reproduction rates. | Polar bears are adapted to cold Arctic conditions — their thick fur and fat layer help them survive. They cannot live in warmer climates, so their distribution is limited to cold regions. |
Water availability | Water is essential for life. The amount of available water affects the types of plants and animals that can survive in an ecosystem. | Cacti and other succulents are adapted to dry desert conditions by storing water and reducing water loss. Amphibians, however, need moist environments to reproduce and cannot live in deserts. |
Soil quality (mineral content and pH) | Soil provides water, minerals, and a place for plants to anchor. Poor or acidic soils can limit plant growth, which affects the whole food web. | Heather grows well on acidic moorland soils, but grasses and many crops do not. This influences which animal species can live in that habitat. |
Oxygen availability (in aquatic habitats) | Some aquatic animals (such as fish) can only survive in water with high oxygen concentrations. | In warm, stagnant ponds, oxygen levels can drop, causing fish and other animals to die or migrate to better-oxygenated areas. |
Pollution | High levels of pollution can damage habitats and kill sensitive species, reducing biodiversity. | Lichens are sensitive to air pollution (e.g. sulfur dioxide). Few lichens are found in areas with high air pollution, so their presence indicates clean air. |
Worked Example
Plants of the same species were grown in tanks with different CO2 levels. Their height was measured after 3 weeks and an average taken.
What conclusion can you draw from this graph about the effect of CO2 concentration on plant growth rate? Explain your answer
Example exam answer
As CO2 concentration increases, average plant height also increases. For example, at 5% CO2, the average plant height was 10cm, but at 30% CO2, the average plant height was 60cm.
This shows that the higher the CO2 concentration, the greater the plant growth rate. This is because CO2 is used by plants for photosynthesis, which allows the plant to produce glucose for energy to grow.
Biotic factors
A biotic factor is a living factor in the environment
e.g. competition, predation and disease
How biotic factors affect the population size and distribution of organisms
Biotic factor | How biotic factor affects community | Example |
|---|---|---|
Availability of food | More food means organisms have a higher chance of surviving and reproducing. This allows populations to increase. When food is scarce, only the best-adapted organisms survive and reproduce. | Rainforest ecosystems have a rich food supply that supports a wide variety of species. Deserts, by contrast, have limited food sources, so fewer species can survive there. |
New predators | In balanced ecosystems, predators catch enough prey to survive but not so many that they wipe out the prey population. If a new predator is introduced, it can disrupt this balance and cause prey populations to decline. | Red foxes were introduced to Australia in the 1800s for recreational hunting and have caused the decline of many native species, such as small mammals and birds. This has also reduced the food supply for native predators. |
New pathogens | If a new pathogen enters an ecosystem, the populations living there may have no immunity or resistance to it, causing their numbers to fall sharply or be wiped out entirely. | Ash dieback disease, caused by the fungus Hymenoscyphus fraxineus, has infected millions of ash trees in Europe since it was introduced from Asia. Many native ash populations have been severely reduced. |
Competition | If two species compete for the same resource(s) and one is better adapted, that species will outcompete the other. The less well-adapted species may decline or even become extinct if its population falls too low to breed successfully. | North American grey squirrels were introduced to the UK in the 1800s and have outcompeted native red squirrels for food and nesting sites. Grey squirrels also carry a virus that red squirrels are not resistant to. |
Worked Example
A study recorded the number of red and grey squirrels in a particular woodland habitat for 20 years. Grey squirrels were introduced to the habitat in year 6 of the study.
What conclusion can you draw from this graph about the effect of introducing grey squirrels to a habitat that is occupied by red squirrels? Explain why this might have occurred.
Example exam answer
As the number of grey squirrels increases, the number of red squirrels decreases. This might have occurred because the two squirrel species are competing for one or more of the same resources.
Grey squirrels are better adapted to use these resources and have outcompeted the red squirrels until eventually there are too few red squirrels left to breed successfully and there are none left in the habitat.
Examiner Tips and Tricks
You should be able to extract and interpret information from charts, graphs and tables relating to the effect of biotic and abiotic factors on organisms within a community
When answering questions that refer to a chart, graph or table, remember to reference specific figures from the data to support your answer, as seen in the example above.
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