Abiotic factors and population size revision notes

Edexcel International A Level Biology (YBI11), chapter 15: Ecology, specification point 5.12. Free lesson.

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Abiotic factors are non-living features of the environment that affect how many organisms live in an area and where they are found. Explain their effects as a chain from the factor, through what happens inside the organism, to survival and reproduction.

What an abiotic factor is

An abiotic factor is a non-living physical or chemical feature of the environment that affects organisms. It can change their numbers and distribution. Predators, competitors and diseases are biotic because they are living. Examples of abiotic factors include temperature, pH, oxygen, mineral ions, light, water, wind and soil type.

How temperature, pH, oxygen and minerals affect organisms

These factors mainly affect enzymes and metabolism, the chemical reactions taking place inside cells. If a reaction slows down or stops, an organism may not grow, survive or reproduce. Temperature: enzymes work more slowly in cold conditions and can denature if it is too hot. Few reptiles, which rely on outside heat, live in cold climates. pH: changes the shape of enzyme active sites and affects which mineral ions are available. Heather grows on acidic soils, while acidified lakes lose many fish species. Oxygen: is needed for aerobic respiration. Waterlogged soil contains…

How light changes the plants on a woodland floor

Light affects photosynthesis. In a beech wood, the amount of light reaching the floor changes as the trees come into leaf, so different plants can grow at different times. In early spring, the canopy is thin and light reaches the floor, so bluebells grow and flower. In late spring, the canopy closes. Less light reaches the ground, and the last bluebells are finishing. In summer, the canopy is dense and the floor is almost bare, apart from a few shade-tolerant plants.

How water, wind and soil type affect plants

These factors mainly affect water supply or water loss. Water is needed for cell reactions, photosynthesis and keeping plants firm, or turgid. Water: species on a high rocky shore must survive drying out. Wind: increases water loss by transpiration, which is evaporation from leaves, and can damage plants. Trees on exposed hilltops are fewer and stunted. Soil type: affects drainage, mineral ions and the amount of air in the soil. Sandy dunes drain quickly, so they suit marram grass, which copes with drought. Be specific about the soil property you mean: drainage, mineral…

Link an abiotic factor to survival

A clear explanation connects the environmental change to its effect inside the organism, then to whether it can survive and reproduce. Carbon for photosynthesis comes from carbon dioxide in the air, which enters through stomata, not from carbon in the soil. Organic matter in soil can still help plants: it holds water and releases mineral ions such as nitrate as it decomposes. In a stream downstream of a sewage outlet, bacteria decomposing sewage use dissolved oxygen for aerobic respiration. Stonefly larvae need oxygen for aerobic respiration too, so too little remains for them to survive.