Abiotic & Biotic Factors

Every habitat — a pond, a wood, a rocky shore — provides a set of conditions that decide which organisms can live there and how many of them survive. These conditions fall into two groups.

Abiotic factors are the non-living parts of the environment — the physical and chemical conditions. The word abiotic literally means "without life". Examples include light, temperature, water, oxygen and carbon dioxide levels, soil pH and mineral ions in the soil.

Biotic factors are the effects caused by other living organisms. The word biotic means "to do with life". Examples include food availability, predators, competition between organisms and disease.

Two important words describe the effect these factors have:

Term	Meaning
Population size	The total number of individuals of one species living in a habitat.
Distribution	Where organisms of a species are found (which areas of a habitat, or which parts of the world).

A factor that helps organisms survive and reproduce usually makes the population larger and lets it spread into more areas. A factor that harms or kills organisms usually makes the population smaller and limits where it can live.

Exam tip. A classic one-mark question is "is this an abiotic or a biotic factor?". Sort it by asking: is this thing alive? Temperature, light and pH are non-living → abiotic. A predator, a competitor or a disease involves other living things → biotic.

Each abiotic factor sets a limit on which organisms can thrive and how big their populations can grow. You should be able to explain the effect of each one named in the specification.

• Light. Plants need light for photosynthesis. More light (up to a point) means more food made, so plants grow better and their populations increase. On a woodland floor the few plants that grow are those adapted to low light, such as ferns and bluebells — light limits their distribution.
• Temperature. Enzymes work fastest at their optimum temperature. If it is too cold, reactions are slow and growth/reproduction fall; if it is too hot, enzymes denature. Each species has a temperature range it can survive in, which decides where it lives — e.g. polar bears in cold regions, cacti in hot deserts.
• Water. All organisms need water for their cells and reactions. In a dry habitat only drought-adapted species survive, so populations of most organisms are small; where water is plentiful, populations can be large.
• Oxygen and carbon dioxide. Animals need oxygen for respiration. In water, dissolved oxygen levels decide which animals live there — fast-flowing streams are high in oxygen and support mayfly larvae, while polluted, low-oxygen water supports far fewer species. Plants also need carbon dioxide for photosynthesis; more CO₂ can increase plant growth.
• Soil pH. Soil that is too acidic or too alkaline stops many plants growing because it affects enzymes and how easily roots can take up minerals. Some plants are adapted to particular pH values — for example, bilberry grows on acidic soils — so pH affects plant distribution.
• Mineral ions. Plants need mineral ions such as nitrate (for proteins) and magnesium (for chlorophyll) from the soil. Where the soil is rich in these minerals, plants grow well and populations rise; poor soils support fewer and smaller plants.

Exam tip. Always link the factor to a process. Light → photosynthesis. Temperature → enzymes. Oxygen → respiration. Nitrate → making proteins/growth. Showing the cause and the effect on numbers scores the explain marks.

Biotic factors come from the living members of a community interacting with one another. The four you must know are below.

• Food availability. When there is plenty of food, organisms survive and reproduce more, so the population grows. When food is scarce, more individuals die or fail to breed, so the population falls. Food availability is often the main thing limiting how big a population can become.
• Predators. A predator is an animal that hunts and eats another animal (the prey). If predator numbers rise, more prey are eaten and the prey population falls. Fewer prey then means less food for the predators, so the predator population falls too — this is why predator and prey numbers often rise and fall in linked cycles.
• Competition. When organisms need the same limited resource, they compete for it. Plants compete for light, water and minerals; animals compete for food, water, space (territory) and mates. The better-adapted competitors survive, so strong competition keeps populations lower and can push weaker species out of an area.
• Disease. Pathogens (disease-causing microorganisms) can spread quickly, especially when a population is large and crowded. An outbreak of disease can kill many individuals and cause a sharp fall in population size.

These factors do not act alone — they usually work together. For example, when a prey population becomes very large, food runs short (food availability), competition increases, and disease spreads more easily in the crowded population, so numbers fall back again.

Exam tip. For predator–prey questions, remember the order of events: more predators → fewer prey → less food for predators → fewer predators → prey recover. Describing this two-way link is the key to the higher marks.

It helps to keep the two outcomes clear in your mind.

• Population size is about numbers — how many individuals of a species live in the habitat. Factors that increase survival and reproduction (good food supply, warmth within range, plenty of light or minerals, few predators or diseases) push the population up. Factors that increase death or reduce breeding (cold, drought, low oxygen, many predators, strong competition, disease) push it down.
• Distribution is about where — which areas a species can occupy. Distribution is mainly limited by whether the conditions are tolerable. A plant adapted to acidic soil is only found where the soil is acidic; a fish needing high oxygen is only found in fast, well-oxygenated water. So distribution is set by the abiotic and biotic conditions an organism can survive in.

This is the diagram you can picture: a population sits at a certain size, with abiotic and biotic factors pushing it up or down like weights on a balance.

Exam tip. Read whether the question asks about how many (population size) or where (distribution) — answer the one being asked, and always justify your answer with a named factor and the process it affects.

In the exam you will often be given data or a scenario — a graph of predator and prey numbers, a table of plant numbers at different soil pH, or a description of an oxygen-poor pond — and asked to explain what is happening. A reliable way to answer is the cause → process → effect chain:

1. Name the factor and say whether it is abiotic or biotic.
2. Link it to a process in the organism (photosynthesis, respiration, enzyme activity, making proteins, finding food, etc.).
3. State the effect on the population size or distribution (numbers rise/fall, found here/not there).

Worked through for two examples:

• Few plants grow under a dense tree canopy. Factor = light (abiotic). Process = less light reaches the floor, so less photosynthesis and less food made. Effect = fewer plants survive there, so plant population is small and only shade-tolerant species grow → limited distribution.
• A pond becomes polluted and fish die. Factor = dissolved oxygen (abiotic), often linked to disease/competition (biotic). Process = low oxygen means fish cannot respire enough. Effect = the fish population falls and oxygen-needing species disappear from that part of the pond.

The key habit: never just name the factor — always say which process it changes and what happens to the numbers or distribution as a result.

Exam tip. When a graph is provided, quote a value or trend from it ("between weeks 4 and 8 the prey number fell from 80 to 30") and then explain it with a factor. Using the data is what lifts an answer into the top band.