Abiotic factors are the non-living physical and chemical conditions of an environment; biotic factors are the living components — other organisms that interact with a species. Together they determine where organisms can survive, how large their populations grow, and how ecosystems change over time.
What are abiotic factors?
Abiotic factors are the non-living elements of an environment that affect the organisms living there. They include physical conditions such as temperature and light, and chemical conditions such as pH and salinity.
| Abiotic factor | How it affects organisms |
|---|---|
| Temperature | Controls metabolic rate; extreme cold or heat can denature enzymes |
| Light intensity | Limits photosynthesis in plants; affects behaviour and reproduction in animals |
| Water availability | Essential for all metabolic reactions; drought reduces population size |
| Oxygen levels | Low dissolved oxygen kills aquatic organisms; altitude affects terrestrial ones |
| Carbon dioxide levels | Limits photosynthesis when below the compensation point |
| Soil mineral content | Nitrate, phosphate, and magnesium affect plant growth and health |
| Soil pH | Affects enzyme activity in root cells and mineral ion solubility |
| Wind speed | Increases transpiration in plants; affects thermoregulation in animals |
| Salinity | High salt concentrations cause osmotic water loss in cells |
What are biotic factors?
Biotic factors are the living components of an environment — the other organisms that affect a species. They include:
- Competition — organisms competing for the same limited resource (food, water, shelter, light, mates, or territory). Competition can be interspecific (between different species) or intraspecific (between members of the same species).
- Predation — predators reduce prey populations directly by consuming individuals.
- Disease — pathogens (bacteria, viruses, fungi, protists) reduce population size and can cause mass die-offs.
- Food availability — if food supply falls, populations of consumers decline.
- Mutualism — both species benefit (e.g., nitrogen-fixing bacteria in legume root nodules; oxpecker birds removing parasites from rhinos).
- Parasitism — one organism benefits at the expense of the host (e.g., tapeworms in the gut; mistletoe on trees).
How do abiotic and biotic factors affect population size?
Population size increases when birth rate and immigration exceed death rate and emigration. Abiotic and biotic factors can shift any of these:
- A drought (abiotic) reduces food supply for herbivores → populations fall.
- A mild winter (abiotic) allows more aphids to survive → aphid population rises → ladybird population rises in response.
- A new predator arrives (biotic) → prey population falls → predator population eventually falls too as food becomes scarce.
Populations rarely grow without limit. Once a population becomes large, intraspecific competition for limited resources increases, slowing growth. The carrying capacity of an environment is the maximum population size it can sustainably support.
What is a predator–prey cycle?
A predator–prey cycle is the pattern of oscillating population sizes seen when a predator and its prey are closely linked. A classic example is the Canadian lynx and snowshoe hare:
- Hare population rises (ample food, mild winters).
- More hares → more food for lynx → lynx population rises.
- More lynx → more hares eaten → hare population falls.
- Fewer hares → less food for lynx → lynx population falls.
- Fewer lynx → less predation pressure → hare population begins to rise again.
The predator population cycle lags behind the prey cycle by roughly one quarter of the period. In graphs, the lynx peak comes after the hare peak. This lag occurs because predator reproduction takes time to respond to increased food availability.
How do scientists measure the effect of abiotic factors on distribution?
Scientists use belt transects and quadrats to sample how populations change across an environmental gradient — for example, from the top of a rocky shore to the waterline, or from a shaded woodland floor into open grassland.
By recording which species occur at different positions along the transect, and simultaneously measuring the relevant abiotic factor (e.g., light intensity, soil pH, moisture), ecologists can build a picture of the abiotic conditions each species tolerates. This is called the species' ecological niche.
How do biotic and abiotic factors interact?
Abiotic and biotic factors rarely act in isolation — they interact in complex ways:
- A high soil nitrate level (abiotic) supports dense plant growth → more food and cover for herbivores (biotic effect).
- A disease outbreak (biotic) reduces a prey population → predators must compete more strongly (biotic) or shift to a new habitat where abiotic conditions may be less favourable.
- Climate warming (abiotic) shifts species ranges poleward, bringing new predators, competitors, and pathogens (biotic) into contact with communities that have not evolved defences against them.
Frequently asked questions
What is the difference between abiotic and biotic factors?
Abiotic factors are the non-living physical and chemical conditions of an environment, such as temperature, light intensity, water availability, pH, and oxygen levels. Biotic factors are the living influences, including predation, competition, disease, food availability, and mutualism. Both types of factor affect where organisms live, how large populations grow, and how ecosystems change over time.
How does temperature affect population size?
Temperature is a key abiotic factor because it controls the rate of enzyme-catalysed reactions, including photosynthesis and respiration. When temperature rises toward the optimum for a species, growth and reproduction rates increase and population size tends to rise. Temperatures above or below the tolerable range reduce survival and reproduction, shrinking populations. For ectotherms (cold-blooded animals) this effect is especially strong, as their body temperature matches the environment.
What is interspecific and intraspecific competition?
Interspecific competition occurs between individuals of different species competing for the same resource — for example, red and grey squirrels both eating acorns in the same woodland. Intraspecific competition is between individuals of the same species — for example, two male stags fighting over a mate, or seedlings of the same tree competing for light. Intraspecific competition intensifies as population size rises and is a key mechanism limiting population growth.
Why does the predator population peak after the prey population?
The predator population lags behind the prey population because reproduction takes time. When prey numbers rise, predators have more food and begin to reproduce more successfully, but the increase in predator numbers only shows up in the data weeks or months later. By the time predator numbers are at their peak, prey numbers have already been depleted, and the predator population then falls in turn. This time lag is the defining feature of predator–prey oscillation.
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