Abiotic factors are non-living physical and chemical conditions, while biotic factors arise from living organisms and their interactions. Both influence a population's distribution, abundance and growth.
This is core content for SL and HL, Subtopic 2.1: Individuals, populations, communities and ecosystems.
A population is a group of organisms of the same species living in the same area at the same time and capable of interbreeding. Whether that population survives and reproduces depends on the environmental conditions and resources it experiences.
| Factor type | Examples | Effect on a population |
|---|---|---|
| Abiotic | Temperature, sunlight, pH, salinity, dissolved oxygen and soil texture | Determines whether organisms can tolerate the physical and chemical conditions of a habitat. |
| Biotic | Competition, predation, herbivory, parasitism, disease and mutualism | Changes survival, reproduction and access to resources through interactions with other organisms. |
For example, the Yellowstone elk population is affected by abiotic conditions such as drought and severe winters. It is also affected by biotic factors such as wolf predation, competition and the availability of plant food.
These factors may operate together. Drought can reduce plant growth, which decreases food availability and intensifies competition among herbivores. Population size may then decline because mortality rises or reproductive success falls.
Many biotic factors are density-dependent: their effects become stronger as population density increases. Competition and disease can therefore create negative feedback, slowing population growth as it approaches carrying capacity, the maximum population size determined by competition for limited resources. Abiotic disturbances such as drought or fire are often density-independent, although this distinction is not absolute.
A common misconception is that human influences form a completely separate category. Humans are organisms, but their activities can alter either type of factor: harvesting directly changes biotic interactions, while pollution may change abiotic conditions such as water pH.
Exam technique: For a command term such as distinguish, state both definitions and make the difference explicit. For explain, link a named factor to its effect on survival, reproduction, distribution or population size.