Every ecosystem is shaped by two broad kinds of influences: biotic factors, which come from living organisms, and abiotic factors, which come from the nonliving physical and chemical environment. Together, they determine where organisms can live, how they survive, how populations change, and how ecosystems function.
The distinction is simple, but the relationship between the two is more complex. A plant, for example, depends on sunlight, water, temperature, and soil conditions while also competing with other plants and interacting with insects, fungi, and animals. An ecosystem exists through the constant interaction of both kinds of factors.
What are biotic factors?
Biotic factors are the living components of an ecosystem and the effects they have on other organisms. They include plants, animals, fungi, bacteria, protists, and other living organisms.
Biotic factors do not refer only to organisms themselves. They also include biological interactions that influence an organism’s survival or reproduction.
For example, a forest contains trees, shrubs, birds, insects, mammals, fungi, and microorganisms. These organisms affect one another in many ways. Plants compete for light and water. Predators consume prey. Pollinators help plants reproduce. Fungi and bacteria decompose dead material and make nutrients available for other organisms.
Some important biotic interactions include competition, predation, herbivory, parasitism, mutualism, and disease.
Competition occurs when organisms use the same limited resource, such as food, water, space, or sunlight. Predation occurs when one organism kills and consumes another. Herbivory is the consumption of plants or algae by animals. Parasitism benefits one organism while harming its host.
Mutualism is an interaction in which both organisms benefit. A familiar example is the relationship between flowering plants and many pollinators: the animal obtains food, while the plant can receive assistance with pollination.
What are abiotic factors?
Abiotic factors are the nonliving physical and chemical components of an ecosystem. They include conditions and resources such as temperature, water, sunlight, air, soil, minerals, salinity, pH, and moisture.
Abiotic conditions vary dramatically from one environment to another. A desert may have intense sunlight, very little available water, and large temperature changes. A freshwater lake has different conditions involving water chemistry, temperature, light penetration, and dissolved substances.
Abiotic factors can determine which organisms are capable of surviving in a particular environment. A plant that requires abundant water may not thrive where precipitation is scarce. An aquatic organism may be unable to survive if water temperature or chemistry moves beyond the range it can tolerate.
Some abiotic factors are resources that organisms need, such as water, oxygen, carbon dioxide, and mineral nutrients. Others are environmental conditions, such as temperature or pH, that affect how organisms function.
Biotic vs. abiotic factors
The easiest way to distinguish the two is to ask whether the factor is living or nonliving.
| Biotic factors | Abiotic factors |
|---|---|
| Plants | Sunlight |
| Animals | Temperature |
| Fungi | Water |
| Bacteria | Air |
| Protists | Soil |
| Predators and prey | Minerals |
| Parasites and pathogens | pH |
| Competition between organisms | Salinity |
| Pollinators | Moisture |
The two categories are different, but they are not independent. Organisms constantly respond to abiotic conditions, while organisms can also modify their physical environment.
How biotic and abiotic factors interact
An ecosystem cannot be understood by looking at living organisms and physical conditions separately. They influence one another continuously.
Consider a plant growing in a grassland. Sunlight provides energy for photosynthesis, while water and nutrients support growth. Those are abiotic factors. At the same time, the plant may be eaten by herbivores, compete with neighboring plants, interact with fungi around its roots, and provide food or shelter for other organisms. Those are biotic influences.
The interactions can also work in the opposite direction. Plants can change abiotic conditions by providing shade, slowing wind, reducing soil erosion, and influencing soil moisture. Trees can contribute organic matter to soil when leaves and branches fall, while decomposers break that material down and return nutrients to the ecosystem.
Animals can also alter physical environments. Burrowing animals can disturb and mix soil, while organisms living in aquatic environments can influence the movement and availability of nutrients.
This means that the boundary between biotic and abiotic factors is useful for classification, but ecosystems themselves operate as interconnected systems.
Abiotic factors can limit where organisms live
Every organism has environmental conditions within which it can survive and reproduce successfully. Changes in abiotic factors can therefore influence the distribution and abundance of species.
Temperature is one important example. Organisms have physiological limits on the temperatures they can tolerate, and temperature can affect processes such as metabolism, growth, development, and reproduction.
Water availability is another major factor. Terrestrial organisms differ greatly in their ability to tolerate dry conditions. Plants in arid environments often have adaptations that help them obtain, store, or conserve water.
Light is particularly important for photosynthetic organisms. The amount and quality of available light can affect plant growth, while changes in light availability can influence which plants are able to compete successfully in a particular habitat.
Soil conditions can also matter. Soil provides plants with water, physical support, and mineral nutrients, while characteristics such as texture, chemistry, and moisture influence which plants can grow there.
No single abiotic factor necessarily determines an organism’s distribution. Several factors can act together, and their effects can depend on the organism’s biology and interactions with other species.
Biotic factors can limit populations too
Living organisms can be just as important as physical conditions in determining whether a population grows or declines.
A population may have abundant food but still remain limited by predators. Plants may have suitable temperature and rainfall but face intense competition for sunlight or nutrients. A species may also be affected by parasites, pathogens, or competitors that reduce survival or reproduction.
These biological interactions can change as population sizes change. If prey become more abundant, predators may have more food available. If predators become more numerous, prey populations may decline. Similarly, a plant population may become more strongly affected by competition when many individuals are growing in the same limited space.
As a result, population size is often shaped by a combination of environmental conditions and interactions among organisms.
Examples in different ecosystems
Forests
In a forest, abiotic factors include temperature, rainfall, sunlight, soil chemistry, moisture, and wind. Biotic factors include trees, shrubs, insects, birds, mammals, fungi, bacteria, and the interactions among them.
The forest canopy creates shade, which changes the amount of light reaching the forest floor. This is an example of a living component altering an abiotic condition. The resulting differences in light can affect which plants grow beneath the canopy.
Freshwater ecosystems
In a lake or stream, important abiotic factors include water temperature, dissolved oxygen, light, water chemistry, flow, and nutrient availability.
Biotic factors include algae, aquatic plants, fish, insects, microorganisms, and other organisms. These species interact through feeding, competition, decomposition, and other biological processes.
Deserts
Desert ecosystems are strongly influenced by limited water availability, temperature, sunlight, soil characteristics, and other abiotic conditions.
The organisms living there—including plants, insects, reptiles, mammals, and microorganisms—have biological interactions that shape the community. Competition for scarce water and food can be particularly important, while predators, herbivores, parasites, and decomposers contribute to the ecosystem’s biological dynamics.
Oceans
Marine ecosystems are influenced by abiotic factors such as temperature, salinity, light availability, depth, water movement, pressure, and nutrient concentrations.
Their biotic components include phytoplankton, algae, invertebrates, fish, marine mammals, microorganisms, and many other organisms. These organisms form food webs in which energy and nutrients move through multiple levels of the ecosystem.
Why the distinction matters
Separating biotic and abiotic factors helps scientists understand why ecosystems have particular structures and why organisms occur where they do.
If a population changes, researchers can ask whether the change is associated with a physical condition, such as temperature or water availability, or with biological interactions, such as predation, competition, disease, or food availability. In many cases, both types of factors are involved.
The distinction is also important for understanding environmental change. A change in an abiotic condition can alter living communities, while changes in organisms can feed back into the physical environment. Ecosystems therefore emerge from interactions between living organisms and the nonliving conditions in which they exist.
