Biotic and Abiotic Factors in Ecosystems

Biotic factors are the living parts of an ecosystem, and abiotic factors are its nonliving physical and chemical parts. In brief: plants, animals, fungi and bacteria are biotic factors, while sunlight, temperature, water, soil and air are abiotic factors. In a pond, the fish and algae are biotic, and the water temperature, oxygen level and sunlight are abiotic.

Every organism depends on both kinds of factors, and the two constantly influence each other. This guide defines each type, lists the most important examples, shows how the factors interact, explains how to measure them in a worked field example and points out common mistakes.

What are biotic and abiotic factors?

The word biotic comes from the Greek word for life, and abiotic means not living. Together, these factors determine which organisms can live in a place and how many of them can survive there.

Ecologists use the terms to describe an ecosystem, which is a community of organisms together with its nonliving environment. A change in one factor often changes others. For example, a drought is an abiotic change that can reduce plant growth, which is a biotic effect, and then affect the animals that eat the plants.

What are the main biotic factors?

Biotic factors include all living organisms and the interactions between them, as well as their products and remains.

• Producers: Plants, algae and some bacteria that make food by photosynthesis.
• Consumers: Animals that eat other organisms, including herbivores, carnivores and omnivores.
• Decomposers: Fungi and bacteria that break down dead material and return nutrients to the soil.
• Interactions: Predation, competition, parasitism, mutualism and disease.

Biotic factors can limit a population. A growing number of predators may reduce the number of prey, and competitors may use up the same food. They can also help, as when bees pollinate flowers.

What are the main abiotic factors?

Abiotic factors are the nonliving conditions and resources of the environment.

• Sunlight: The energy source for photosynthesis. Its intensity and duration vary with latitude, season and shade.
• Temperature: It affects the rate of chemical reactions in cells. Each species tolerates a limited range.
• Water: Availability of water, rainfall and humidity strongly affect distribution of life.
• Soil: Its texture, mineral content, acidity (pH) and ability to hold water determine which plants grow.
• Air and dissolved gases: Oxygen and carbon dioxide levels are important on land and in water.
• Wind, fire and salinity: Wind dries organisms and disperses seeds, fire resets many habitats, and salt concentration limits the species in estuaries.

Biotic and abiotic factors compared

How do biotic and abiotic factors interact?

The two groups are linked in cycles of cause and effect. Abiotic factors set the limits for life, and living organisms change the abiotic conditions in return.

Trees shade the ground, which lowers soil temperature and raises humidity. Roots hold soil together and reduce erosion. Beavers build dams that change water flow and create ponds. Decomposers release minerals from dead material, which changes soil fertility.

Ecologists also use the idea of a limiting factor. This is the factor that is in shortest supply relative to the needs of a population and therefore restricts its growth. In a dry region, water is often the limiting factor, and in a shaded forest floor, light may be limiting.

Worked example: comparing two sites in a woodland

A class compares a shaded site under dense trees (site A) with a sunny clearing (site B) 20 meters away. They measure three abiotic factors and count one biotic factor.

1. Measure light with a light meter at ground level at three points per site and take the mean.
2. Measure soil temperature at 5 centimeters depth with a thermometer, again at three points.
3. Measure soil moisture by weighing a soil sample, drying it and weighing it again. The percentage of water is the difference divided by the original mass, times 100.
4. Count the number of moss plants in 10 quadrats of 0.25 square meters at each site and calculate the mean per quadrat.

Suppose a 100 g soil sample from site A loses 28 g after drying, and a 100 g sample from site B loses 14 g. The moisture is 28 percent at A and 14 percent at B. If mosses average 12 per quadrat at A and 3 per quadrat at B, the data suggest that moss is more abundant where it is shaded and moist.

The result shows a correlation but not proof of cause. Light, moisture and temperature differ together, so a student cannot tell which factor matters most without additional tests that change one factor at a time.

How did ecologists come to study these factors?

In the 1800s, botanists such as Alexander von Humboldt described how plant communities change with altitude and climate. Justus von Liebig proposed that growth is limited by the scarcest resource, an idea known as the law of the minimum.

In 1911 Victor Shelford extended this with the law of tolerance, which says that each species has a range of conditions it can tolerate, with an optimum in the middle. These ideas still underlie the study of how species are distributed.

Why do these factors matter in practice?

Farmers test soil pH and moisture and choose crops suited to local conditions. Fish farmers monitor oxygen and temperature, because low oxygen can kill stock rapidly.

Climate change is altering abiotic factors such as temperature and rainfall, which shifts the range of many species. Predicting these changes requires knowing which factors limit each species and how its biotic partners and competitors will respond.

Which mistakes are common?

Mistake 1: Counting dead organisms as abiotic

Dead wood, fallen leaves and bones come from living things. Ecologists often treat them as part of the biotic component, or as organic matter, and they feed decomposers.

Mistake 2: Listing an organism as an abiotic factor

A beaver is biotic, even though its dam changes the water. Only the physical conditions, such as flow rate and depth, are abiotic.

Mistake 3: Treating the factors as independent

Temperature, light and moisture influence each other and the organisms. Real ecosystems need explanations that include these links.

Mistake 4: Concluding that a correlation proves a cause

Finding more moss where it is shaded does not show that shade is the cause. Controlled comparison is needed.