Ecosystems, abiotic and biotic factorsEdexcel A-Level Biology A: Revision notes

Ecologists use precise terms for the levels of organisation in nature:

• A habitat is the place where an organism lives, such as a pond or a woodland floor.
• A population is all the organisms of one species living in a habitat at the same time, and able to interbreed.
• A community is all the populations of different species living in the same habitat at the same time.
• An ecosystem is a community together with the non-living (abiotic) factors in which it lives, and the interactions between them. Ecosystems can be as small as a rock pool or as large as a tropical rainforest.

For example, in a woodland, all the badgers form a population, all the plants, animals, fungi and microorganisms form the community, and the community together with the soil, light and climate form the ecosystem.

Abiotic factors are the non-living, physical and chemical features of the environment. They control which organisms can live in a habitat, and how many, because each species can tolerate only a certain range of each factor.

Examples include:

• temperature, which affects the rate of enzyme-controlled reactions such as photosynthesis and respiration
• light intensity, which limits the rate of photosynthesis, and so the distribution of plants
• pH of soil or water, which affects enzymes and the availability of ions
• water availability or humidity, and salinity
• oxygen concentration in water or soil, and wind
• soil type and mineral content

Abiotic factors can change with time (day and night, the seasons) and from place to place.

Biotic factors are the effects of other living organisms. They include:

• Competition for resources such as food, light, water or space. Intraspecific competition is between members of the same species. Interspecific competition is between different species.
• Predation: predator and prey numbers rise and fall together, with the predator population lagging behind that of the prey.
• Parasitism and disease, which reduce the numbers of the host.
• Mutualism, in which both species benefit.

The numbers (abundance) and distribution of a species depend on the combined effect of abiotic and biotic factors. For example, a barnacle may be absent from one part of a shore because it cannot tolerate drying out (abiotic) and from another because a faster-growing species outcompetes it (biotic).

The niche of a species is its role in the ecosystem: where it lives, what it eats, when it is active and how it interacts with other species (biotic) and with the abiotic factors it can tolerate.

The concept accounts for distribution and abundance:

• Each species is adapted to a particular niche, so it is found only where the abiotic conditions and resources suit it.
• Two species with the same niche compete strongly, and the one that is better adapted will outcompete the other, which may be reduced in numbers or excluded from the habitat.
• Species with different niches can live in the same habitat, because they use different resources or live in different parts of it, and so compete less.

A species can therefore be common in one area and absent in another, because the niche is available in one and not in the other.

To study the distribution (where organisms are found) and abundance (how many) in a habitat, ecologists take samples:

• Random sampling with a quadrat avoids bias. Lay two tape measures at right angles to make a grid, and use random numbers from a calculator or table as coordinates for each quadrat.
• Systematic sampling is used when conditions change in a direction, such as up a shore or away from a path. Lay a line transect and place a quadrat at regular intervals, or use a belt transect (a strip of quadrats).

Abundance can be given as density (number per m²), frequency (the proportion of quadrats in which the species is found) or percentage cover, which is useful for plants that are difficult to count.

Worked example: 30 quadrats each of area 0.25 m² contain 120 daisies in total. Mean = 120 ÷ 30 = 4 per quadrat, so density = 4 ÷ 0.25 = 16 m⁻². For a field of 500 m², the estimated population = 16 × 500 = 8 000.

Abiotic factors are measured at each sampling point, so that changes in abundance can be linked to them:

• light intensity: light meter
• temperature: thermometer or temperature probe
• soil moisture: soil moisture meter, or the mass lost when a soil sample is dried
• pH: pH probe or soil indicator
• wind speed: anemometer

To make the results reliable, take a large sample and take several readings for each factor, using the mean. Take readings at the same time of day and in similar weather.

A correlation between an abiotic factor and the abundance (for example tested using Spearman's rank correlation) suggests a link but does not prove cause and effect, because other factors may also change.