Investigating ecosystems: biotic and abiotic factors, sampling techniques for measuring abundance and distribution, and the use of indicator species to monitor environmental conditions.

What this dot point is asking

The SQA wants you to describe an ecosystem in terms of its biotic (living) and abiotic (non-living) factors, explain how scientists sample an ecosystem to measure the abundance and distribution of organisms, and explain how indicator species are used to monitor environmental conditions. This is the practical, field-skills foundation of the whole Living Environment area.

Biotic and abiotic factors

The factors that affect organisms split into two groups:

• Abiotic factors are the non-living, physical and chemical conditions. The ones the SQA expects you to measure include light intensity, temperature, pH, dissolved oxygen, soil moisture, wind speed and humidity. Each can be measured with a sensor or probe (a light meter, thermometer, pH meter, oxygen probe or moisture meter).
• Biotic factors are the effects of other living organisms. They include competition for resources, predation, grazing or herbivory, disease and parasitism, and the availability of food.

Both sets of factors interact to determine the abundance (how many) and distribution (where) of a species. For example, a plant might be abundant only where light, soil moisture and pH all suit it and grazing pressure is low.

Sampling: measuring abundance and distribution

You cannot count every organism in an ecosystem, so scientists take representative samples and scale up. The SQA expects you to know three techniques and when each is used.

Quadrats

A quadrat is a square frame of known area placed on the ground. You count the number of a target species inside it, or estimate its percentage cover. Quadrats are placed at random positions (for example using random coordinates) so the sample is unbiased, and the mean count is scaled up to the whole study area.

Transects

A transect is a line laid across the habitat, often where conditions change (such as from the top to the bottom of a rocky shore). Quadrats placed at intervals along it show how the community changes as an abiotic factor changes, revealing zonation and the distribution of species along the gradient.

Capture-mark-recapture (the Lincoln index)

For mobile animals, a first sample is captured, marked harmlessly and released. After they mix back into the population, a second sample is taken. The proportion of marked individuals recaptured is used to estimate the total population:

where $N$ is the estimated population, $M$ is the number marked in the first sample, $C$ is the total caught in the second sample, and $R$ is the number of marked animals recaptured.

Reliability, validity and bias

Higher rewards candidates who can judge a method, not just describe it.

• Reliability comes from a large sample size and repeating the sampling, so the mean is close to the true value and chance has less effect.
• Validity comes from controlling or accounting for other variables, so you measure what you intend to measure.
• Bias is removed by random placement of quadrats; choosing where to put them by eye tends to favour patches with more (or fewer) organisms.

Indicator species

Indicator species let scientists monitor an ecosystem cheaply and continuously, because the living community integrates conditions over time in a way that a one-off chemical test cannot. In rivers, mayfly and stonefly nymphs need clean, well-oxygenated water and vanish under organic pollution, while bloodworms and sludge worms tolerate low oxygen, so the mix of species present scores the water quality. Lichens are sensitive to sulfur dioxide, so the types growing on trees indicate air quality.

Examples in context

Example 1. Riverfly Partnership monitoring. Volunteer groups across the UK sample river invertebrates monthly and score them against tolerance bands. A sudden drop in mayfly numbers flags a pollution incident, such as a sewage discharge, far sooner and more cheaply than continuous chemical monitoring, because the invertebrate community reflects oxygen conditions over the preceding weeks.

Example 2. Rocky-shore transects. A belt transect from the high-tide mark to the low-tide mark shows clear zonation: lichens and periwinkles high up where exposure to air is greatest, then barnacles and limpets, then seaweeds lower down. The transect makes the link between an abiotic gradient (exposure and desiccation) and the distribution of species visible and measurable.

Try this

Q1. Name two abiotic factors and two biotic factors that affect where a plant grows. [2 marks]

• Cue. Abiotic: light, temperature, pH, soil moisture (any two). Biotic: competition, grazing, disease (any two).

Q2. A student wants to compare plant abundance in a shaded woodland and an open field. State one way to make the comparison fair. [1 mark]

• Cue. Use the same quadrat size and the same number of randomly placed quadrats in each habitat.