QCE Biology - Unit 3 - Biodiversity and populations

Distribution, Abiotic and Biotic Factors | QCE Biology

Learn how abiotic and biotic factors affect species distribution, abundance, microhabitats and ecosystem classification.

Updated 2026-05-18 - 6 min read

Distribution, abiotic and biotic factors is part of the way QCE Biology turns living systems into evidence students can describe, analyse and evaluate. The safest way to study it is to connect each term to a data pattern, a biological mechanism and a limitation.

Core explanation

Abiotic factors

Abiotic factors are non-living conditions such as light, moisture, temperature, salinity, pH, soil nutrients and shelter. They shape where organisms can survive before interactions with other organisms are even considered.

Biotic factors

Biotic factors come from living organisms. Competition, predation, disease, food supply and mutualistic partners can increase or reduce abundance in particular parts of the ecosystem.

Microhabitats

A single ecosystem can contain small local habitats with different conditions. A fallen log, shaded creek edge or exposed rock can support different organisms from the surrounding area.

Classifying ecosystems

Classification systems such as Specht and Holdridge use measurable vegetation or climate data so ecosystems can be compared, monitored and managed consistently.

Distribution, abundance and scale

Distribution describes where organisms occur. Abundance describes how many organisms occur. Population distribution describes the spatial pattern of individuals, while population density combines abundance and area by expressing abundance per unit area or volume, such as 14 barnacles per square metre.

| Pattern | What it looks like | Likely biological explanation | | --- | --- | --- | | Clumped distribution | Individuals occur in patches | Resources, shelter or mates are patchy | | Uniform distribution | Individuals are evenly spaced | Competition, territorial behaviour or allelopathy | | Random distribution | Position is unpredictable | Resources are widely available and interactions are weak |

Spatial scale matters. A species can look clumped at the scale of a whole forest but uniform within one favourable patch. Temporal scale also matters because abundance may change daily, seasonally or after disturbances such as fire, flood, disease or human clearing.

Abiotic factors include climate, light intensity, humidity, wind exposure, water availability, salinity, pH, dissolved oxygen, pollutants and substrate type. Substrate is the surface or material an organism lives on or in, such as sand, rock, bark, soil or coral. Biotic factors include predators, competitors, pathogens, food species, pollinators, seed dispersers and humans.

Ecosystem classification in more detail

Ecosystem classification turns complex habitats into comparable categories. Specht-style vegetation classification uses structural formation: the height and projective foliage cover of the dominant plant layer. This can separate closed forest, open forest, woodland, shrubland, grassland and other vegetation structures. The point is not to memorise every category; it is to understand that measurable vegetation structure can classify ecosystems consistently.

Holdridge life zones use climate variables such as temperature, precipitation and potential evapotranspiration to predict broad ecosystem types. This is useful for comparing global patterns such as rainforest, desert, grassland and tundra, but it is a coarse model and may miss local soil, fire, drainage and disturbance effects.

Population size is the total number of individuals in a defined population. Population density is population size divided by area or volume. Population distribution is the spatial pattern of those individuals. A study can therefore show an unchanged population size but a changed distribution if individuals become more clumped around remaining water or shelter.

When describing ecosystem boundaries, name the evidence used: vegetation structure, climate, dominant species, substrate, hydrology, salinity or disturbance history. A boundary on a map is a model; real ecosystems often grade into each other through transition zones.

How to use this in data questions

Start by identifying what has been measured. In Biology, a graph or table is rarely just asking for a trend; it is asking whether you can connect the trend to a process. Quote enough data to show the pattern, then use the concept language from the syllabus. If the evidence is limited, name the limitation precisely: sample size, sampling method, uncontrolled variables, measurement precision, population choice or the time scale of the data.

A useful study habit is to turn each heading into a data prompt. Ask what you would expect to happen if the relevant variable increased, decreased or was removed. For ecology topics, think about abundance, distribution, biodiversity, biomass and carrying capacity. For genetics topics, think about genotype, phenotype, gene expression, allele frequency and inheritance pattern. For evolution topics, think about variation, selection pressure, gene flow, isolation and relatedness.

When a question asks you to evaluate, do not just list problems with the experiment. Link the limitation to the confidence of the conclusion. For example, a small sample size matters because a few unusual individuals can distort the pattern. An uncontrolled abiotic factor matters because it gives another possible explanation for the same biological trend. This is the difference between naming a limitation and using it scientifically.

Worked example

Common exam traps

Other traps to watch for:

• using a general word when a syllabus term is available
• ignoring units, sample size or time scale
• treating a model as a perfect copy of the real ecosystem or cell
• writing a memorised paragraph that does not use the given data

Quick check

Sources

• QCAA Biology subject page
• QCAA Biology 2025 syllabus
• OpenStax Biology 2e: Ecology of Ecosystems
• OpenStax Biology 2e: Biogeography
• Australian Museum: Biodiversity