Biotic Factors

The four named living things that shape every community — and the precise spec wording you cannot afford to paraphrase. Lesson 7.02.

SPEC 4.7.1.3 • PAPER 2

The Other Half of the Story

Lesson 7.01 covered the non-living things shaping communities — light, temperature, oxygen. This lesson handles the living half: predators, food, disease, and competitors.

Why this matters

AQA mark schemes routinely test the biotic-vs-abiotic distinction with "sort into two columns" style questions. Get the categorisation wrong and you lose marks before you even reach the explanation — examiner reports flag this as a high-error question type.

In the next 10 minutes you'll know the four named biotic factors verbatim, why "outcompeting" is more than just competing, and how to write the PEE chain for any biotic-factor question.

What You'll Be Able To Do

Objective 1

Recall the four named biotic factors using verbatim spec wording — including the breeding-threshold qualifier on the fourth.

Objective 2

Distinguish biotic factors from abiotic factors in any context — the classification trap.

Objective 3

Apply PEE to "explain how a change in a biotic factor affects this community" questions, including predator-prey graph interpretation.

The 4 Named Biotic Factors

A biotic factor is a living factor affecting a community. The spec names exactly four — and the wording is precise.

01Availability of foodAffects all consumers — herbivores, carnivores, omnivores

02New predators arrivingNative prey may have no defences — population can crash

03New pathogensNo built-up immunity → rapid spread, high mortality

04One species outcompeting another…so the numbers are no longer sufficient to breed(verbatim spec)

Wording trap

Factor 4 has a specific consequence in the spec wording: numbers fall below the level needed to breed. Don't write "outcompeted, so they die" — the spec point is about the breeding threshold. Lose the qualifier, lose the mark.

Biotic vs Abiotic — The Precision Test

The classic AQA question: given a list of factors, sort them into biotic and abiotic columns. Examiner reports flag this as a high-error question type. The rule is simple but the application catches students out.

BIOTIC (living)ABIOTIC (non-living)

Food / availability of foodLight intensity

Predators / consumers / carnivoresTemperature

Disease / pathogens / bacteria / fungi causing diseaseMoisture levels (water availability)

Competitors (same or different species)Soil pH and mineral content

Microorganisms / microbes / parasitesWind intensity and direction

Amount of dead / decaying matter (in soil)Carbon dioxide (plants) · Oxygen (aquatic animals)

Test for any factor

Ask: "Was this once alive, or is it caused by something alive?" Yes → biotic. No → abiotic. Dead leaves on the soil = biotic (came from a living thing). Soil pH = abiotic (a chemical property).

Predator-Prey Dynamics

The canonical biotic-factor question style. Two populations rise and fall in a coupled cycle. You need to read the graph and explain the lag.

The four-stage cycle

Prey rises → more food for predators → predators rise (with a lag).

Predators peak → over-hunt the prey → prey falls.

Prey low → predators run out of food → predators fall.

Predators low → less predation pressure → prey recovers → cycle repeats.

Mark-scheme essentials

The predator peak follows the prey peak (lag, not in sync). Predator amplitude is usually smaller. State both observations explicitly when reading the graph — examiners credit each separately.

New Pathogens — Why They're Devastating

A pathogen is a microorganism that causes disease. When a new pathogen reaches a population that has never encountered it, the effect can be catastrophic.

The mechanism

New pathogen arrives in a population.

The population has no built-up immunity and no genetic resistance.

The disease spreads rapidly through the population.

Many individuals die or become too sick to reproduce → population falls.

Over generations, individuals with some resistance survive and breed → eventually the population may recover with greater resistance (Module 6 link: natural selection).

Context for application questions

Examples examiners use: ash dieback (a fungal pathogen affecting ash trees in the UK), and similar introductions of diseases into populations without prior exposure. The exam doesn't require specific named diseases — it requires the mechanism: no immunity → rapid spread → population fall.

Outcompeting — The Breeding Threshold

Factor 4 is the most subtle. Read the spec wording exactly:

Spec wording

"One species outcompeting another so the numbers are no longer sufficient to breed."

This is more than ordinary competition. The key idea: when a species' numbers drop too low, individuals can't find mates — even if some survive, the population can't recover.

The mechanism

Two species share overlapping resource needs (food, habitat, light, etc.).

One species is better adapted — uses resources more efficiently, breeds faster, etc.

The better competitor takes most of the resources; the weaker species gets less.

The weaker species' population falls below the breeding threshold — too few individuals to find mates.

The population cannot recover → may go locally extinct.

Classic UK example

Grey squirrels (introduced from North America) outcompete native red squirrels for food and habitat. Across much of England and Wales, red squirrel numbers have fallen below the level needed to sustain a breeding population — Scotland and isolated refuges in northern England remain the strongholds.

Availability of Food — The Chain Effect

The most directly testable biotic factor. When food becomes scarce or abundant, populations of consumers follow.

Worked mechanism — food becomes scarce

Food availability decreases (e.g., plant population falls due to drought, or prey population crashes).

Consumers have less energy for movement, growth, and reproduction.

Some individuals die from starvation; others fail to reproduce successfully.

Consumer population falls.

Knock-on effect: the next trophic level above loses food too.

Why this overlaps with predator-prey

"Less food" and "fewer prey" are often the same event from different angles. In a fox-rabbit system, a fall in rabbit population is a fall in fox food availability. The mark scheme will usually accept either framing — but pick one and be consistent.

Worked Example — Apply PEE

A typical 4-mark Paper 2 question on a biotic factor change.

Question: A new fungal pathogen reaches a forest and begins to infect a species of oak tree. Explain how this could affect the population of squirrels that live in the forest. 4 marks

P

Point: The pathogen is a biotic factor. Squirrels depend on oak trees for food (acorns) and shelter — this is interdependence.

E

Evidence: The oak trees have no built-up immunity to the new pathogen, so many die. Acorn production falls and there is less food available for squirrels.

E

Explain: With less food, squirrels cannot get enough energy for survival and reproduction, so some die and others fail to breed — the squirrel population decreases.

Mark scheme map

Four creditable points: (1) identify biotic factor (pathogen / food availability); (2) no immunity / mechanism of effect on oak; (3) link to squirrel resource (food); (4) effect on squirrel population. Hit all four = 4/4.

Examiner Alerts — Biotic Traps

Alert 1

Don't confuse biotic with abiotic. Predators are biotic. Disease is biotic. Food (which came from a living thing) is biotic. Temperature, pH, and moisture are abiotic. Sort-into-columns questions punish miscategorisation directly.

Alert 2

The breeding-threshold qualifier. For Factor 4 (outcompeting), the spec is specific: numbers fall below the level needed to breed. Don't write "they die out" — write "the population becomes too small to breed."

Alert 3

Predator-prey graphs need both observations. When asked to interpret oscillating curves, state explicitly that the predator peaks after the prey, AND that predator amplitude is smaller. Each is a separate mark.

Common Errors — Wrong vs Right

Wrong

"Disease is an abiotic factor."

Right

Disease is biotic — caused by a living pathogen (bacterium, virus, fungus, protist).

Wrong

"The grey squirrels outcompeted the reds, so the reds died."

Right

"…so red squirrel numbers fell below the level needed to breed." Use the spec wording.

Wrong

"New pathogens always kill all the population."

Right

Many die because there is no built-up immunity, but resistant individuals may survive and breed.

Wrong

"Predator and prey populations rise and fall together."

Right

The predator population peaks after the prey (a lag). Both peaks aren't simultaneous.

Your Turn

Question:

4 marks

Hint: identify the biotic factor, link to the resource limitation, then to the population — and remember the breeding threshold for Factor 4.

P

Point: The beetle is a biotic factor. The beetle and mice compete for the same food (seeds) — interspecific competition.

E

Evidence: The beetle is described as more efficient at finding seeds, so the beetle takes more of the available food and the mice get less.

E

Explain: Mice have less food → less energy for survival and reproduction → some die and others fail to breed → mouse population falls. If numbers fall below the level needed to breed, the local population may not recover.

✓

Marks: biotic factor / competition (1) · mechanism (beetle more efficient → less food for mice) (1) · effect on individuals (less energy / fewer offspring) (1) · effect on population, incl. breeding threshold (1) = 4/4.

Recap — Three Takeaways

The 4 biotic factors.

Food · new predators · new pathogens · outcompeting (so numbers no longer sufficient to breed).

Biotic vs abiotic.

Living = biotic (food, predators, disease, competitors). Non-living = abiotic (light, temp, pH, oxygen).

Predator-prey lag.

Predators peak after prey. State the lag AND the smaller amplitude — both are separate marks.

Next lesson

7.03 — Adaptations

Structural, behavioural, and functional adaptations — and how extremophiles survive impossible conditions. Spec 4.7.1.4, Paper 2.

Lesson Summary Card

Key Definitions

Biotic factor = a living factor affecting a community.

Abiotic factor = a non-living factor affecting a community.

Pathogen = a microorganism that causes disease.

Interspecific competition = competition between different species for the same resources.

The 4 Biotic Factors (verbatim spec)

Availability of food

New predators arriving

New pathogens

One species outcompeting another so the numbers are no longer sufficient to breed

Predator-Prey Pattern

Two oscillating populations, coupled.

Predator peaks after prey (the lag).

Predator amplitude is usually smaller .

Four-stage cycle: prey↑ → predator↑ → prey↓ → predator↓ → repeat.

Top exam tip

For "explain how [biotic factor] affects [population]" questions, the chain is: biotic factor → resource / interaction → effect on individuals → effect on population. For Factor 4, end with the breeding threshold.