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In a predator-prey cycle, abundant prey lets predator numbers grow, then more predators reduce prey numbers. Free notes and 13 questions answered.
Analyzing Changes in Communities is part of Adaptations, interdependence and competition in GCSE AQA Higher Biology.
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In a predator-prey cycle, abundant prey lets predator numbers grow, then more predators reduce prey numbers. Free notes and 13 questions answered.
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Use this lesson to stay inside Adaptations, interdependence and competition while connecting the detail back to the wider Ecology topic.
Read the core explanations from Analyzing Changes in Communities before testing yourself from memory.
Imagine a forest where suddenly all the wolves disappear. What happens next? The deer population explodes, plants get overgrazed, and the entire ecosystem shifts. This is community change in action.
Communities are constantly changing as biotic factors - the living parts of ecosystems - interact with each other. When one species is affected, it creates a ripple effect throughout the entire community.
Trap: Many students confuse biotic and abiotic factors. Remember: biotic = living (bacteria, birds, bears), abiotic = non-living (temperature, rainfall, soil pH).
Five main biotic factors drive community changes. Each one can dramatically alter population sizes and species composition.
Predation: When predator numbers change, prey populations respond inversely. More foxes means fewer rabbits. Fewer sharks means more fish.
Disease: Pathogens can wipe out entire populations. Dutch elm disease killed millions of elm trees, changing forest communities across Europe.
Competition: When resources are limited, species compete. The better competitor thrives while others decline or disappear locally.
Food availability: Changes in food sources directly affect consumer populations. Poor acorn years mean fewer squirrels the following spring.
Parasitism: Parasites weaken host populations, making them vulnerable to other pressures like predation or disease.
Predator-prey relationships create predictable cycles. When prey is abundant, predator populations grow. As predators increase, they reduce prey numbers. With less food, predator numbers then fall, allowing prey to recover.
This creates the classic predator-prey cycle: prey peaks first, followed by predator peaks. The cycles are usually out of phase by about a quarter cycle.
Worked example: Analyzing Predator-Prey Data
A study tracked lynx and snowshoe hare populations over 20 years. In year 5, hares peaked at 8000. In year 7, lynx peaked at 400. Explain this pattern.
Answer: The 2-year lag shows typical predator-prey dynamics where predator populations respond to changes in prey availability with a time delay.
Competition occurs when organisms need the same limited resources. There are two types: intraspecific (within species) and interspecific (between species).
Intraspecific competition is usually more intense because organisms of the same species need identical resources. When red deer numbers increase, they compete more fiercely for the same food plants.
Interspecific competition can lead to competitive exclusion - one species outcompetes another completely. Grey squirrels outcompeted red squirrels in most of Britain because they're better at digesting acorns.
The intensity of competition depends on resource availability. In drought years, competition for water becomes fierce. In good years, competition relaxes and populations can grow.
Disease can devastate communities rapidly. Unlike gradual changes from competition, disease outbreaks cause sudden population crashes that ripple through the entire ecosystem.
Parasites work more slowly but can be equally devastating. They weaken hosts, making them more vulnerable to predation and less able to compete for resources.
Disease effects depend on population density. Crowded populations spread disease faster - this is why disease often follows population booms, creating boom-bust cycles.
Some diseases are species-specific, while others jump between species. Ash dieback disease specifically targets ash trees, but bird flu can affect multiple bird species.
Exam questions often give you data about population changes and ask you to explain the patterns. Here's how to tackle them systematically.
Command word focus: 'Explain' questions need the word 'because' - you must give reasons. 'Describe' questions just need you to state what happens without explaining why.
Common mistakes: Don't confuse correlation with causation. Just because two populations change at the same time doesn't mean one caused the other. Look for biological mechanisms that make sense.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
In a predator-prey cycle, abundant prey lets predator numbers grow; more predators then reduce prey numbers; with less food, predator numbers fall, letting prey recover. Prey peaks first and predator peaks follow, usually out of phase by about a quarter cycle.
Competitive exclusion is when interspecific competition leads one species to outcompete another completely. Grey squirrels outcompeted red squirrels in most of Britain because they are better at digesting acorns.
On a predator-prey graph, the predator peak lags behind the prey peak. If snowshoe hares peak at 8000 in year 5 and lynx peak at 400 in year 7, high hare numbers gave lynx abundant food to reproduce, then more lynx reduced the hares, and lynx numbers will decline as hares fall.
Disease can cause sudden population crashes that ripple through the whole ecosystem, as Dutch elm disease killed millions of elm trees across Europe. Crowded populations spread disease faster, which is why disease often follows population booms, creating boom-bust cycles.
Parasites weaken host populations more slowly than disease, but they can be equally devastating, making hosts more vulnerable to other pressures such as predation or disease and less able to compete for resources.
Predator numbers peak after prey numbers because high prey numbers provide abundant food, so predators reproduce successfully and their population grows with a time delay. For example, snowshoe hares peaked in year 5 at 8000 and lynx peaked in year 7 at 400, a 2-year lag.
The five main biotic factors that change communities are predation, disease, competition, food availability and parasitism. Each can dramatically alter population sizes and species composition, creating a ripple effect through the community.
Dutch elm disease killed millions of elm trees, changing forest communities across Europe. It shows how a pathogen can wipe out entire populations and alter whole ecosystems.
Disease is more common after a population boom because crowded populations spread disease faster. This is why disease often follows population booms, creating boom-bust cycles.
Predator and prey cycles are usually out of phase by about a quarter cycle: prey peaks first, followed by the predator peak. As predators increase they reduce prey numbers, then with less food predator numbers fall, letting prey recover.
Poor acorn years mean fewer squirrels the following spring, because changes in food sources directly affect consumer populations. Food availability is one of the main biotic factors driving community change.
Ash dieback disease specifically targets ash trees, so it is species-specific, while bird flu can affect multiple bird species. Some diseases affect only one species, while others jump between species.
Two populations changing at the same time does not prove one caused the other; correlation is not causation. A biological mechanism that makes sense is needed before saying one population change caused another.