Analyzing Changes in Communities Revision Notes | GCSE AQA Higher Biology

1. Why Communities Change

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).

2. The Big Players in Community Change

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.

3. Predator-Prey Dynamics

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.

1. Lynx peak occurs 2 years after hare peak — Predator populations always lag behind prey populations
2. High hare numbers provide abundant food for lynx — More food allows lynx to reproduce successfully
3. Increased lynx numbers reduce hare population through predation — More predators means higher predation pressure
4. Lynx numbers will decline as hare numbers fall — Less food means lynx struggle to survive and reproduce

Answer: The 2-year lag shows typical predator-prey dynamics where predator populations respond to changes in prey availability with a time delay.

4. Competition and Resource Limitation

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.

5. Disease and Parasitism Effects

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.

6. Exam Technique - Analyzing Community Changes

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.