Studying ecosystems and predator-prey relationships Revision Notes | GCSE AQA Higher Biology

1. Why Study Ecosystems?

Imagine you're a wildlife manager trying to save an endangered butterfly. How many are left? Where do they live? What's eating them? Without answers, your conservation efforts are just guesswork.

Ecologists face this challenge daily. They need precise methods to count organisms, map their distribution, and understand the relationships between species. This isn't just academic curiosity — it's essential for conservation, agriculture, and managing our environment.

The tools they use are surprisingly simple: a square frame called a quadrat and a measuring tape for transects. But the insights they reveal are profound. Let's see how these basic tools unlock the secrets of ecosystems.

2. Quadrats: Counting Life in Squares

Picture a 1m × 1m square frame thrown randomly onto a meadow. Everything inside that square gets counted — every daisy, every blade of grass, every beetle. This is quadrat sampling, and it's how ecologists turn vast, complex ecosystems into manageable data.

The key word is 'random'. You can't just pick the prettiest spots or avoid the muddy areas. Random sampling eliminates bias and gives you a true picture of what's really there. Ecologists use random number generators to choose coordinates, ensuring their samples represent the whole ecosystem.

But what exactly do you measure? For mobile animals like insects, you count individuals. For plants, you might measure percentage cover — what fraction of the quadrat area each species occupies. A large oak tree might cover 80% of a quadrat, while scattered daisies might cover just 5%.

Worked example: Calculating Population Density from Quadrat Data

A student samples a grassland using 10 quadrats, each 0.5m × 0.5m. The total number of clover plants counted across all quadrats is 45. Calculate the population density of clover.

1. Area = 0.5m × 0.5m = 0.25m² — Each quadrat is a square, so area = length × width
2. Total area = 10 quadrats × 0.25m² = 2.5m² — Multiply number of quadrats by area of each
3. Population density = Total organisms ÷ Total area — This gives organisms per unit area
4. Population density = 45 ÷ 2.5 = 18 clover plants per m² — Always include units in your final answer

Answer: 18 clover plants per m²

3. Transects: Following the Trail of Change

Quadrats tell you what's in one spot, but ecosystems aren't uniform. Walk from a pond edge to dry land, and you'll see species change dramatically. Water lilies give way to reeds, then to grasses, then to trees. This is where transects come in.

A transect is like drawing a line across your ecosystem and recording everything that touches it. There are two main types: line transects (record only what touches the line) and belt transects (record everything within a strip, like a series of connected quadrats).

The power of transects lies in revealing patterns. As you move along the transect, you're usually following some environmental gradient — perhaps moving away from water, up a slope, or across different soil types. The species you find change in predictable ways, telling the story of how organisms adapt to their environment.

**Trap**: Many students think you can choose where to place your transect based on where it 'looks interesting'. Actually, transect placement should be systematic — often running perpendicular to the environmental gradient you're studying, like from water's edge inland.

4. The Dance of Predators and Prey

In the Canadian wilderness, lynx hunt snowshoe hares. For nearly a century, fur trappers have kept records of both species. The data reveals something remarkable: their populations rise and fall in perfect cycles, like dancers following the same rhythm.

Here's how the cycle works: When hare numbers are high, lynx have plenty to eat. Well-fed lynx reproduce successfully, and lynx numbers increase. But more lynx means more hunting pressure on the hares. Hare numbers start to fall.

As hares become scarce, lynx struggle to find food. Hungry lynx reproduce less successfully, and some starve. Lynx numbers drop. With fewer predators around, the surviving hares face less hunting pressure. Hare numbers begin to recover, and the cycle starts again.

This creates the characteristic predator-prey cycle: prey numbers peak first, followed by predator numbers. Then prey numbers crash, followed by predator numbers. The cycle typically takes several years to complete.

**Trap**: Students often think predator and prey numbers rise and fall together. Actually, there's always a time lag — predator numbers follow prey numbers, not the other way around. The prey population drives the cycle.

5. Putting It All Together: Ecological Surveys

Real ecological surveys combine multiple methods. You might use transects to map how species change across a habitat, then use quadrats at specific points along the transect to get detailed abundance data. This gives you both the big picture and the fine detail.

The data you collect tells stories. A transect from woodland edge to centre might show how light-loving plants give way to shade-tolerant species. Quadrat data might reveal that certain plants only grow where soil pH is just right. Predator-prey surveys might explain why rabbit numbers crashed last year.

But ecological surveys have limitations. Quadrats only sample a tiny fraction of the total habitat. Mobile animals might move in and out of quadrats. Some species are hard to identify or count accurately. Weather, season, and time of day all affect what you find.

That's why ecologists use statistical methods to account for uncertainty and always repeat their surveys. They also combine different techniques — camera traps for mammals, mist nets for birds, pitfall traps for ground-dwelling invertebrates. Each method has strengths and weaknesses.

6. Exam Success: Getting Full Marks

Ecology questions often test your ability to analyse data and suggest improvements to methods. Examiners love asking about sources of error and how to make surveys more reliable. Here's how to nail these questions.

**Command word focus**: 'Describe' questions want you to state what happens in sequence. 'Explain' questions need you to give reasons why. 'Evaluate' questions require you to weigh up advantages and disadvantages before reaching a conclusion.

For calculation questions, always show your working clearly. Write the formula, substitute the values, then calculate the answer with units. Even if your final answer is wrong, you can still get method marks for the correct approach.

Remember that ecological surveys are tools for understanding nature, not just academic exercises. The methods you've learned help scientists track endangered species, monitor pollution effects, and predict how ecosystems will respond to climate change. Every quadrat thrown and every transect walked contributes to our understanding of the living world.