Ecosystems and Populations » Distribution Investigation Methods

Introduction to Distribution Investigation Methods

When biologists want to study ecosystems, they need to find out where different species live and how many there are. This is called investigating distribution. Since we can't count every single organism in a habitat (imagine trying to count every daisy in a field!), we use special sampling methods to get reliable estimates.

Key Definitions:

• Distribution: Where organisms are found within a habitat or ecosystem.
• Population: All the organisms of one species living in the same area at the same time.
• Sampling: Taking measurements from a small part of the habitat to estimate what's happening in the whole area.
• Population density: The number of organisms per unit area (e.g., per m²).
• Frequency: How often a species appears in the samples taken.

Quadrat Sampling

Quadrats are square frames used to sample plants and slow-moving animals like snails. They help us study what's living in specific areas without having to count everything in the entire habitat.

Using Quadrats Effectively

A quadrat is typically a 1m × 1m square frame, though smaller ones (like 0.5m × 0.5m) can be used for detailed work. The key is to place them randomly to avoid bias - you don't want to accidentally choose only the areas with lots of flowers!

🎲 Random Sampling

Use random number generators or throw the quadrat over your shoulder to ensure unbiased placement. This gives every part of the habitat an equal chance of being sampled.

📈 What to Count

Count individual organisms, estimate percentage cover for plants, or record presence/absence. Choose the method that works best for your species.

📊 Sample Size

Take at least 10 quadrat samples, but more is better for accuracy. The more variable your habitat, the more samples you need.

Transect Methods

Transects are straight lines across a habitat used to study how species distribution changes from one area to another. They're perfect for investigating transitions between different habitats, like from woodland to grassland.

Types of Transects

There are two main types of transects, each with different advantages depending on what you're studying.

📏 Line Transects

A measuring tape is laid across the habitat and you record what's touching the line at regular intervals (e.g., every metre). Quick and simple, but gives limited data.

▦ Belt Transects

Quadrats are placed at regular intervals along the line, giving much more detailed information about species abundance and distribution patterns.

Capture-Recapture Method

This clever technique estimates populations of mobile animals that you can't easily count directly, like beetles, fish, or small mammals. It's based on the idea of marking some animals, releasing them, then seeing how many marked ones you catch later.

How Capture-Recapture Works

The method involves three main steps: capture and mark, release and wait, then recapture and count. The proportion of marked animals in your second sample helps estimate the total population size.

🐛 Step 1: Capture & Mark

Catch animals safely and mark them with harmless paint, tags, or clips. Count how many you've marked before releasing them.

⌛ Step 2: Wait & Mix

Allow time for marked animals to mix back into the population. Usually wait a few days to a week depending on the species.

📜 Step 3: Recapture

Catch another sample and count how many are marked versus unmarked. Use the Lincoln Index formula to calculate population size.

Lincoln Index Formula:
Population estimate = (Number marked in first sample × Total caught in second sample) ÷ Number of marked animals recaptured

Avoiding Sampling Bias

Bias occurs when your sampling method accidentally favours certain areas or species, making your results unreliable. Good scientists work hard to minimise bias and make their results as accurate as possible.

Common Sources of Bias

Understanding where bias comes from helps you avoid it. Here are the main culprits and how to deal with them.

⚠ Personal Bias

Unconsciously choosing 'interesting' areas or avoiding difficult terrain. Solution: Use random sampling methods and stick to your planned sampling points.

🕑 Temporal Bias

Only sampling at certain times of day or seasons. Solution: Consider when your target species are most active and sample accordingly.

Calculating Population Data

Once you've collected your data, you need to process it to get meaningful results. The main calculations involve working out density, frequency and population estimates.

Key Calculations

These calculations turn your raw data into useful information about species distribution and abundance.

Population Density = Total number of organisms ÷ Total area sampled

Frequency = (Number of samples containing the species ÷ Total number of samples) × 100

Mean = Sum of all values ÷ Number of samples

📈 Example Calculation

10 quadrats (each 1m²) contained: 5, 3, 7, 0, 2, 8, 4, 1, 6, 4 daisies respectively. Mean = 40 ÷ 10 = 4 daisies per m². Frequency = 9/10 × 100 = 90%.

Practical Considerations

Successful fieldwork requires careful planning and attention to detail. Weather, equipment and safety all play important roles in getting good results.