Organisms and Environment » Distribution Investigation

What you'll learn this session

Study time: 30 minutes

How to design and conduct ecological distribution investigations
Methods for measuring organism abundance and distribution
Sampling techniques including quadrats and transects
How to analyse and present ecological data
Factors affecting the distribution of organisms in different habitats

Introduction to Distribution Investigations

Ecologists study how organisms are distributed in their environment and what factors affect where they live. Distribution investigations help us understand these patterns and the relationships between organisms and their habitats.

Key Definitions:

Distribution: The way in which organisms are arranged in an area or habitat.
Abundance: The number of individuals of a species in a given area.
Habitat: The place where an organism lives, which provides the conditions it needs to survive.
Population: All the individuals of a species living in a particular area at a particular time.

🌍 Why Study Distribution?

Understanding where organisms live helps us:

Identify environmental factors affecting species
Monitor changes in ecosystems over time
Predict how species might respond to environmental changes
Develop conservation strategies for endangered species

🔬 Scientific Method in Ecology

Distribution investigations follow the scientific method:

Ask a question about distribution patterns
Form a hypothesis to explain these patterns
Design an investigation to test your hypothesis
Collect and analyse data
Draw conclusions based on evidence

Planning a Distribution Investigation

Before heading into the field, you need a clear plan for your investigation. This ensures you collect reliable data that can answer your research question.

Research Questions and Hypotheses

Good ecological investigations start with clear questions such as:

How does the distribution of dandelions change with distance from a footpath?
Is there a relationship between soil pH and the abundance of earthworms?
How does the distribution of limpets change from the upper to lower shore?

From these questions, you can form testable hypotheses:

"The abundance of dandelions will decrease as distance from the footpath increases due to reduced soil compaction."

Case Study Focus: Rocky Shore Zonation

Rocky shores show clear patterns of species distribution in zones from high to low tide. Different organisms are adapted to survive at different levels of the shore. For example, limpets and barnacles can withstand longer periods out of water and are found higher up, while seaweeds like kelp need to be submerged most of the time and are found lower down. This zonation pattern can be investigated using a line transect from the high tide mark to the low tide mark.

Sampling Techniques

Since it's usually impossible to count every organism in a habitat, we use sampling techniques to estimate population sizes and distribution patterns.

🧭 Quadrats

Square frames (usually 0.5m × 0.5m or 1m × 1m) placed randomly or systematically to sample an area. Used to count organisms or measure percentage cover of plants.

Best for: Slow-moving or stationary organisms like plants, lichens and some invertebrates.

📏 Line Transects

A line (usually a tape measure) laid across an area where environmental conditions change, such as from a hilltop to a valley or from high to low tide on a beach.

Best for: Studying how distribution changes along an environmental gradient.

🎯 Belt Transects

A combination of line transect and quadrats, where quadrats are placed at regular intervals along a transect line.

Best for: Detailed study of changes in distribution across a gradient while capturing more data than a simple line transect.

Collecting Data in the Field

When conducting your investigation, you'll need to collect both biotic (living) and abiotic (non-living) data to understand what factors might be influencing distribution.

🐾 Biotic Measurements

Frequency: The percentage of quadrats in which a species appears
Abundance: The number of individuals of a species
Percentage cover: The percentage of ground covered by a plant species (viewed from above)
Density: The number of individuals per unit area

🌞 Abiotic Measurements

Light intensity: Using a light meter
Temperature: Air, water, or soil temperature
Soil pH: Using a soil testing kit
Soil moisture: Using a moisture meter
Wind speed: Using an anemometer

Random vs. Systematic Sampling

The way you position your quadrats or transects is crucial for collecting reliable data:

Random sampling: Quadrats are placed using random coordinates or by throwing the quadrat. This avoids bias but may miss important patterns.
Systematic sampling: Quadrats are placed at regular intervals (e.g., every 5 metres along a transect). This ensures coverage across an environmental gradient.

Practical Tip: Creating Random Coordinates

To generate random coordinates for quadrat placement:

Mark out your study area with a grid system
Use a random number generator or table of random numbers to select coordinates
Place your quadrat at each set of coordinates and record your data

For a truly random sample, you need enough quadrats to represent the area (usually at least 10).

Analysing and Presenting Your Data

After collecting your data, you need to analyse it to identify patterns and relationships between species distribution and environmental factors.

Data Analysis Techniques

Common methods for analysing ecological data include:

Calculating means, medians and ranges for your measurements
Creating graphs to visualise relationships (e.g., scatter plots to show correlation between an environmental factor and species abundance)
Statistical tests to determine if observed patterns are significant (e.g., Spearman's rank correlation)
Creating distribution maps or kite diagrams to show how species abundance changes along a transect

📊 Graphing Your Results

Different types of graphs are suitable for different data:

Bar charts: Good for comparing abundance of different species
Line graphs: Show changes in a variable along a transect
Scatter graphs: Show correlation between two variables (e.g., light intensity and plant abundance)
Kite diagrams: Show changes in multiple species along a transect

⚠ Sources of Error

Be aware of potential issues that could affect your results:

Insufficient sample size
Seasonal variations (time of year/day)
Weather conditions during sampling
Observer bias in identifying or counting organisms
Equipment inaccuracies

Factors Affecting Distribution

Many factors can influence where organisms are found. Understanding these helps explain the patterns you observe.

🌏 Abiotic Factors

Light intensity
Temperature
Water availability
Soil pH and nutrients
Wind exposure
Oxygen levels (in water)

🐦 Biotic Factors

Competition for resources
Predation
Disease
Availability of food
Presence of pollinators
Symbiotic relationships

🏠 Human Impacts

Pollution
Habitat destruction
Introduction of invasive species
Climate change
Trampling and soil compaction

Drawing Conclusions

The final step is to interpret your results and draw conclusions about what factors are influencing the distribution patterns you observed.

Linking Data to Explanations

When explaining your findings, consider:

How your results support or contradict your hypothesis
Which environmental factors correlate most strongly with species distribution
How the adaptations of organisms might explain their distribution
Alternative explanations for the patterns you observed
How your findings compare with published research

Case Study: Dandelion Distribution in a School Field

Students investigated dandelion distribution across their school field using quadrats placed at 5m intervals along three transects. They found dandelion density was highest near paths and decreased with distance. They also measured soil compaction and found a positive correlation with dandelion abundance. This supported their hypothesis that dandelions thrive in compacted soil, likely because their deep taproots give them an advantage over other plants in these conditions. This simple investigation demonstrated how human activity (walking) can influence plant distribution through its effect on soil conditions.

🧠 Test Your Knowledge!