Schnabel Method for Estimating Population Size

What is the Schnabel Method?

The Schnabel Method is a capture-recapture technique used to estimate the size of an animal population. It's particularly useful when the population is large and marked animals can be reliably identified between trapping sessions.

How to Use This Calculator

1. Enter the number of animals captured in each of five trapping sessions in the left input fields.
2. Enter the number of animals that were recaptured (re-captured) in each session in the right input fields.
3. Click the "Calculate" button to estimate the population size.
4. The calculator will display the estimated population size, standard error, and 95% confidence interval.

Understanding the Inputs

For each session, you need to record two values:

• Captured: The number of animals captured during that session and marked for identification.
• Recaptured: The number of marked animals that were recaptured during that session.

How the Formula Works

The Schnabel Method uses the following formula to estimate population size:

Population Size (N) = (Σ(r_i × c_i)) / Σ(r_i)

Where:

• Σ = Sum across all sessions
• r_i = number recaptured in session i
• c_i = number captured in session i

This formula assumes that the proportion of marked animals in the population is equal to the proportion of marked animals recaptured in each session.

Limitations

The Schnabel Method provides a good estimate when:

• Marking does not affect the behavior of animals
• There is no immigration or emigration between sessions
• Sampling is random
• The population is closed (no births, deaths, or other changes)

For more accurate results, consider using multiple sessions and ensuring that the recapture rates are consistent across all sessions.