Calculator inputs
Use the form below to estimate demographic migration rates and optional gene flow migration from FST and effective population size.
Example data table
This sample shows how seasonal wildlife monitoring data can be organized before calculation.
| Study site | Initial population | Final population | Immigrants | Emigrants | Births | Deaths | Time | Net migrants |
|---|---|---|---|---|---|---|---|---|
| North Wetland | 1,200 | 1,295 | 160 | 70 | 45 | 30 | 12 months | 90 |
| South Wetland | 860 | 820 | 40 | 85 | 22 | 17 | 12 months | -45 |
| River Edge | 430 | 468 | 52 | 21 | 18 | 11 | 6 months | 31 |
| Forest Patch | 1,540 | 1,498 | 76 | 110 | 35 | 43 | 12 months | -34 |
Formula used
1) Net migrants
Net migrants = I − E
2) Gross movement rates
Gross immigration rate = I / t
Gross emigration rate = E / t
3) Net migration rate
Net migration rate = (I − E) / t
4) Per-capita migration rate
Per-capita net migration rate = (I − E) / (Navg × t), where Navg = (N₀ + N₁) / 2.
5) Net migration percentage
Net migration % = [(I − E) / Navg] × 100
6) Population accounting check
Observed change = N₁ − N₀
Expected change = B − D + I − E
7) Optional gene flow estimate
m ≈ [(1 / FST) − 1] / (4Ne). This rearranges the classic island-model approximation and is best treated as a simplified genetics estimate, not a field census substitute.
How to use this calculator
- Enter the starting and ending population counts for the same biological population.
- Add the number of immigrants and emigrants recorded during the study period.
- Include births and deaths when you want a stronger population accounting check.
- Enter the study duration and choose the matching time unit.
- Add study area if you want density-adjusted movement.
- Add FST and effective population size only for a simplified population-genetics migration estimate.
- Press the calculate button to show the result above the form, inspect the results table, and review the chart.
- Use the CSV or PDF buttons to export your computed results for reports, worksheets, or lab documentation.
FAQs
1) What does migration rate mean in biology?
It describes how quickly individuals move into or out of a population over a defined period. It can be expressed as gross immigration, gross emigration, or net migration.
2) Why does this calculator use average population?
Average population provides a more balanced denominator when population size changes during the study. That makes per-capita rates more stable than using only the starting count.
3) What is the difference between gross and net migration?
Gross immigration counts only incoming individuals. Gross emigration counts only outgoing individuals. Net migration combines both, showing whether the population gained or lost individuals overall.
4) Can I leave births and deaths at zero?
Yes. The calculator will still compute movement-based migration results. Births and deaths mainly improve the consistency check between observed population change and expected change.
5) What does a negative migration rate indicate?
A negative value means more individuals left than entered during the study period. In ecology, that often signals outflow, dispersal pressure, or less favorable local conditions.
6) When should I use the FST and Ne fields?
Use them for simplified population-genetics interpretation when you already have an FST estimate and an effective population size value. They are not required for field-count migration analysis.
7) Can I compare two habitats with this tool?
Yes. Run the calculator separately for each habitat using the same time unit and similar survey methods. Then compare net rates, per-capita rates, and density-adjusted rates.
8) Why might observed and expected changes differ?
Differences can come from incomplete counts, detection bias, unrecorded births or deaths, missed movement events, or mismatched study boundaries across sampling periods.