Population Estimation
Estimate population size using mark-recapture and trap data.
Overview
Population estimation methods calculate total population size from sample data. FieldEco supports several mark-recapture approaches and effort-based metrics.
Lincoln-Petersen Estimator
Classic two-sample mark-recapture method.
Method
- Session 1: Capture, mark, and release M individuals
- Session 2: Capture C individuals, R are recaptures
Formula
N = (M × C) / R
Where:
- N = population estimate
- M = marked in first sample
- C = total caught in second sample
- R = recaptures
Assumptions
- Closed population (no births, deaths, migration)
- Equal catchability
- Marks not lost
- Short time between samples
Example
- Marked 50 mice (M = 50)
- Caught 60 in second sample (C = 60)
- 10 were recaptures (R = 10)
- N = (50 × 60) / 10 = 300 mice
Chapman Estimator
Bias-corrected version of Lincoln-Petersen.
Formula
N = ((M+1)(C+1) / (R+1)) - 1
When to Use
- Small sample sizes
- Few recaptures
- More accurate than LP for small R
Notes
- Less biased than LP
- Preferred for most applications
- Standard errors available
Schnabel Estimator
Multiple mark-recapture sessions.
Formula
N = Σ(Ct × Mt) / Σ(Rt)
Where:
- Ct = caught in session t
- Mt = marked before session t
- Rt = recaptures in session t
Advantages
- Uses all capture data
- More precise than two-sample
- Reduces variance
Requirements
- 3+ capture sessions
- Cumulative marking
- Closed population (or model violations)
CPUE Analysis
Catch Per Unit Effort - relative abundance metric.
Formula
CPUE = Total Captures / Total Effort
Effort Units
| Method | Effort Unit |
|---|---|
| Traps | Trap-nights |
| Mist nets | Net-hours |
| Fishing | Hook-hours |
Example
- 25 captures in 100 trap-nights
- CPUE = 0.25 captures/trap-night
Applications
- Compare between sites
- Track changes over time
- Index of abundance
Trap Success Rate
Proportion of traps catching animals.
Formula
Trap Success = (Successful Traps / Total Trap-nights) × 100%
Interpretation
| Success Rate | Interpretation |
|---|---|
| <5% | Low population or poor placement |
| 5-15% | Moderate |
| 15-30% | Good |
| >30% | High population |
Using Analysis in FieldEco
Running Mark-Recapture Analysis
- Complete trap survey with check sessions
- Ensure individuals are marked
- Open Analysis → Population Estimation
- Select estimator method
- View results
Data Requirements
- Individual-level capture data
- Session dates/times
- Mark identification
- Capture history per individual
Results Include
- Population estimate
- Standard error
- 95% confidence interval
- CPUE summary
Best Practices
Study Design
- Plan multiple sessions
- Use adequate trap density
- Allow sufficient time between sessions
- Mark individuals uniquely
Assumptions to Check
- Population closure
- Equal catchability
- Mark retention
- Independence of captures
Reporting
Include:
- Estimator used
- Sample sizes (M, C, R)
- Effort (trap-nights)
- Confidence intervals
- Assumption violations
Model Violations
| Violation | Effect | Solution |
|---|---|---|
| Mark loss | Underestimate | Use permanent marks |
| Trap-happy | Overestimate | Use robust designs |
| Trap-shy | Underestimate | Vary bait/location |
| Open population | Variable | Use open models |
Related Documentation
- Traps Survey - Data collection
- Diversity Indices - Community metrics
- Field Tools - GPS for trap mapping