Distance sampling is a widely used method for estimating the abundance and density of biological populations. It involves counting objects of interest, such as birds, in a sample of line transects or points, and modeling the probability of detecting objects as a function of distance from the transect or point. Some key aspects of distance sampling are:
Overview of Distance Sampling
Distance sampling is based on the idea that the probability of detecting an object, such as a bird, decreases with increasing distance from the transect or point. So, to estimate density, we need to model this “detection function”, which gives the probability of detection as a function of distance. The key assumptions are:
- Objects directly on the transect or point are detected with certainty (probability = 1).
- Detections are independent events.
- Distances to objects are measured accurately.
Based on a sample of detected distances and the modeled detection function, we can estimate the number of objects in covered regions and the effective strip width (or radius for points). This allows us to estimate density as:
D = n / (L * ESW)
Where:
- D = Density
- n = Number of detected objects
- L = Total length of transects
- ESW = Effective strip width (average detection width)
Distance sampling methods provide less biased and more accurate density estimates compared to simple plot sampling, by accounting for imperfect detection. The key is modeling the detection function well.
Methods for Estimating the Detection Function
There are several common ways to model the detection function in distance sampling:
1. Histogram Approaches
Bins distances into intervals and models detection probability in each distance interval. Simple but can be biased if few detections at large distances.
2. Parametric Key Functions
Fits a parametric model with particular shape. Some common examples are the half-normal, hazard-rate, and uniform models. Provides smoothing and robustness.
3. Non-parametric Models
Empirical step functions or spline models that make no assumptions about shape. Flexible but variable at far distances where detections are few.
Distance Sampling Analysis Process
The key steps in a distance sampling analysis are:
- Design survey with line transects or points.
- Conduct field work and record distances to detections.
- Explore data and truncation distances.
- Select detection function model.
- Fit model and estimate key parameters.
- Estimate density and abundance.
Careful field methods, exploration of distance data, and selection of the detection function model are key factors that influence accuracy and precision of estimates.
Design Considerations
Some important design considerations for distance sampling include:
- Straight, randomly placed line transects are ideal.
- Transects should be long enough and numerous enough to get sufficient detections at various distances.
- Observers need training on distance estimation, object detection, and protocols.
- Stratification can improve precision.
- Paired observer methods can estimate detection probability on the line or point.
Advantages of Distance Sampling
Some key advantages of distance sampling:
- Accounts for imperfect detection unlike plot sampling.
- More accurate and precise density and abundance estimates.
- Relatively simple field methods.
- Can be used for many taxa in various habitats.
- Analysis methods and software available.
Challenges and Considerations
Some challenges and considerations when using distance sampling:
- Requires moderate sample sizes and sufficient detections at range of distances.
- Detection function modeling can be tricky.
- Accurate distance measurements in field.
- Violations of key assumptions can cause bias.
- Costly surveys over large remote regions.
- Advanced analyses needed for some data types.
Uses for Birds
Distance sampling is an excellent method for estimating densities and abundances of bird populations. Some examples of using distance sampling for birds include:
- Estimating territorial songbird densities in forests.
- Waterfowl abundance in wetlands.
- Shorebird density on coastal mudflats.
- Seabird populations.
- Raptor density in open habitats.
Distance sampling accounts for the decline in detectability with distance for visually and aurally detected birds. It works well for most bird species that can be detected at reasonable distances from transects or points.
Analysis Software
Specialized software is available to assist with distance sampling analysis, model fitting, and estimation. Popular options include:
- Distance – The standard software for distance sampling analysis.
- R packages – Options like Distance, MRDS, and unmarked provide analysis tools within R.
- CoPACE – Multi-taxa model fitting and abundance estimation in Windows GUI.
- Density – Simpler distance sampling analysis focused on line transect surveys.
These tools automate fitting of detection functions, diagnostic plots, and estimating densities and abundances. They provide a range of options for analysis while maintaining the key distance sampling framework.
Case Study: Wood Thrush Density Estimation
As an example, distance sampling was used to estimate Wood Thrush density in a forest study area in Maryland, USA (Wilson et al 2015). The key steps were:
- 43 line transects (4-5 km long) were placed randomly in the 40 km2 study area.
- Two observers walked the transects, recording distances to visual and aural detections.
- Detection data were explored and truncated at 125 m distance.
- A half-normal key function with cosine series expansion was fit.
- Model estimated an effective strip width of 63.2 m.
- Density was estimated as 2.08 birds/ha, for an abundance estimate of 8,320 in the area.
This example illustrates the field and analytical process for estimating bird density by distance sampling of line transects in a forest habitat.
Conclusion
In summary, distance sampling is an effective method for estimating densities and abundances of biological populations like birds while accounting for imperfect detection. It involves counting objects in sampled transects or points, modeling the detection function, and estimating an effective area or distance for density estimates. Good survey design, field methods, and analytical approaches are needed. When implemented well, distance sampling provides accurate density and abundance estimates for many bird populations across various habitats and landscapes.
