What is distance sampling?

Definition

Distance sampling is a field survey and analysis technique designed to estimate the density or population size of species

It is ideal for situations where:

  • You cannot detect all individuals in an area
  • You cannot identify individuals from their markings or vocalisations
  • You can gather observations 📝 using line transect surveys or point counts

Alternatives

Alternatives to distance sampling for estimating population size are:

  • Capture-Mark-Recapture (CMR) using data from:
    • Camera trapping 📷 or direct sightings for species with individual markings
    • Trapping and marking individuals such as birds 🐦 or rodents 🐭
    • Estimating home range size 🏠 and overlap of known individuals

Did you count every individual?

You can calculate density as number of individuals divided by area surveyed, if you are able to reliably:

  1. Count all individuals within a survey area (a total count)
  2. Measure the size of area you have surveyed

You know from your own experience that it’s virtually impossible to count all individuals in an area with certainty

It is inevitable that some individuals are missed, in at least some surveys, because of:

  • Intervening vegetation 🌿 or landscape features 🌄
  • Poor weather ☔ or difficult survey conditions 🌊
  • The species’ secretive 💨 or nocturnal 🌘 behaviour

This is especially true for the rare or elusive species we target in conservation

Distance sampling allows for uncertainty in whether you detected all the individuals

What area did you survey?

It can also be difficult to draw a boundary around the precise area that you have surveyed

Survey methods differ in how much of the landscape they allow you to survey. You might see further with aerial ✈️ or vehicle 🚚 ⛵ surveys compared to transects on foot 👢

However, with all surveys there is uncertainty about how far away you can detect species effectively, so you don’t know precisely what area you have covered

Even the same method, such as line transects, can give you different coverage in different places or times, as visibility changes according to vegetation, weather and terrain

Benefits of distance sampling

Distance sampling is designed to overcome these difficulties in estimating density

Distance sampling enables us to:

  1. Calculate the probability of detecting an individual based on its distance from the transect line or point.  We can use this to determine what proportion of the population we detected, and thus the total population size
  2. Use our field sightings to accurately estimate the area that we have surveyed

Our calculation of density is an estimate, because it is based on a sample of the population and area, rather than a total count

As well as our density estimate, we calculate standard errors and confidence intervals, which communicate our level of certainty about our estimate (how accurate we believe it is)

Key concepts

This raises several key concepts that we’ll explore in more detail in this Theory course:

  • Detection and false absences (failure to detect)
  • Using information from repeat surveys to estimate detectability

In the Analysis with R course, we’ll look at:

  1. Using R to model the density of animals, in the Analysis module
  2. Incorporating environmental and survey-specific data into our models to investigate correlates of spatial and temporal variation in density, in the Covariates module
  3. Using output to compare models and evaluate the importance of covariates, in the Use output module