Collecting field data for distance sampling involves:
Surveys can occur:
As well as live animals, you can survey for animal signs such as:
Translate signs to density
If you’re estimating the density of animals signs, you need to combine that with information on sign creation and decay rates to estimate population density
Encounter rate is the number of sightings per unit of distance you walk along your transects
For example:
Encounter rates will obviously vary between species and sites, and can also be affected by many other variables
Encounter rates are useful because they provide us with information on the efficiency of our survey technique. We learn more about encounter rates in the Survey design course, as we use them to help us determine the distance that we need to survey in order to collect sufficient sightings for analysis
When analysing line transect or point count data, it’s important to distinguish between replicate and repeat surveys
Replicates are transects in different locations, and therefore sampling a different part of the population
The number of replicate transects is your total sample size
Repeats are return visits to the same transect
By repeating a survey of the same transects you collect more sightings to help construct your detection function, but you are not increasing your sample size or the precision of your density estimates
We learn more about replicates versus repeats in the Analysis and Survey design courses
Distance sampling relies on four assumptions for trustworthy results
Random placement: Transects are randomly placed with respect to the distribution of animals
The animals themselves do not need to be distributed at random, as long as the transects are randomly placed in the environment, and do not follow features such as roads that may themselves influence the animals’ locations
Detection is certain: You will always detect animals (or sign) that are directly on the transect line
In other words, detection probability on the transect line itself is 1
No movement: The animal is detected before it moves in response to the observer
Your record of the animal must match its true location during the survey, so that you can accurately model the pattern of detectability at different distances from the transect. Movements in response to the observer before detection will make your density estimate less reliable
Accurate measurements: Distances and angles to the animal are measured accurately
Alternatively, animals are assigned to the correct distance interval, for example 0-5m, 5-10m etc
Covariates are environmental variables that influence density and/or detection of your species
Covariates can be:
To meet the assumptions of distance sampling, probability of detection and density must either be: