The importance of survey design in distance sampling

The importance of survey design in distance sampling: field evaluation using domestic sheep
Tom A. Porteus, Suzanne M. Richardson, and Jonathan C. Reynolds

Wildlife Research 38(3) 221-234 http://dx.doi.org/10.1071/WR10234

Wildlife managers and researchers regularly use distance sampling. Here is a paper that emphasizes the importance of survey design in distance sample. Management decisions are often made based on these estimates. Without knowledge of true population size it is not possible for wildlife biologists to evaluate how biased the estimates can be if survey design is compromised.

Here the researchers did a clever field evaluation using domestic sheep. Their aims were to use distance sampling to estimate population size for domestic sheep free-ranging within large enclosed areas of hill country. By comparing estimates against actual numbers, they examined how bias and precision are impaired when survey design is compromised

Both line and point transect sampling were used to derive estimates of density for sheep on four farms in upland England. In Stage I they used limited effort and different transect types to compromise survey design. In Stage II they increased effort in an attempt to improve on the Stage I estimates. They also examined the influence of a walking observer on sheep behaviour to assess compliance with distance sampling assumptions and to improve the fit of models to the data.

The results clearly demonstrated that distance sampling can lead to biased and imprecise density estimates if survey design is poor, particularly when sampling high density and mobile species that respond to observer presence. The researchers say in Stage I, walked line transects were least biased; point transects were most biased. Increased effort in Stage II reduced the bias in walked line transect estimates. For all estimates, the actual density was within the derived 95% confidence intervals, but some of these spanned a range of over 100 sheep per km².

The researchers have clearly shown that survey design is vitally important in achieving unbiased and precise density estimation using distance sampling. Adequate transect replication reduced the bias considerably within a compromised survey design.

The researchers sign off like this “Management decisions based on poorly designed surveys must be made with an appropriate understanding of estimate uncertainty. Failure to do this may lead to ineffective management.”

Tailpiece: We had sent mail to one wildlife warden regarding the poor design of some of his census techniques. We had also requested him to contact well versed scientists to set things right. He never responded. This post is intended to show him and his ilk how design flaws can throw the entire process out of kilter.