Workshop 1: Introduction to Distance Sampling

Introduction

The general objective of this 4 day workshop is to give wildlife managers, ecologists and statisticians a solid grounding in the basic methods for design and analysis of distance sampling surveys. Teaching will be a combination of lectures, computer sessions and discussion groups.

The course is intended for scientists who are carrying out population assessments of wildlife. We anticipate a mix of marine and terrestrial mammalogists, ornithologists and fisheries biologists, plus statisticians who have an interest in the topic. To gain maximum benefit from the workshop, biologists and ecologists should have some quantitative training, including basic statistical methods (point and variance estimation and confidence intervals), and some experience of survey design would be useful. Some familiarity with the concepts of line or point transect sampling would be an advantage, although no prior knowledge of these topics is assumed.

Much of the teaching will be through use of PCs, and some knowledge of Microsoft Windows will be assumed. Participants are welcome to bring their own portable computers (see Information for participants for minimum requirements).

Distance sampling

The term ‘distance sampling’ covers a range of methods for assessing wildlife abundance:

• line transect sampling, in which the distances sampled are distances of detected objects (usually animals) from the line along which the observer travels
• point transect sampling, in which the distances sampled are distances of detected objects (usually birds) from the point at which the observer stands
• cue counting, in which the distances sampled are distances from a moving observer to each detected cue given by the objects of interest (usually whales)
• trapping webs, in which the distances sampled are from the web centre to trapped objects (usually invertebrates or small terrestrial vertebrates)
• migration counts, in which the ‘distances’ sampled are actually times of detection during the migration of objects (usually whales) past a watch point

Workshop content

The workshop will concentrate primarily on line and point transect sampling. The concepts of distance sampling will be explained and the assumptions of the methods discussed. Although the basic theory will be covered, the focus of the workshop will be on practical application of the methods.

The workshop will start with an introduction to wildlife population assessment methods, and demonstration of how line and point transect methods are generalizations of sample count methods (strip counts and point counts respectively). The underlying theory and assumptions of both line and point transect sampling will be covered, and the relative merits of the two approaches in different circumstances discussed. Computer sessions will follow, to train participants in the use of Distance for straightforward data sets.

More complex issues will then be covered. Special methods are required when animals occur in groups or “clusters”. For example, size bias can occur – that is large clusters have a higher probability of detection than small clusters, so that population size is overestimated. Methods for adjusting for this bias will be given. Another issue is stratification, which is used to improve the precision of estimates when animal abundance, detection probability or clustering varies over time or space. Good survey design is an essential ingredient of a successful survey so design issues and field methods will be covered in detail. Some specialized applications of distance sampling such as cue counting, migration counts, trapping webs and indirect counts (e.g., dung or nests) will be mentioned.

Automated survey design (using Distance to assist in determining where survey effort will be allocated) as well as the incorporation of covariates in modelling of detection functions have moved from the advanced workshop to this introductory workshop.

Participants will be taught how to use the newest, windows-based version of the industry-standard program Distance. Exercises in applications of Distance will be set, and participants are encouraged to bring (a subset of) their own data sets for preliminary analysis during the workshop. Towards the end of the workshop, some time is put aside for informal discussion groups composed of participants with common interests or problems. In the past, these groups have included the following topics: population trend analysis; transect surveys in rain forest; marine mammal surveys; songbird surveys; seabird surveys; and dung surveys.

Participants interested in going beyond the basics of distance sampling are encouraged to register for the advanced workshop, which takes place directly after this workshop finishes.

The workshop will begin at 09:00 on Tuesday, 20th with registration at CREEM from 08:45. The workshop will finish at 17:00 on Friday, 23rd.

Materials provided

The following publications and software will be provided to participants at no additional cost.

• Introduction to Distance Sampling: Estimating Abundance of Biological Populations. S.T. Buckland, D.R. Anderson, K.P. Burnham, J.L. Laake, D.L. Borchers and L. Thomas. Oxford University Press, 2001.
• Lecture notes containing all of the overheads and slides shown during the workshop. This material supplements and extends the distance sampling text book.
• Distance software