Chapter 9 - Wildlife population estimates by census and distance measuring techniques

Counting the number of sightings forms the basis for estimating density for many animal groups. This is particularly the case for large or easily seen animals such as birds, large grassland mammals, whales, crocodillians and large, active insects such as butterflies. While it may be possible to count animals from a suitable vantage point or while moving along a transect, the count can only be converted to a density estimate if the area scanned can be estimated. This simple approach (see Chapter 4) is often difficult to undertake for two reasons, firstly, it may not be possible to estimate sufficiently accurately the area scanned and secondly, not all of the animals present may have been spotted. Distancing sampling methods have been developed to allow for these problems by assuming that the likelihood that an individual will be observed will decline in a mathematically definable way with distance. The methods discussed here are also useful for small, sessile or slow moving organisms such as barnacles, corals and molluscs and some can also be applied to data collected non-visually, for example a bird census may be based on bird song or an electric fish survey on the detection of the distinctive electrical signals. One problem , as can be the case with a visual survey, is to ensure that the same individual is only counted once. Distance sampling and nearest neighbour methods give absolute estimates of animal density. These represent the fourth and final approach to absolute population estimation the others are mark-recapture (Chapter 3), removal trapping (Chapter 7) and direct counting of a unit volume or area of habitat (Chapters 4 to 6). We briefly introduce wildlife census methods here rather than with other direct counting methods because they are the natural starting point for from which to introduce distance sampling methods.

By convention a census is defined as the counting of all the individuals belonging to the group of interest within a defined area and a survey when only a proportion are counted. When the study area can be divided into quadrates which can vary in size or shape, e.g., a rectangular 10 ha area of forest or a 0.1 m2 core or grab sample then a census can be made of a randomly selected set and standard statistical theory used to estimate average density and its variance for the entire area. Survey methods such as distance sampling and nearest neighbour aim to estimate density using observations on the distance between animals or from a selected line or point to the animals and thus do not require the worker to accurately map out or define the sampling area. They are thus particularly appropriate for the estimation of population density for large animals living at low density in difficult to traverse habitat. For example, distance sampling using a Fourier series is frequently the method of choice for estimating population size of primates in neotropical forest. The animals are counted while walking along specially cut forest trails. The high density of the forest and the low density and mobility of the monkeys would make it futile to try to census a number of pre-defined quadrats.

9.1 Census methods 9.2 Point and line survey methods 9.2.1 Indices of abundance using transects 9.2.2 Methods based on flushing 9.2.3 Line transect methods: the Fourier series estimator 9.2.4 Point transects 9.3 Spatial distribution and plotless density estimators 9.3.1 Closest individual or distance method 9.3.2 Nearest neighbour methods