Geoscience Reference
In-Depth Information
4
Line Transe
ct Sampling
Jorge Navarro and Raúl Díaz-Gamboa
4.1 Introduction
Line transect sampling is intended not only for the estimation of the abun-
dance per unit area of rare, mobile, difficult-to-detect animals but also is of
value for the study of rare, difficult-to-detect plants, intertidal organisms,
and so on (Burnham et al., 1980). The technique is related to variable cir-
cular plot sampling and is sometimes called distance sampling (Buckland
left al
.,
2001). With line transect sampling, the basic idea is that an observer
moves along a line through a study area, looking to the left and right for the
animal or plant of interest. Line transects are walked, flown, or otherwise
traversed, and the perpendicular distances to all detected items of interest
are recorded. Combining these data with the assumption that all items on
the line are detected, it is possible to correct the estimates of abundance per
unit area for the items not detected. When an individual of the species of
interest is detected, it is recorded, usually together with its distance from
the line, because it is assumed that individuals that are far from the line are
harder to detect than those that are close. This is one of the specialized ways
that ecologists can use to estimate the density or the total number of animals
or plants in a study area when it is not possible to simply count all the indi-
viduals and the standard sampling methods considered in Chapter 2 are for
some reason not practical. For example, if the study area is very large and the
species of interest is rare, then a random sample of quadrats in the study area
may contain no individuals. However, if a long line is traveled through the
area, then some individuals might be detected.
The size of items or other variables might influence the probability of
detection, and such variables are sometimes included in the model that is
used to correct for the items missed. The accurate estimation of the density
or number of animals is always difficult using specialized methods like line
transect sampling, and with all such methods, the final estimate obtained
47
