Geoscience Reference
In-Depth Information
5
Removal an d Change-in-Ratio Methods
Lyman McDonald and Bryan Manly
5.1 Introduction
This chapter covers two methods for estimating the size of animal popu-
lations. The first is the removal method, which relies on the capture and
removal of animals where the number of animals removed has a noticeable
impact on the numbers found in later samples. Alternatively, the methods
apply if animals can be uniquely marked or tagged and the proportion of
unmarked animals found in later samples decreases substantially.
The second is the change-in-ratio method, which relies on the removal of some
of one type of animal (e.g., males), leading to a substantial change in the propor-
tion of that type in the population and in later samples. Alternatively, the ani-
mals can be uniquely marked and tagged so that the proportion of unmarked
animals of one type (e.g., males or juveniles) in later samples changes.
Clearly, the removal method is related to the methods covered in further
chapters on mark-recapture sampling, and data can be analyzed by the more
complex methods presented in those chapters. In this chapter, we recommend
analysis using the computer software program CAPTURE at the interactive
website http://www.mbr-pwrc.usgs.gov/software/capture.html (White et
al., 1978, 1982; Otis et al., 1978; Rexstad and Burnham, 1991). However, we also
present a linear regression method to give an intuitive understanding of the
information in the data. The regression method is simpler to understand and
will provide approximately the same results as the more complex method.
5.2 Removal Method
The simplest removal method involves taking a series of samples from an
animal population and removing or marking the animals that are captured.
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