Graphical and demographic synopsis of the captive cohort method for estimating population age structure in the wild

Abstract

The purpose of this paper is to complement the literature concerned with the captive cohort method for estimating age structure including (1) graphic techniques to visualize and thus better understand the underlying life table identity in which the age structure of a stationary population equals the time-to-death distribution of the individuals within it; (2) re-derive the basic model for estimating age structure in non-stationary population in demographic rather than statistical notation; and (3) describe a simplified method for estimating changes in the mean age of a wild population.

Copyright © 2012 Elsevier Inc. All rights reserved.

Figures

Fig. 1

Graphic showing the equality of the age distribution in a stationary population (upper) and the distribution of times-to-death of its members (lower). The implied age-specific mortality rates (qx) are 1/4, 1/6, 4/5 and 1.00 for age classes 0, 1, 2, and 3, respectively. Each segment labeled a, b, c or d in the top bar graphs corresponds to the fraction of the age class of the population that dies in the intervals, 0, 1, 2, and 3 respectively (bottom bar graphs). For example, the percentage of the standing population that die in each segment labeled ‘a’ (top graph) sum to 40% in the time-to-death class 0 (bottom graphic). Or alternatively, as the population ages the number of individuals in segments a, b, c, and d in age class 0 (top graph) die off at times 0, 1, 2 and 3, respectively as shown in the time-to-death distribution (bottom graph).

Figs. 2a,b

Pre- and post-capture patterns depicted for the horizontal life-lines of 100 hypothetical medflies. In the inset in Fig. 2a each tic in the horizontal life lines depict the ages in which the individual medflies are captured, the left and right portions representing the pre- and post-capture segments, respectively. The pre-capture life days of individuals are connected to the post-capture life-days in Fig. 2a. These segments are decoupled from the post-capture life days and rank ordered top-to-bottom from shortest to longest to show their equivalency in Fig. 2b.

Fig. 3

Schematic depicting hypothetical age distributions of younger (Population A) and older (Population B) populations and their corresponding post-capture death distributions, labelled A’ and B’, respectively. Note that the average age of death of members in Population A is greater than the average age of death for the members in Population B because the members are more youthful on average in the former population (redrawn from Carey et al., 2012).

Fig. 4

Mean post-capture longevity regressed on mean population age in the Mediterranean fruit fly (redrawn from Fig. 2 in Carey et al., 2012).