Static life tables

The difficulties of constructing a cohort life table for an organism with overlapping generations are eased somewhat when the organism is sessile. In such a case, newly arrived or newly emerged individuals can be mapped, photographed or even marked in some way, so that they (or their exact location) can be recognized whenever the site is revisited subsequently. Taken overall, however, practical problems have tended to deter ecologists from constructing cohort life tables for long-lived iteroparous organisms with overlapping generations, even when the individuals are sessile. But there is an alternative: the construction of a static life table. As will become clear, this alternative is seriously flawed - but it is often better than nothing at all.

An interesting example emerges from Lowe's study of red deer on Rhum. As has already been explained, a large proportion of the deer that died from 1957 to 1966 could be aged reliably. Thus, if, for example, a fresh corpse was examined in 1961 and was found to be 6 years old, it was known that in 1957 the deer was alive and 2 years old. Lowe was therefore eventually able to reconstruct the age structure of the 1957 population: age structures are the basis for static life tables. Of course, the age structure of the 1957 population could have been ascertained by shooting and examining large numbers of deer in 1957; but since the ultimate aim of the project was the enlightened conservation of the deer, this method would have been somewhat inappropriate. (Note that Lowe's results did not represent the total numbers alive in 1957, because a few carcasses must have decomposed or been eaten before they could be discovered and examined.) Lowe's raw data for red deer hinds are presented in column 2 of Table 4.3.

Remember that the data in Table 4.3 refer to ages in 1957. They can be used as a basis for a life table, but only if it is assumed that there had been no year-to-year variation prior to 1957 in either the total number of births or the age-specific survival rates. In other words, it must be assumed that the 59 6-year-old deer alive in 1957 were the survivors of 78 5-year-old deer alive in 1956, who were themselves the survivors of 81 4-year olds in 1955, and so on. Or, in short, that the data in Table 4.3 are the same as would have been obtained if a single cohort had been followed.

Number of individuals

Smoothed

Age (years)

observed of age x

x

ax

lx

dx

Qx

lx

dx

Qx

1

129

1.000

0.116

0.116

1.000

0.137

0.137

2

114

0.884

0.008

0.009

0.863

0.085

0.097

3

113

0.876

0.251

0.287

0.778

0.084

0.108

4

81

0.625

0.020

0.032

0.694

0.084

0.121

5

78

0.605

0.148

0.245

0.610

0.084

0.137

6

59

0.457

0.047

-

0.526

0.084

0.159

7

65

0.504

0.078

0.155

0.442

0.085

0.190

8

55

0.426

0.232

0.545

0.357

0.176

0.502

9

25

0.194

0.124

0.639

0.181

0.122

0.672

10

9

0.070

0.008

0.114

0.059

0.008

0.141

11

8

0.062

0.008

0.129

0.051

0.009

0.165

12

7

0.054

0.038

0.704

0.042

0.008

0.198

13

2

0.016

0.008

0.500

0.034

0.009

0.247

14

1

0.080

- 0.023

-

0.025

0.008

0.329

15

4

0.031

0.015

0.484

0.017

0.008

0.492

16

2

0.016

-

-

0.009

0.009

1.000

Table 4.3 A static life table for red deer hinds on the island of Rhum, based on the reconstructed age structure of the population in 1957. (After Lowe, 1969.)

Table 4.3 A static life table for red deer hinds on the island of Rhum, based on the reconstructed age structure of the population in 1957. (After Lowe, 1969.)

Having made these assumptions, the lx, dx and qx columns were constructed. It is clear, however, that the assumptions are false. There were actually more animals in their seventh year than in their sixth year, and more in their 15th year than in their 14th year. There were therefore 'negative' deaths and meaningless mortality rates. The pitfalls of constructing such static life tables (and equating age structures with survivorship curves) are amply illustrated.

Nevertheless, the data can be useful. Lowe's aim was to provide a general idea of the population's age-specific survival rate prior to 1957 (when culling of the population began). He could then compare this with the situation after 1957, as illustrated by the cohort life table previously discussed. He was more concerned with general trends than with the particular changes occurring from 1 year to the next. He therefore 'smoothed out' the variations in numbers between ages 2-8 and 10-16 years to give a steady decline during both of these periods. The results of this process are shown in the final three columns of Table 4.3, and the survivorship curve is plotted in Figure 4.11. A general picture does indeed emerge: the introduction of culling on the island appears to have decreased overall survivorship significantly, overcoming any possible compensatory decreases in natural mortality.

Notwithstanding this successful use of a static life table, the interpretation of static life tables generally, and the age structures from which they stem, is fraught with difficulty: usually, age structures offer no easy short cuts to understanding the dynamics of populations.

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Responses

  • nicodemo
    What is static lafe table?
    3 years ago
  • girmay
    What is lacking in a static life table?
    1 year ago
  • Olga
    Which of the following is not a process or assumption used to produce a static life table?
    12 months ago
  • Beau
    How to make a static life table?
    6 months ago

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