Cohort life tables

Constructing a cohort life table for species that breed repeatedly is more difficult than constructing one for an annual species. A cohort must be recognized and followed (often for many years), even though the organisms within it are coexisting and intermingling with organisms from many other cohorts, older and younger. This was possible, though, as part of an extensive study of red deer (Cervus elaphus) on the small island of Rhum, Scotland (Lowe, 1969). The deer live for up to 16 years, and the females (hinds) are capable of breeding each year from their fourth summer onwards. In 1957, Lowe and his coworkers made a very careful count of the total number of deer on the island, including the total number of calves (less than 1 year old). Lowe's cohort consisted of the deer that were calves in 1957. Thus, each year from 1957 to 1966, every one of the deer that was discovered that had died from natural causes, or had been shot under the rigorously controlled conditions of this Nature Conservancy Council reserve, was examined and aged reliably by examining tooth replacement, eruption and wear. It was therefore possible to identify those dead deer that had been calves in 1957; and by 1966, 92% of this cohort had been observed dead and their age at death therefore determined. The life table for this cohort of hinds (or the 92% sample of it) is presented in Table 4.2; the survivorship curve is shown in Figure 4.11. There appears to be a fairly consistent increase in the risk of mortality with age (the curve is convex).

the species composition of seed banks

Figure 4.10 Species recovered from the seed bank, from seedlings and from mature vegetation in a coastal grassland site on the western coast of Finland. Seven species groups (GR1-GR7) are defined on the basis of whether they were found in only one, two, or all three stages. GR3 (seed bank and seedlings only) is an unreliable group of species that are mostly incompletely identified; in GR5 there are many species difficult to identify as seedlings that may more properly belong to GR1. None the less, the marked difference in composition, especially between the seed bank and the mature vegetation, is readily apparent. (After Jutila, 2003.)

Table 4.2 Cohort life table for red deer hinds on the island of Rhum that were calves in 1957. (After Lowe, 1969.)

1

1.000

0

0

2

1.000

0.061

0.061

3

0.939

0.185

0.197

4

0.754

0.249

0.330

5

0.505

0.200

0.396

6

0.305

0.119

0.390

7

0.186

0.054

0.290

8

0.132

0.107

0.810

9

0.025

0.025

1.000

Proportion of original Proportion of original cohort surviving to the cohort dying during

Age (years) beginning of age-class x age-class x Mortality rate x lx dx qx

1000

500 400 300

500 400 300

Hinds on Rhum in 1957 (static)

Yearling hinds on Rhum in 1957 (cohort)

Age (years)

Figure 4.11 Two survivorship curves for red deer hinds on the island of Rhum. As explained in the text, one is based on the cohort life table for the 1957 calves and therefore applies to the post-1957 period; the other is based on the static life table of the 1957 population and therefore applies to the pre-1957 period. (After Lowe, 1969.)

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