To monitor and quantify survival, we may follow the fate of individuals from the same cohort within a population: that is, all individuals born within a particular period. The life table then records the survivorship of the members of the cohort over time (Figure 4.9). The most straightforward life table to construct is a cohort life table for an annual species. Putting to one side the caveats raised above, annual life cycles take approximately 12 months or rather less to complete (Figure 4.6b, c). Usually, every individual in a population breeds during one particular season of the year, but then dies before the same season in the next year. Generations are therefore said to be discrete, and each cohort is distinguishable from every other; the only overlap of generations is between breeding adults and their offspring during and immediately after the breeding season.
an annual life table for a plant
Two very simple life tables, for inland and coastal subspecies of the annual plant Gilia capitata, growing in California, USA are shown in Table 4.1. Initial cohorts of around 750 seeds were followed from seed germination to the death of the last adult.
Table 4.1 Two cohort life tables for the annual plant Gilia capitata. One is for the ‘inland’ subspecies, G. capitata capitata, and one for the ‘coastal’ subspecies, G. capitata chamissonis, growing at an inland site in Napa County, California, USA and being easily distinguishable morphologically, despite being cross‐fertile. Cohorts of seeds were planted at the beginning of the season in 1993 and the life cycle divided simply into seeds, plants that emerged from those seeds, and emerged plants that went on to flower. Other column entries are explained in the text. Source: After Nagy & Rice (1997).
| Stage (x) | Number alive at the start of each age class ax | Proportion of original cohort surviving to the start of each age class lx | dx | qx | log ax | log lx | kx | Number of female young produced by each age class Fx | Number of female young produced per surviving individual in each age class mx | Number of female young produced per original individual in each age class lxmx |
|---|---|---|---|---|---|---|---|---|---|---|
| Inland subspecies: | ||||||||||
| Seed (0) | 746 | 1.00 | 0.66 | 0.66 | 2.87 | 0.00 | 0.47 | 0 | 0 | 0 |
| Emergence (1) | 254 | 0.34 | 0.25 | 0.74 | 2.40 | –0.47 | 0.59 | 0 | 0 | 0 |
| Flowering (2) | 66 | 0.09 | 1.82 | –1.05 | 28,552 | 432.61 | 38.29 | |||
| Coastal subspecies: | ||||||||||
| Seed (0) | 754 | 1.00 | 0.73 | 0.73 | 2.88 | 0.00 | 0.57 | 0 | 0 | 0 |
| Emergence (1) | 204 | 0.27 | 0.25 | 0.91 | 2.31 | –0.57 | 1.03 | 0 | 0 | 0 |
| Flowering (2) | 19 | 0.03 | 1.28 | –1.60 | 8645 | 455.00 | 11.47 |
a cohort life table for marmots
Even when generations overlap, if individuals can be marked early in their life so that they can be recognised subsequently, it is feasible to follow the fate of each year’s cohort separately. It may then be possible to merge the cohorts from the different years of a study to derive a single, ‘typical’ cohort life table. An example is shown in Table 4.2 of females from a population of the yellow‐bellied marmot, Marmota flaviventris. The population was live‐trapped and marmots marked individually from 1962 through to 1993 in the East River Valley of Colorado, USA and it was this that allowed each individual to be assigned, whenever it was caught, to its own cohort.
Table 4.2 A cohort life table for female yellow‐bellied marmots, Marmota flaviventris in Colorado, USA. The columns are explained in the text. Source: After Schwartz et al. (1998).
| Age class (years) x | Number alive at the start of each age class ax | Proportion of original cohort surviving to the start of each age class lx | dx |
|---|