- Age-specific, Cohort, or Dynamic life tables: data are collected by following a cohort throughout life. This is rarely possible with natural populations of animals.
- Static or time-specific tables: age-distribution data are collected from a cross-section of the population at one particular time or during a short segment of time, such as through mortality data.
- Composite tables: data are gathered over a number of years and generations using cohort or time-specific techniques. This method allows the natural variability in rates of survival to be monitored and assessed.
So, why do we care about life tables? The main value of a life table lies in what it tells us about the population's strategy for survival. So, life tables help us to understand the dynamics of populations. For example, time-specific life tables are valuable to a manager of exploited populations because they show the existence of strong year classes or help identify weak age classes.
Here is an example of a population life table for a Darwin finch:
| Life table for one Darwin finch, the Galapagos cactus finch (Geospiza scandens)* | |||
| age class** (x) | probability of surviving to age x (lx) | average number of fledgling daughters (mx) | product of survival and reproduction (Σlxmx) |
| 0 | 1.0 | 0.0 | 0.0 |
| 1 | 0.512 | 0.364 | 0.186 |
| 2 | 0.279 | 0.187 | 0.052 |
| 3 | 0.279 | 1.438 | 0.401 |
| 4 | 0.209 | 0.833 | 0.174 |
| 5 | 0.209 | 0.500 | 0.104 |
| 6 | 0.209 | 0.833 | 0.174 |
| 7 | 0.209 | 0.250 | 0.052 |
| 8 | 0.209 | 3.333 | 0.696 |
| 9 | 0.139 | 0.125 | 0.017 |
| 10 | 0.070 | 0.0 | 0.0 |
| 11 | 0.070 | 0.0 | 0.0 |
| 12 | 0.070 | 3.500 | 0.245 |
| 13 | 0 | — | — |
| R0 = 2.101 | |||
| Net reproductive rate = R0 = Σlxmx = 2.101 Mean generation time = T = (Σxlxmx)/(R0) = 6.08 years Intrinsic rate of natural increase of the population = r = approximately 1nR0 / T = 2.101/6.08 = 0.346 | |||
No comments:
Post a Comment