A “life table” is a kind of bookkeeping system that ecologists often use to keep track of stage-specific mortality in the populations they study. It is an approach that is especially useful in entomology where developmental stages are discrete and mortality rates may vary widely from one life stage to another. From a pest management standpoint, it is very helpful to know when (and why) a pest population suffers high mortality — this is usually the time when it is most vulnerable. By managing the natural environment to maximize this vulnerability, pest populations can often be suppressed without any other control methods.
To create a life table, an ecologist follows the life history of many individuals in a population, keeping track of how many offspring each female produces, when each one dies, and what caused its death. After amassing data from different populations, different years, and different environmental conditions, the ecologist summarizes this data by calculating average mortality within each developmental stage.
For example, in a hypothetical insect population, an average female will lay 200 eggs before she dies. Half of these eggs (on average) will be consumed by predators, 90% of the larvae will die from parasitization, and three-fifths of the pupae will freeze to death in the winter. (These numbers are averages, but they are based on a large database of observations.)
A life table can be created from the above data. Start with a cohort of 200 eggs (the progeny of Mrs. Average Female). This number represents the maximum biotic potential of the species (i.e. the greatest number of offspring that could be produced in one generation under ideal conditions). The first line of the life table lists the main cause(s) of death, the number dying, and the percent mortality during the egg stage. In this example, an average of only 100 individuals survive the egg stage and become larvae. The second line of the table lists the mortality experience of these 100 larvae: only 10 of them survive to become pupae (90% mortality of the larvae). The third line of the table lists the mortality experience of the 10 pupae — three-fifths die of freezing. This leaves only 4 individuals alive in the adult stage to reproduce. If we assume a 1:1 sex ratio, then there are 2 males and 2 females to start the next generation.
If there is no mortality of these females, they will each lay an average of 200 eggs to start the next generation. Thus there are two females in the cohort to replace the one original female — this population is DOUBLING in size each generation!!
In ecology, the symbol “R” (capital R) is known as the replacement rate. It is a way to measure the change in reproductive capacity from generation to generation. The value of “R” is simply the number of reproductive daughters that each female produces over her lifetime:
- If the value of “R” is less than 1, the population is decreasing — if this situation persists for any length of time the population becomes extinct.
- If the value of “R” is greater than 1, the population is increasing — if this situation persists for any length of time the population will grow beyond the environment’s carrying capacity. (Uncontrolled population growth is usually a sign of a disturbed habitat, an introduced species, or some other type of human intervention.)
- If the value of “R” is equal to 1, the population is stable — most natural populations are very close to this value.
A typical female of the bubble gum maggot (Bubblicious blowhardi Meyer) lays 250 eggs. On average, 32 of these eggs are infertile and 64 are killed by parasites. Of the survivors, 64 die as larvae due to habitat destruction (gum is cleared away by the janitorial staff) and 87 die as pupae because the gum gets too hard. Construct a life table for this species and calculate a value for “R”, the replacement rate (assume a 1:1 sex ratio). Is this population increasing, decreasing, or remaining stable?