Fecundity schedules and basic reproductive rates

The fecundity schedule in Table 4.1 (the final three columns) begins with a column of raw data, Fx: the total number of seeds produced during each period. This is followed in the next column by mx: the individual fecundity or birth rate, i.e. the mean number of seeds produced per surviving individual. Although the reproductive season for the Phlox population lasts for 56 days, each individual plant is semelparous. It has a single reproductive phase during which all of its seeds develop synchronously (or nearly so). The extended reproductive season occurs because different individuals enter this phase at different times.

Perhaps the most important summary term that can be extracted from a life table and fecundity schedule is the basic reproductive rate, denoted by R0. This is the mean number of offspring (of the first stage in the life cycle - in this case seeds) produced per original individual by the end of the cohort. It therefore indicates, in annual species, the overall extent by which the population has increased or decreased over that time. (As we shall see below, the situation becomes more complicated when generations overlap or species breed continuously.)

There are two ways in which R0 can be computed. The first is from the the basic reproductive formula: rate, R0

i.e. the total number of seeds produced during one generation divided by the original number of seeds (X Fx means the sum of the values in the Fx column). The more usual way of calculating R0, however, is from the formula:

i.e. the sum of the number of seeds produced per original individual during each of the stages (the final column of the fecundity schedule). As Table 4.1 shows, the basic reproductive rate is the same, whichever formula is used.

The age-specific fecundity, mx (the fecundity per surviving individual), demonstrates the existence of a preproductive period, a gradual rise to a peak and then a rapid decline. The reproductive output of the whole population, Fx, parallels this pattern to a large extent, but also takes into account the fact that whilst the age-specific fecundity was changing, the size of the population was gradually declining. This combination of fecundity and survivorship is an important property of Fx values, shared by the basic reproductive rate (R0). It makes the point that actual reproduction depends both on reproductive potential (mx) and on survivorship (lx).

In the case of the Phlox population, R0 was 2.41. This means that there was a 2.41-fold increase in the size of the population over one generation. If such a value were maintained from generation to generation, the Phlox population would grow ever larger and soon cover the globe. Thus, a balanced and realistic picture of the life and death of Phlox, or any other species, can only emerge from several or many years' data.

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