My book says,

If in a pond there are 20 lotus plants last year and through reproduction 8 new plants are added, taking the current population to 28, we calculate the birth rate as 8/20 = 0.4 offspring per lotus per year.


If 4 individuals in a laboratory population of 40 fruitflies died during a specified time interval, say a week, the death rate in the population during that period is 4/40 = 0.1 individuals per fruitfly per week.

What does per lotus per year, and, per fruitfly per week imply?

  • $\begingroup$ Please rewrite the quote. Images are not searcheable and can pose problems to visually impaired people $\endgroup$ – Remi.b Jan 2 '18 at 18:17
  • $\begingroup$ 0.4 offspring per lotus per year means that each lotus produces on average 0.4 offspring every year. $\endgroup$ – Remi.b Jan 2 '18 at 18:18
  • $\begingroup$ @Remi.b All should be good now. :) $\endgroup$ – Charles Jan 2 '18 at 20:44
  • $\begingroup$ Thanks! +1 You might want to comment on my answer if something is still unclear to you. $\endgroup$ – Remi.b Jan 2 '18 at 20:50
  • $\begingroup$ I'm not the OP. =P $\endgroup$ – Charles Jan 2 '18 at 21:00

I will use an ever so slightly more specific scenario than what your book offers.

At the beginning of 2017, there were twenty lotus plants in a pond. By the first day of 2018, eight of those twenty lotus plants reproduced. Using this information, we now want to describe the birth rate of lotus plant population.

To do this, we need the number of plants that produced offspring, and, the population size at the beginning of 2017.

$$\frac{no. \ of \ parent \ lotus \ plants}{initial \ population \ size} = \frac{8}{20} = .4$$

This means that, when considering the twenty lotus plants that first existed at the beginning of 2017, each of those twenty plants contributed an average of 4/10s of a lotus plant offspring for that year.

To generalize this, we say, "a birth rate of .4 offspring per lotus plant per year."

The same method and persective can also be applied to the fruitflies, however, they're considering death rate.

  • 1
    $\begingroup$ Hey Thanks, this was comprehensible. Could you please explain the meaning of 'per capita' also please? $\endgroup$ – Shreya Varshney Jan 3 '18 at 11:24
  • $\begingroup$ @ShreyaVarshney Per capita is just another way of saying Per lotus plant. :) $\endgroup$ – Charles Jan 3 '18 at 11:27
  • $\begingroup$ @Charles , the numerator should be the "number of offspring." Per capita birth rate is the average number of offspring at the next time step produced by mothers at a particular time. Your equation is the proportion of number mothers to total population at a particular time. $\endgroup$ – Chris Moore May 8 at 16:54

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