This video, we're going to talk about life tables and how they display aspects of demographic information. A life table is really just a comprehensive statistical table that shows a population's survivorship and mortality and reproductive rates of a cohort. A cohort is defined as a group of same species individuals born into a population right around the same time, and so they're all about the same age. Life tables often only include data on females when they're looking at sexually reproducing organisms, since only females produce offspring, and therefore, females tend to have a greater impact on population dynamics in comparison to males. Notice down below, we're showing you an example of a life table for female fruit flies, Drosophila melanogaster.

In this life table, it's analyzing a cohort of 500 female fruit flies that are all about the same age, between 0 to 10 days of age. It analyzes and follows these fruit flies through time until all of these fruit flies are dead. Along the way, it collects and gathers all of the statistical data that you can see here in this table. It is important for you to be able to understand life tables, so let me walk you through it a little bit. Notice on the far left, we have the age interval in days, then we have the number of organisms alive at the start of the age interval, then we have the overall survivorship at the start of the interval or the proportion of organisms that are still alive at the start of that age interval, then we have the overall mortality rate or the overall death rate between age intervals, and then we have the fecundity or the capacity for the organisms to reproduce.

Here in this specific scenario, we're showing you the average number of female offspring per female, and so let me slide out of the way so that you can see these numbers here. Now what we're going to do now is fill in these interactive blanks throughout this table, so that you can again better understand this table and how the values are collected. Here we're looking at the mortality rate of this specific age group between 0 to 10 days. What we need to realize is that initially, there were 500 of these female Drosophila fruit flies, but then in the next age interval, there were only 366, which means that 134 of them died. If you calculate 134500, what you get is an answer of 0.268, or in other words, 26.8% of the 500 organisms died in this age interval.

Next, what we have over here is the survivorship at the start of the age interval 20 to 30 days, and so what we need to realize is that there are 280 organisms still alive at the start of this age interval, and so if you calculate 280500, the initial number, what you get is an answer of 0.560, or in other words, 56% of all the organisms that started are still alive at the start of this age interval, 20 to 30 days. Last but not least, we have this blank here, which is going to be the number of organisms alive at the start of this 30 to 40 day interval. Notice that the survivorship is 0.248, or in other words, 24.8 percent of the initial number of organisms are still alive here. If you calculate 0.248500, what you get is an answer of 124, and that is the number of organisms alive at the start of this 30 to 40 day interval. Really, that's all there is to these life tables.

Moving forward, we'll be able to get some practice applying these concepts, and I'll see you all there.