50. Population Ecology
Population Demography
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This video, we're going to talk about the demographic information in survivorship curves. And so a survivorship curve is really just a graphical representation of a life table, and so it really is just a graph of the number of living individuals in a cohort, which we plot on the y axis of the curve, surviving to each age group, which we plot on the x axis of the curve. Now the y axis of a survivorship curve is often expressed on a logarithmic scale, which is important because it allows for a wide range of values to be plotted onto the same graph. Now, really, there are only 3 general shapes of survivorship curves, which we call type 1, type 2, and type 3 survivorship curves, and we can see them down below in this graph. So notice that the blue curve that we're showing you here represents a type 1 survivorship curve. And so in a type 1 survivorship curve, there's very low mortality rates early on in life, allowing most individuals that are born to survive to adulthood. But then later in life, the mortality rates increase significantly. Now an example of a species that exhibits type 1 survivorship curve are human beings like you and I. Now a type 2 survivorship curve is being shown here in red. And so instead of having low and then high mortality rates, with a type 2 survivorship curve, the mortality rates are pretty similar throughout the entire species' lifetime, creating a straight line as you can see here. And an example of a species that displays type 2 survivorship is this northern cardinal that you can see here. Now last but not least, we have a type 3 survivorship curve, which is essentially the opposite of a type 1 survivorship curve. And so instead of having really low mortality rates early in life, there are very high mortality rates early in life, and so most individuals that are born will not survive to adulthood. But then once the individuals reach a certain age, the mortality rates decrease significantly, and very few of them can actually reach adulthood and, you know, older ages of life. And an example of a species that displays type 3 survivorship is the American bullfrog that you can see here. Now down below, we do have a very important note, which is that survivorship is actually a spectrum, and so most populations actually exhibit characteristics that fall somewhat in between these three classic types, type 1, type 2, and type 3. And so these are just very recognizable survivorship curves. But, again, species in reality can display all kinds of different shapes, and so they can fall in between these values. So this here concludes our lesson on survivorship currents, and we'll be able to get some practice moving forward. So I'll see you on our next video.
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