POP: Lesson - Survivorship Curves (Topic 3.3) 📖

⏳ Estimated Reading Time: 2 - 4 minutes

Explain survivorship curves.

Survivorship Curves

A diagram titled "FROG LIFE CYCLE" shows the four stages of a frog's life: eggs, tadpole, froglet, and adult. Below the title, illustrations of each stage are labeled and connected by arrows indicating progression. The "Eggs" stage depicts a cluster of round eggs, "Tadpole" shows an aquatic larva with a tail, "Froglet" displays a small creature with legs and a tail, and "Adult" features a fully developed frog with distinct limbs and no tail. — ***Frogs begin as eggs, turn into tadpoles, then change into tadpoles with legs (froglets), and finally change into adult frogs.***

An organism’s life history is the progression of changes an organism undergoes during its life.

A graphic titled "SURVIVORSHIP CURVES" shows three types of survivorship curves. The x-axis is labeled “Age (Percent of Lifespan)” and the y-axis is labeled “Number of Survivors”:

Orange Type I (K-strategists): Starts high and drops steeply, indicating most live to old age (e.g., humans, elephants).
Blue Type II: Shows a constant decline, indicating an equal chance of dying at any age (e.g., birds, rodents).
Green Type III (r-strategists): Starts low and flattens, indicating most die young, but survivors live long (e.g., oysters, insects). — ***This curve shows us what it looks like for a cohort of different types of species. There are 3 types of survivorship curves: Type I (orange), Type II (blue), and Type III (green).***

A survivorship curve is a graph showing the number remaining from a group all born at the same time, a cohort, decreasing over time until their lifespan is met.

Essential Knowledge

A survivorship curve is a line that displays the relative survival rates of a cohort - a group of individuals of the same age - in a population, from birth to the maximum age reached by any one cohort member. There are Type I, Type II, and Type III curves.

Let's explore the 3 types of survivorship curves below. Click through to learn more about which organisms are most likely to follow which curve. Which one do you think best fits humans?

Type I
Type II
Type III

Type I Survivorship Curve

A graphic of Survivorship curves, the x-axis is labeled “Age (Percent of Lifespan)” and the y-axis is labeled “Number of Survivors”. The orange curve is called Type I (K-strategists) and it starts high and declines steeply.

Type I survivorship curves represent K-strategists. If you recall, these are organisms that have few offspring and exhibit high levels of parental care. You can see in the curve that most of the offspring survive and tend to die of old age or later in life.

Examples of Type I species are humans, gorillas, polar bears, humans, elephants, and other charismatic megafauna.

Type III Survivorship Curve

The image shows a graph titled “SURVIVORSHIP CURVES” on a grid background. The x-axis is labeled “Age (Percent of Lifespan)” and the y-axis is labeled “Number of Survivors”. Type III (r-strategists): A green curve that starts with a steep decline, indicating low early survival rates, then flattens as survivors age.

Type III survivorship curves represent r-strategists. Remember, these are organisms that produce lots of offspring but have little to no parental care. These organisms tend to have high levels of loss early in life. This is why these species tend to have many offspring – they “expect” many of their offspring to die. Once these organisms pass that large loss early in their life, they tend to survive well towards their end of their life. These species tend to have short lifespans, however.

Examples of Type III organisms are most fish species, many invertebrates (like oysters and insects), and frogs.

Essential Knowledge

Survivorship curves differ for K-selected and r-selected species, with K-selected species typically following a Type I or Type II curve and r-selected species following a Type III curve.

AP Exam Tip

You should be able to draw the shape of the three types of survivorship curves from memory and know which belongs to K- and r-selected species.

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