Name(s): _________________________________

Case Study: Survivorship and Population Models

Part 1: Hatchling Exodus
Amy was very excited to be staying at a resort in
Florida where loggerhead turtles lay their eggs.
She had carefully read the literature that outlined
the rules for her stay. Rules that were in place so
that visitors would not interfere with the nesting
females. Though her cabin was next to the beach,
there were no exterior lights and she was warned
to not use flashlights or flash photography. The
beach was also to be kept clear of any toys or
furniture that may block a female from a nesting site. Amy was surprised to learn that
loggerhead turtles cannot back up.
At night, Amy and her tour group were allowed to carefully walk the beach and observe a
female turtle as she laid her eggs. The massive 275 pound loggerhead had a shell that
was almost 3 feet long. The huge reptile used her hind flippers to dig a trench and lay over
100 eggs. The tour guide explained that the hatching success rate of an in situ nest has a
success rate of 80-100%, but a relocated nest will only have about a 60% success rate.
"Why would anyone want to relocate the nest?" Amy asks.
The tour guide answered, "Sometimes the nests are in high traffic areas, or areas that
have been compromised. During the gulf oil spill, thousands of turtles nests were
relocated." She shares a link with them on her iPad showing how painstaking it is to
relocate the nests. (Use the QR code to view the video or wait for your instructor to show it
to you.)
"What happens after the loggerhead turtles hatch?"
The tour guide sighed as she relayed that the odds of their survival is really low. "Many of
the hatchlings die before they ever reach the ocean, they fall prey to birds and crabs.
Some become disoriented and go the wrong direction."
"Where do they go if they do make it to the ocean?"
"That's the strange thing," answered the guide, "Biologists didn't really know what
happened during the decade after they were hatched. They even called that period of time
'the lost years.'"
A member of the group raised his hand and speaking quietly so as not to disturb the
female, shared additional information. "I'm from the University of Florida, and I know that
they've managed to attach solar trackers to some of the juvenile turtles. It seems that the
turtles enter the North Atlantic Gyre which is part of the Gulf Stream. They live in the
seaweed there, using it as food and shelter."
In the back of the group, someone whispered: "Like in Finding Nemo!"
Amy watched as the female turtle finished laying her clutch of eggs and pushed sand over
the nest to cover them. This female must have beaten all the odds to return to this beach
to lay her eggs many years later.
http://video.nationalgeographic.com/video/news/us-oil-spill-turtle-relocation-vin

Answer the questions on the next page

1. What reproductive challenges do female sea turtles face?
Things on the beach, oil spill

2. What factors must be considered when relocating turtle nests?

Human inference, population

- based on the context of the reading, what does "in situ" mean?
In place
3. Some conservation efforts remove eggs and raise the hatchlings in captivity to ensure
their survival. Do you think money should be spent for captive breeding programs? Why or
why not?

Part 2: Survivorship Curves

When Amy returned from her vacation, she decided to look into sea turtles to find out how
many of them actually survived those early years. She discovered very little data on sea
turtles as teenagers, but she recalled how the guide had referred to that time period as the
"lost years." One data set did include turtle hatchling numbers and then numbers of turtles
who returned to the beach a decade later.
Sea Turtle Data Individuals
5. Calculate the survivorship by dividing the Present (or Survived Survivorship
number that survived by the initial hatched)
population (hatched). Complete the table to Time 0 (eggs 1100 1100 1.0
show survivorship at each time frame. laid)
Day 1 1100 1000 1000/1100 = .91
Day 2 1000 600 600/1000 = .6
6. Create a graph that has % of organisms
surviving (survivorship) on the Y axis, and Day 3 600 230 230/600 = .38

time on the X axis. (Year 15 can be treated Day 4 230 80 80/230 = .35

as a distant point on the graph) Day 5 80 18 18/80= .23

Year 15 18 16 89
 As Amy looked at the graph she found it odd that a whole species would invest so much
into a clutch of eggs, and have so few survivors. She took the data to her biology
instructor to see if Dr. Franklin could shed some light on this. As he looked over the data,
he nodded slowly. "I think I can explain what is going on here."
"In the animal kingdom there are different strategies for reproduction. Remember that
evolution favors behaviors or traits that ensure the survival of the species. Sometimes this
means that the adults have a large number of offspring where only a few are expected to
survive. These type of animals are called "R-Strategists" like the loggerhead sea turtles.
As long as a few of the offspring survive to reproductive age, then there is continuity within
the species."
"What does the "R" stand for?" asked Amy.
"Well, that is a little more complicated. There is a formula for population growth, where r
represents the maximum reproductive capacity. This basically means that the turtles
would be at maximum if so many of them didn't die that first year of life. Imagine what
would happen to the sea turtle population if there were no predators!"
"There would be a lot of sea turtles out there. Probably too many for the environment to
support."
"Exactly." Replied the professor. "Fortunately, very few organisms reach their biotic
potential."
"What is biotic potential?" Asked Amy.
"That’s the maximum rate the population can grow given optimal conditions. It basically
means the growth rate if there were no limiting factors like food, space, or water."
Amy felt the topic was veering off track. She really wanted to understand why evolution
would select for organisms that would lay hundreds of eggs but only have a few that
survived the first year of life. It seemed like a waste of resources. "But what about the
turtles, why lay so many eggs to just have them all die?"
The professor thought about how to explain it and then went to a blackboard. He quickly
sketched a graph. "You see, there are three types of survivorship curves. One of these
looks like the graph you made of the sea turtles."

7. Which type of survivorship pattern is

observed with the sea turtles?
Type 3

8. What other animals can you think of would be

R strategists?
Fishes and spiders

9. What kind of survivorship curve would you expect to see in humans? Explain how you
came to this conclusion.
type 1 because most babies survive but
die as they get older
10. In some cases, animals reach their biotic potential (if for a short period of time) and
wreak havoc on the environment. What is biotic potential and why would reaching it create
problems for the environment?

11. The three types of curves are also described as: early loss, late loss, and constant
loss. Explain why these descriptions work to describe the trends in the graph.

12. Propose an organism that would have a Type 2 curve. Defend your choice by
describing this animal's life pattern using the data and growth curve as a guide. (If you are
really stuck, use google to look up survivorship curves)

Part 3: Population Models

Amy became really interested in how populations work and how different groups of
organisms evolved different strategies for reproduction. Though Dr. Franklin mentioned
that the "R" was part of an equation, she decided to look up the actual equation. A simple
google search located the equation that Dr. Franklin had referred to:

Where (dN/dt) is the rate of increase of the population and N is the population size, r is the
intrinsic rate of increase. K is the carrying capacity of its local environmental setting. Thus,
the equation relates the rate of change of the population N to the current population size
and is the theoretical maximum rate of increase of a population per individual.
The equation was developed from the r/K selection theory which relates to organisms that
trade off between quantity and quality of offspring. If "r" was the maximum growth rate and
referred to animals like sea turtles, then there must be a different type of strategy that
referred to animals like humans or whales that had very few offspring but invested in years
of parental care.
Another internet search revealed that animals that invest more heavily into fewer offspring
are called K-strategists. Traits associated with these animals are large body size, long
life expectancy, production of fewer offspring and more parental care.
Amy sighed deeply, that equation looked like the differential equations she learned in
math class. She barely remembered how to work those types of problems. Her roommate,
who is a math major, looked at the equation and quickly sketched what the graph would
look like. Amy took one look at the graph and realized she'd seen it before. "Oh, that's
a logistic growth curve!"
 Her roommate looked puzzled and Amy
explained further. "Logistic growth curves
occur when there is a limiting factor in the
environment. A population grows until it
levels out, usually because the population
runs out of resources. Some populations,
like humans, are in exponential growth,
they haven't leveled out."
Her roommate shrugged and went back to
calculus. "I guess humans will reach that
point some day...."
13. "K" refers to the carrying capacity.
Based on the graph and the text, suggest a defintion for "carrying capacity." (If you are
stuck, you could google it.): the maximum number turtles in one place
14. What factors would affect the carrying capacity of turtles?
Low on food, place to lay their eggs

15. The following growth curve shows a population of crab in areas where loggerheads
are known to gather. Crabs are a food source for the turtles who can use their strong jaws
to crack their shells.
Based on the graph, what is the carrying
capacity for crabs in the area being
studied. Propose a reason that the
population dropped in 2010.
They was getting hunted
What does the population curve of crabs
tell you about the loggerhead turtle
populations in the same area?

That their is a whole lot of turtles

there

16. Summarize: Using all of the information from the case study, summarize the
relationship between the following terms:
carrying capacity | r-strategist | survivorship | biotic potential | logistic growth curve |