Download the tortoise and the cactus – teacher handout

THE TORTOISE AND THE CACTUS – TEACHER HANDOUT
Grade Level:
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High-school biology
Objectives:
The students will:
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Identify selective pressures that act on organisms
Identify adaptations and state how they help organisms survive and reproduce in the face of selective pressures
Outline the “steps” of acquiring adaptations (these are the steps of natural selection)
Describe how two species (Galápagos tortoise and Opuntia cactus) can be major selective pressures on each another
Make inferences about the historical biogeography of the Galápagos tortoises based on the distribution of Opuntia forms
Make predictions about how the growth forms of Opuntia may respond to the extirpation of tortoises on some of the islands
Background Information:
The students should already be familiar with the following topics:
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Hypothesis testing
Selective pressures
Adaptations
The steps of acquiring adaptations (natural selection)
Time Requirement: (Includes time for a short introduction to the Galápagos and a follow-up discussion)
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Two 70-minute blocks or three 45-minute class periods
Materials:
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Student handouts (1 per student)
Evaluation:
Check student handout. Answers, even if technically incorrect, should be defensible.
Teacher Handout: The tortoise and the cactus.
Page 1
THE TORTOISE AND THE CACTUS – TEACHER HANDOUT
The Galápagos are dry volcanic islands, which arose from the seafloor west of South America sometime between 3 and 6
million years ago. They were never connected to South America, but have plants and animals that originally came from
there. One example is the genus of cactus called Opuntia (which includes the prickly-pear cactus). Another example is the
giant Galápagos tortoise (Testudo elephantopus).
Figure 1.Cactus from South America (Opuntia
dillenii) [Illustration from Watson, W. (1889)]
Figure 2. Tortoise from South America
(Geochelone chilensis) [Illustration from
Wayne and Burke. (1997)]
Question: How could the turtle have traveled from South America to the Galápagos Islands?
Answers could include:
•
Swimming
•
Rafting
o
Some students may note that a pregnant /gravid female would have been most likely to establish a new
population.
Teacher Handout: The tortoise and the cactus.
Page 2
Question: Scientists have proposed two ways that the cactus may have traveled
to the Galápagos from South America. What do you think they are (Hint: One way
involves an animal)?
a. Floating
b. Seed dispersal in the gut of an animal (e.g. bird)
Some students may suggest that the turtle from the previous
question carried it.
Question: How could scientists test the hypothesis that the Galápagos tortoises
(Testudo elephantopus) and the cacti (Opuntia species) originally came from
South America?
Answers will vary, but may include:
•
Compare to similar species from South America
•
Check the fossil record
Figure 3. Map of South
America and the Galápagos
Islands [Illustration from
Moen]
Adaptations of the Opuntia
The Galápagos tortoise is one of the main predators of Opuntia. The fruits and pads of the cactus provide both food and water for the
tortoise. Although the cactus has populations on all of the islands, the turtles are only found on some of the islands.
Question: Name two selective pressures that Opuntia have to deal with on the Galápagos Islands.
a. Lack of water (from introductory paragraph)
b. The predatory tortoises
Teacher Handout: The tortoise and the cactus.
Page 3
Listed below are some of the characteristics of the various Opuntia species on the Galápagos Islands. Some of the characteristics
are found on Opuntia that live on the same islands as tortoises and some of the characteristics are found on Opuntia that do not live
with tortoises. Put a checkmark next to the characteristics that you think are adaptations the Opuntia have for living with the turtles.
Explain how these adaptations help the cactus survive and reproduce in the presence of turtles.
Table 1. Adaptations the Opuntia have to living with turtles (Biggs, 1990).
Adaptation
Characteristic
to turtles
Plant grows tall (arborescent form) with the edible fruits and
pads on top (See Figure 4)
√
How it helps the cactus survive/reproduce in the
presence of turtles
Protects them from being eaten by the tortoises
Spines are weak or absent
Plant grows low along the ground (decumbent form. See
Figure 5)
√
√
√
Sharp spines on the lower part of the plant fall off as it
grows. They are replaced with a thick, tough bark.
Young plants are very spiny and grow fast.
The seeds of the plant have a thick husk that can (and
must) pass through an animal’s intestines before
germinating (sprouting). The fruits fall to the ground once
ripened.
The seeds of the plant cannot survive passing through an
animal’s intestines.
Teacher Handout: The tortoise and the cactus.
Protects them from being eaten by the tortoises
Protects them from being eaten by the tortoises,
even though they are within reach
The seeds are protected from being eaten by
tortoises by their thick husks, even after landing
on the ground.
Page 4
Figure 4. Arborescent
form of the Opuntia
[Illustration from National
Center for Mathematics
and Science (2002)]
Figure 5. Recumbent form of
the Opuntia [Illustration from
National Center for
Mathematics and Science
(2002)]
Choose two of the characteristics that you put a checkmark next to in Table 1.
• Write them in the spaces below labeled A and B.
• Fill in the “Steps to getting adaptation” column (from your notes).
• Fill in the steps for getting adaptation A and B.
Table 2. How the Opuntia adapted to living with turtles.
Step
Steps to getting adaptation
Characteristics
A. Plant grows tall with pads and fruits
B. Plants have seeds with thick husks
on top
1
Organisms produce more offspring than
can survive to reproduce.
Opuntia produce more offspring than
can survive to reproduce.
Opuntia produce more offspring than
can survive to reproduce.
2
Offspring vary, and some of the variation
is due to genetic differences.
Selective pressures are present.
Opuntia vary in height, some of which is
due to genetic differences
Tortoise predation is a selective
pressure
Taller Opuntia (with pads and fruits on
top) are more likely to survive and
reproduce, and pass this trait on to
offspring.
Opuntia vary in husk thickness, some
of which is due to genetic differences
Tortoise predation is a selective
pressure
Opuntia with thicker seed husks are
more likely to survive and reproduce (if
eaten by a tortoise), and pass this trait
on to offspring.
3
4
Organisms with beneficial heritable
variations are more likely to survive,
reproduce, and pass the trait on to
offspring.
Teacher Handout: The tortoise and the cactus.
Page 5
Question: Assume that all the adaptations Opuntia have, for
living with turtles, cost the cactus precious energy. Write a
hypothesis that predicts which growth form (decumbent or
arborescent) a cactus would have if it lived on islands that
have never had turtles.
Opuntia that live without tortoises should have a
decumbent growth form, because it would save them
precious energy.
Question: Look at the map (Figure 6) and state whether or
not you were correct. Explain how you could tell.
Partially. All of the islands with turtles have arborescent
forms of Opuntia. However, some of the islands without
tortoises also have arborescent forms.
It turns out that the turtles were hunted to extinction on four
of the islands in the last two hundred years.
Question: Which islands probably used to have turtles, but
no longer do? How could you tell?
Rabida, Pinzon, Santa Fe, and Santa Maria probably
used to have turtles. Their Opuntia are the arborescent
forms.
Question: Predict how the Opuntia on these islands may
change now that they do not have to live with turtles.
They may become decumbent because they no longer
have the selective pressures to stay tall.
T
T
Illustration modified from
Biggs (1990).
Teacher Handout: The tortoise and the cactus.
Page 6
Adaptations of the Testudines
Although they are all fairly dry, some islands of the Galápagos have more moisture than others. The cooler, wetter islands have
many plant species that the turtles can eat. On the drier islands, there is almost no ground vegetation and the turtles have to rely on
eating the arborescent Opuntia in order to survive.
Question: Name two selective pressures on Opuntia that are stronger on the drier islands than on wetter islands.
• Less water
• More predation by tortoises
Question: Name two selective pressures on tortoises that are stronger on drier islands than on wetter ones.
• Less water
• Fewer plants to eat
• Arborescent Opuntia
It is said that you can tell which island a Galápagos tortoise is from by the shape of its shell. There are two basic shapes: large
dome-shaped shells (Figure 7) and smaller saddleback shells (Figure 8).
The advantages of being a saddleback
The shell of the saddleback tortoise is raised above the neck and flared around the back legs (see Figure 8).
Question: How do you think this helps the saddleback tortoise? (Hint: Remember the selective pressures that they have to deal with.)
Students should note that saddleback tortoises could better reach the arborescent Opuntia.
Teacher Handout: The tortoise and the cactus.
Page 7
Figure 7. Dome-shaped shell.
Figure 8. Saddle-shaped shell
Photograph
from
Humphrey
(2003)
Photograph from
Humphrey (2003)
Illustration from
National Center for
Mathematics and
Science (2002)
Photo from
Doubleday (2004)
Teacher Handout: The tortoise and the cactus.
Illustration from
National Center for
Mathematics and
Science (2002)
Photo from
Townsend
(1925)
Page 8
The Advantage of Being a Tall Opuntia
Figure 9. Tall and short
arborescent Opuntia.
There are two heights of arborescent Opuntia: tall and short. The
tall forms are more common on the drier islands.
Question: Why would the Opuntia on the drier islands need to grow
taller than those on the wetter islands?
They need to keep their fruits out of reach of the tortoises.
Illustration modified from National Center
for Mathematics and Science (2002)
Table 3. Fill in the following table.
Steps to getting adaptation
Tortoise: saddleback shell shape
Opuntia: tall arborescent form
1
Organisms produce more offspring than
can survive to reproduce.
Tortoises produce more offspring than
can survive to reproduce.
Opuntia produce more offspring than
can survive to reproduce.
2
Offspring vary, and some of the variation
is due to genetic differences.
Opuntia vary in height, some of which is
due to genetic differences
3
Selective pressures are present.
4
Organisms with beneficial heritable
variations are more likely to survive,
reproduce, and pass the trait on to
offspring.
Tortoise offspring vary. Because of
genetic differences, some will have a
more pronounced saddleback shell
shape than others.
Reaching the tall arborescent Opuntia
is a selective pressure.
Tortoises with a more saddleback shell
shape will be more able to reach taller
Opuntia. They are then more likely to
survive, reproduce, and pass this trait
onto their offspring.
Teacher Handout: The tortoise and the cactus.
Tortoise predation is a selective
pressure
Taller Opuntia (with pads and fruits on
top) are more likely to survive and
reproduce, and pass this trait on to
offspring.
Page 9
Question:
On a separate sheet of paper, answer the following in paragraph form:
•
How did the Opuntia adapt to living with turtles (summarize what you wrote in Table 1)?
o Should include adaptations checked in Table 1.
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How did the amount of moisture affect the food available to the turtles?
o Less water results in fewer plants, which is what turtles eat.
•
How and why did the turtles on the dry islands develop saddleback shells (from Table 3)?
o Why? – to reach the major food source, arborescent Opuntia.
o How? – students should summarize the steps outlined in Table 3.
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How and why did the arborescent Opuntia on the dry islands get taller than those on the wet islands (from Table 3)?
o Why? – to keep their fruits out of reach of the hungry tortoises
o How? – students should summarize the steps they outlined in Table 2
Teacher Handout: The tortoise and the cactus.
Page 10
References
Biggs, A. L. (January, 1990). Coevolution in the Galápagos: an example for the biology classroom. The American Biology Teacher.
Doubleday, E. (March, 2004). Galapagos Hawk on Galápagos Tortoise The Galápagos Islands Y.C.A.S. Photo Gallery Slide and
Photo Contest. York County Audobon Society. Retrieved October 30, 2005 from
http://www.yorkcountyaudubon.org/photogallery/photogallery_jpg/slides/HawkTortoise.jpg
Fritts, T.H. (1984). Evolutionary divergence of giant tortoises in Galápagos. Biological Journal of the Linnean Society, 21, 165 – 176.
Fritts, T.H. 1983. Morphometrics of Galapagos tortoises: Evolutionary implications. In: R.I. Bowman, M. Berson, and A.E. Leviton
eds. Patterns of Evolution in Galapagos Organisms, pp. 107-122. San Francisco, CA:Pacific Division, AAAS.
Humphrey, R. (2003, August). A Sample of Fact Sheets Developed for the San Diego Zoological Society Intranet, Galápagos
Tortoise, Geochelone nigra. Colorado.edu. Retrieved October 30, 2005, from
http://spot.colorado.edu/~humphrey/fact%20sheets/tortoise_galapagos/tortoise.htm
Moen, J. Galápagos Islands. Worlatlas.com. Retrieved October 30, 2005, from
http://www.worldatlas.com/webimage/countrys/samerica/galap.htm
National Center for Mathematics and Science. (2002). Modeling for Understanding in Science Education: Natural Selection.
Wisconsin Center for Education Research, University of Wisconsin-Madison. Retrieved October 30, 2005 from
http://www.wcer.wisc.edu/ncisla/muse/naturalselection/materials/section1/lesson1A/handouts/handout2.html
Townsend, C.H. (1925). The Galápagos tortoises in their relation to the whaling industry [Online]. New York Zoological Society, New
York Aquarium Nature Series, New York. Reprinted from Zoologica v.4, no.3, 1925. Retrieved October 30, 2005 from
http://www.du.edu/~ttyler/ploughboy/townsendgaltort.htm#bibnote
W. M. White, W.M (2001, January). Galápagos geology on the web. Cornell.edu. Retrieved October 30, 2005, from
http://www.geo.cornell.edu/geology/Galapagos.html
Watson, W. (1889). Cactus Culture for Amateurs Being Descriptions of the Various Cactuses Grown in this Country, with Full And
Practical Instructions for their Successful Cultivation [Online]. L. Upcott Gill, London. Available online from Project
Gutenberg.org. Retrieved October 30, 2005, from http://www.gutenberg.org/dirs/1/3/3/5/13357/13357-h/13357-h.htm#opuntia
Wayne, K.F. and Burke, R.L. (eds.). (1997). Crocodilian, Tuatara, and Turtle Species of the World: An Online Taxonomic and
Geographic Reference [Online].Association of Systematics Collections, Washington, D.C. Retrieved October 30, 2005 from
http://www.flmnh.ufl.edu/natsci/herpetology/turtcroclist/chklst9.htm
Teacher Handout: The tortoise and the cactus.
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