Lesson 6 Overview
DRIVING QUESTION: How do we know that ecosystems change over
time?
LEARNING GOAL:
• Students collect and analyze historical climate data to make a claim about past climate events.
TOTAL TIME:
200 minutes
LESSON SUMMARY: Students will examine past and present distributions of a species
and then examine relative dating techniques of sediments and fossils. Students will use pollen stratigraphy data and oxygen isotope data to make a claim about past climate events.
MATERIALS:
• Candy bar, cake, or pastry with layers
• Plastic knife
LESSON 6: How do we know that ecosystems change over time?
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BEFORE YOU BEGIN LESSON 6:
Part 1
You may want to familiarize yourself with the past and present species
distributions on the SPECIES interface before introducing this to the class.
Read through the options for the relative age activity to determine which your
class can do. You’ll need to either arrange for your class to go outside to a rock
outcropping or snow pit. Or you can bring in some cake or candy to demonstrate
layers and relative age.
Part 2
You may want to review the pollen diagram that students will interpret so you
understand how historical pollen data is presented.
Part 3
You may want to review the information on oxygen isotope ratios in ice cores so
that you can help answer student questions.
LESSON 6: How do we know that ecosystems change over time?
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Part 1: Comparing Past and Present Distributions
Have your class review an example species for which there are data on past distributions (10,000
years ago). The class will compare that past distribution with a current distribution map.
Lead a class discussion on how these distributions are different and why they may be different.
● How are these distributions different from one another?
● Why do you think these distributions are different: did the biology (requirements) of the species
change or has the environment in that area changed?
● Given what you know about the distribution of the species now, how have abiotic conditions
changed from 10,000 years ago to today?
LESSON 6: How do we know that ecosystems change over time?
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Part 1: Comparing Past and Present Distributions
Students will use information on past and present distributions of a species to construct a justified
prediction for the scientific question: What must the climate have been like in the places where
scientists found fossils of the species?
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Part 1: Relative Age - Layers of Life
Lead a class discussion to begin the relative age activity. Consider how your laundry accumulates
over the week, perhaps in a hamper or in a pile on the floor.
● How can you tell which shirt was worn at the beginning of the week?
● Which is the “youngest” pair of socks (i.e. worn at the end of the week).
● Build an explanation that describes how the order of clothes in a pile of laundry represents
time.
● What could happen to the pile of laundry to make it difficult to interpret which layer of
clothes came first or last?
There are two options to illustrate relative age after the class discussion. If you have appropriate
geologic outcrops to show sedimentary layers or if you can dig a snow pit that shows layers, use
Option 1. If not, use Option 2.
Option 1: Use the natural environment to illustrate relative age.
○ Dig a snow pit to show different snow layers or look at a sedimentary rock
outcrop to view sedimentary layers.
○ How can you tell which layer formed first?
○ Which is the youngest layer? How can you tell?
○ What would have to happen to disrupt the order of layers in the snow or
sedimentary rock?
Option 2: Use a candy bar, cake, or other layered treat to illustrate relative age.
○ Students examine cross sections of a candy bar, cake, or other layered treat to
model the order of formation of sediment layers. Use any kind of candy or treat
that has distinct layers.
○ How can you tell which layer formed first?
○ Which layer was added last? How can you tell?
○ What could you do to disrupt the order of layers in the candy or treat.
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Part 2: Pollen in Lake Sediments
Once the students have a grasp of relative dating of layers, they will examine data on sedimentary
layers of pollen. Pollen sediments in the bottom of lakes and ponds are used to understand how local
floras have changed over time.
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Part 2: Pollen in Lake Sediments
Pollen can be identified so that changes over time between different landscape types can be tracked.
For example, the majority of pollen in a sediment can change from primarily pine pollen to primarily
grass pollen, indicating that the local flora has changed from primarily pine forest to open grassland.
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Part 2: Pollen in Lake Sediments
Students interpret the pollen stratigraphy diagram and answer questions to help them interpret the
data.
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Part 3: Oxygen Isotopes Data from Ice Cores.
Students have been examining pollen data that illustrates relatively local conditions over time
(a flora around a lake or pond and the area immediately around a tree). Students then turn their
attention to data on oxygen isotopes present in ice cores. Oxygen isotope ratios are a characteristic of
Earth’s atmosphere so these data are a good representative of global climate. The ratio of two oxygen
isotopes: O16 to O18, varies with temperature, so trapped oxygen bubbles in ice can be analyzed to
determine global average temperature at the time that the oxygen bubble was trapped.
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Part 3: Oxygen Isotopes Data from Ice Cores.
These activities are concluded with a synthesis of these historical data. Students will be asked to
explain how different kinds of data are used to understand how the Earth’s climate and habitats have
changed over time.
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Optional Extension: Candy Half Life Lab
If you want to go more in depth on how to date sediments, rocks, or fossils, this is an extension
activity that examines how scientists estimate absolute age (rather than relative age). Using M&M’s,
students can model radioisotope decay to understand how isotope decay is used to measure time.
Candy Half-Life Lab
Definitions
Isotope: number of neutrons vary but protons stay the same. An unstable or radioactive, isotope
is called the parent material. Isotope that results from the decay and becomes stable is called the
daughter material.
Half-life: Way to express the rate of radioactive decay; it is the amount of time needed for one
half of the nuclei in a sample to decay to its stable isotope
Prelab Activity
1. Take 2 Twizzlers and a piece of graph paper
2. Create an x- and y-axis (it has to be at least as tall as the twizzler).
3. Put a Twizzler on the paper parallel to the y-xis and the left side of the twizzler touching the
y-axis. Mark the height and leave the twizzler on the paper
4. Break the other Twizzler in half. Place one piece on the graph and leave it there. Mark the
height.
5. Break the other piece in half, place it next to the other Twizzler and mark the height.
6. Continue to break the piece in half until it is too small to break. Line each piece of Twizzler
next to the other and continue to mark the height.
7. Connect the lines of the graph
Lab Activity:
Use candy pieces and model the decay of a typical isotope with respect to half-life.
Materials:
100 pieces of M&M’s, Skittles, or pennies
2 styrofoam plates
Napkin or paper towel
Pen or pencil
Graph paper
Ruler
Clean hands
Hypothesis: Make a hypothesis about what you think will happen when we do the M&M activity
based on your knowledge of half-life and the pre-lab Twizzler activity.
Procedures: 1. With a partner count out 100 pieces of candy into one plate. (If possible, remove yellow candies
as they are hard to see the “M” (or “S”).)
2. Place all 100 candies with the letter facing up on the plate.
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