Summer Research Program for Science Teachers

August 2004

Naomi Cook

Humanities Preparatory High School

Manhattan

Struggle for Existence and Population Growth in Aphids

Introduction:  Aphids are great insects to study in the Living Environment classroom.  They reproduce asexually and sexually and it is very easy to work with them.  By doing background research, carrying out the experiment and writing up a comprehensive research paper that follows the checklist, students will be covering many of the important concepts in the Living Environment curriculum.  This project was modified from the following website:

http://insected.arizona.edu/gg/overview.html

Objectives:  Students will…

• Understand what the struggle for existence means for a population
• Appreciate the importance and sources of genetic variation within a species
• Be able to identify carrying capacity
• Identify adaptations that aphids have for their environment
• Make connections between struggle for existence and the larger idea of evolution
• Design a hypothesis based on background research
• Design an experiment that tests their hypothesis
• Collect and present data that helps to prove or disprove their hypothesis

Grade Level:

This can be done with any high school life science class.  It can also be carried out with modification in a middle school class.

National Science Standards:

TEACHING STANDARD A:
Teachers of science plan an inquiry-based science program for their students.

• Select teaching and assessment strategies that support the development of student understanding and nurture a community of science learners.

TEACHING STANDARD B:
Teachers of science guide and facilitate learning.

• Focus and support inquiries while interacting with students.
• Orchestrate discourse among students about scientific ideas.

TEACHING STANDARD D:
Teachers of science design and manage learning environments that provide students with the time, space, and resources needed for learning science.

• Structure the time available so that students are able to engage in extended investigations.
• Create a setting for student work that is flexible and supportive of science inquiry.

TEACHING STANDARD E:
Teachers of science develop communities of science learners that reflect the intellectual rigor of scientific inquiry and the attitudes and social values conducive to science learning.

·         Nurture collaboration among students.

·         Structure and facilitate ongoing formal and informal discussion based on a shared understanding of rules of scientific discourse.

Time Required:

This lab requires two, 50 minute periods to get the experiment started.  Students will be collecting data for about 10 minutes every class period for one month.  You may want to dedicate one or two class periods to getting students started on their paper.

Teacher preparation:

You will have to start growing the pea plants about 3 weeks prior to the beginning of the experiment.  Each group of 3-4 students should have one pea plant. They can be potted in deli containers.   Plants should be grown under plant lights and there should be extra plants in case something goes wrong.  Aphids should be ordered about 2 weeks in advance.  Upon receiving the aphids, they should be moved to a healthy pea plant and maintained throughout the duration of the experiment.  

Procedure:

            Pre-lesson activity

1. Have students perform research on aphids either as homework or in the classroom.  They should use the aphid worksheet to guide their research.  The questions address specific content that will help them better understand the purpose of the experiment.

Day 1

2. Have students read the excerpt from Darwin’s Origin of Species on struggle for existence.  What are the resources that organisms struggle for?  Why can only a limited amount of organisms survive in an environment?  Explain to the students who Thomas Malthus was and how Darwin built on his idea to come up with the struggle for existence.  Science is often a process that involves building on someone else’s ideas.  You can use the example of Charles Darwin not understanding the mode of inheritance when he came up with his theory of evolution.  Illicit from the students how the struggle for existence within an environment can drive evolution.  Darwin writes, “Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product.”  He did not have the vocabulary that we have today, but what ecological principle is he describing?  Go over the aphid worksheet.  Focus on the aphid reproductive cycle.  This is a form of adaptation.  What are the benefits of reproducing asexually?  Why would the aphids also reproduce sexually?  What are the sources of genetic variation?  Why is genetic variation important for a species?  Break students up into groups of 3 or 4.  These will be their research groups for the next month.  Pose the following question to students:
   1. Adult female aphids give birth to 6-8 nymphs (Aphids unlike other insects, give birth to live young, not eggs) every day until she has produced about 100 offspring.  Nymphs mature into adults in 10 days.  Assume that we start with one female that is sexually mature.  If all the aphid offspring survive, how many aphids would we theoretically have at the end of 28 days?  Do you think this will happen?  Based on this, create a hypothesis.  What will happen to the population of aphids in an isolated environment over a period of one month, if we start with one sexually mature female?

Day 2

3. Have students design their experiment for the next month.  Explain that each group has a potted pea plant and one sexually mature female pea aphid.  They should use the Experimental Design Worksheet to plan out and set up the experiment. 

Days 3-30

4. Students should count how many aphids are on their pea plant; observe the condition of the plant and record observations.  They will also need to water plants.  It may become possible to explain sampling methods if the population is too much to count completely.  Make sure students are careful with water.

Assessment

5. Have students write a paper on the experiment.  Refer them to the checklist.

Aphid Worksheet

Name:  ____________________________________

We will be working with aphids for the next month.  You will be designing an experiment that will examine the struggle for existence among an isolated population of aphids and how this affects population growth.  Since you will be developing a very close relationship with these creatures, you should know as much as you can about them! 

Please find the answers to the following questions…

These websites might be helpful:

http://www.gov.mb.ca/agriculture/crops/insects/fad14s00.html

http://ohioline.osu.edu/hyg-fact/2000/2031.html

http://www.ext.vt.edu/departments/entomology/factsheets/gaphids.html

1. What type of creature are pea aphids?

2. Where do pea aphids live?

3. What do pea aphids eat?

4. How do pea aphids reproduce?

5. What are some adaptations that pea aphids have to deal with their environment?

6. Explain the symbiotic relationship that pea aphids have with ants.

Experimental Design Worksheet

Name:  ________________________________

1. What is your hypothesis?

You have the following materials:

• One potted pea plant
• One sexually mature female aphid that can give birth to live young asexually
• 2 liter coke bottles
• scissors
• craft glue
• sticks
• nylon netting (or organza)

Design a set up for your experiment. 

2. What variables can affect population growth in natural populations?

(food & water availability, predators, immigration, emigration)

3. How are you going to keep these variables constant in your experiment, so that you are really testing your hypothesis?

(water same amount, don’t add any more food than the one pea plant, keep aphids in a contained environment, don’t add or take any aphids out)

4. How are you going to design a contained environment?  Aphids shouldn’t be able to get in or out, but they need oxygen.

5. Design a data table that can organize results for the next 28 days.  Each time you come to class, count the number of aphids on your plant. 

(Data table should contain a section for observations of the plant)

6. Get your plan approved by the teacher.

Aphid research paper checklist

• Address the question we are looking at.

• Background (some things you may want to cover)
  • What is the struggle for existence?
  • Who came up with it (you may want to refer to Darwin and Malthus).  How did they come up with the idea?  We went over this in class.
  • Darwin speaks of 3 struggles in the Origin of Species.  What are they?
  • Who survives best in the struggle for existence?
  • We will be studying population growth and the struggle in aphids.  What are aphids?
  • In what type of environment do aphids live?
  • What do they eat?
  • How do they reproduce?  Why is their reproductive strategy important with regards to evolution?  What are sources of variation?
  • State your hypothesis
    • Adult female aphids give birth to 6-8 nymphs (Aphids unlike other insects, give birth to live young, not eggs) every day until she has produced about 100 offspring.  Nymphs mature into adults in 10 days.  Assume that the one female we put in the bottle was sexually mature.  If all the aphid offspring survive, how many aphids would we theoretically have at the end of 28 days?  Do you think this will happen?  Use this background to explain your hypothesis.

• Procedure
  • What did we do?  How did you set up the experiment?  How did you start it?  How did you collect data?
  • What are some variables that affect population growth?  (Again, we did this in class)  How did you control for these variables?

• Results
  • Put all results in a table and then a graph.  Make sure you label everything.  The table and graph should have titles and labels for units and axes.  Please use Excel to design the tables and graph.

• Analysis
  • Did you prove your hypothesis correct?
  • Compare the population numbers with the state of the plant. 
  • What type of struggle were you witnessing in this experiment?
  • We studied the struggle for existence in a closed unrealistic environment with aphids.  How does it affect ecosystems on the earth and the interaction of species?  How does it affect the environment of the earth?  What do you think population growth would look like in the real world?  Would it look like your curve?  Why or why not?  Bring in the idea of carrying capacity.  Now is a great time to explain the struggle for existence within the context of evolution.   What does it have to do with evolution?  You should integrate concepts of adaptations and natural selection and genetic variation. 
  • What are some further questions you have on the struggle for existence?  This often becomes an afterthought, but it’s an important part of your paper.
• Bibliography
  • Keep track of all sources you use. 
  • One already is the Origin of Species.

I should premise that I use the term Struggle for Existence in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny. Two canine animals in a time of dearth, may be truly said to struggle with each other which shall get food and live. But a plant on the edge of a desert is said to struggle for life against the drought, though more properly it should be said to be dependent on the moisture. A plant which annually produces a thousand seeds, of which on an average only one comes to maturity, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. The missletoe is dependent on the apple and a few other trees, but can only in a far-fetched sense be said to struggle with these trees, for if too many of these parasites grow on the same tree, it will languish and die. But several seedling missletoes, growing close together on the same branch, may more truly be said to struggle with each other. As the missletoe is disseminated by birds, its existence depends on birds; and it may metaphorically be said to struggle with other fruit-bearing plants, in order to tempt birds to devour and thus disseminate its seeds rather than those of other plants. In these several senses, which pass into each other, I use for convenience sake the general term of struggle for existence.

A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life. It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial increase of food, and no prudential restraint from marriage. Although some species may be now increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them.

There is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty-five years, and at this rate, in a few thousand years, there would literally not be standing room for his progeny. Linnaeus has calculated that if an annual plant produced only two seeds and there is no plant so unproductive as this and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. The elephant is reckoned to be the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase: it will be under the mark to assume that it breeds when thirty years old, and goes on breeding till ninety years old, bringing forth three pairs of young in this interval; if this be so, at the end of the fifth century there would be alive fifteen million elephants, descended from the first pair.

The Origin of Species, Chapter 3 – The Struggle for Existence by Charles Darwin

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