How Do Populations Grow

GT BIOLOGY 2008

SR.ECO.7B

HOW DO POPULATIONS GROW?

INTRODUCTION

A population is a group of individuals of the same species living in the same place at a given time. Ecologists study the size and rate of growth of populations in order to predict how they will change with time. By learning how and why a population thrives or dies out in a particular area, ecologists can understand the delicate balance of the earth’s ecosystems and how humans affect this balance.

In this activity, we will model a population of lily pads in a pond. Imagine that the lily pads can double in number every day. Water plants such as lily pads grow so rapidly and are so hard to eliminate that they are a daily problem for citizens in many countries. In Thailand, for example, the cities are built around waterways. People often commute to work using boats and ferries instead of cars. The outboard engines frequently get clogged with lily pads.

Food, water, and living space are some of the resources individuals need to live and reproduce to their maximum potential. If these resources are unlimited, a population may continue to grow. Continuing growth will generally occur only when a population is very small relative to the available resources or very aggressive in taking resources away from other populations. In most ecosystems, however, resources and space are typically limited.

OBJECTIVE

When students have completed this lesson, they will be able to distinguish between exponential and logistic population curves in order to explain how limiting factors and the carrying capacity of an environment control the growth of populations.

MATERIALS

64 index cards

Ruler

Masking tape

PROCEDURE

Part I: Lily Pad Population Study

1. Work in groups of 3 to 4 students. Imagine that your lab table is the surface of the pond in

your backyard. Using a ruler, measure a square, 48" X 48" on your desk or lab table. Place

masking tape down to mark the edges of your "pond."

2. Get 64 index cards from your teacher. Pretend that each card is a lily pad.

3. Lay one card in the corner of the "pond" to represent the first lily pad. Record in the day 1

row of Table 1.

4. Now pretend that one day has passed. Double the number of lily pads in your pond. Record

in the day 2 row of Table 1.

5. Pretend another day has passed and double your lily pad population again. Record in the

day 3 row of Table 1.

6. Keep on doubling the population until half of the entire surface of the pond has been filled.

Record your population growth in Table 1.

TABLE 1: LILY PAD POPULATION

Questions:

1. How many lily pads does it take to fill half of your pond?

2. How many days passed before half of the pond was filled with lily pads?

3. How many more days will pass before the entire pond has been filled?

Graph of your Data:

Now make a graph of your results in Table 1. Keep in mind: What is the independent variable? What is the dependent variable? What is the title of your graph? Do you connect the dots or make a line of best fit?

Questions:

4. Is this graph linear? If not what is its shape?

5. What accounts for this shape? (What kind of growth is occurring in your lily pad population?)

6. What are some characteristics of this kind of growth curve?

7. Can your lily pad population grow like this forever? Why or why not?

8. What are some factors that affect how much your lily pad population can grow?

Part II: Looking at Lily Pad Carrying Capacity

As you may have predicted, there is a maximum number of lily pads that can grow in your pond. This is called the carrying capacity. Several factors limit the carrying capacity of any environment, including climate, food and water availability, physical space, disease, and predators.

The figure below shows what happens to a population when it reaches the carrying capacity of the surrounding environment. This is called an “S curve” because it is roughly shaped like an “S.”

Questions:

1. Examine the first half of the “S curve” shown in the above graph (point A). What is the shape of this graph?

1. What happens at point B?

1. What is the carrying capacity of the population (about how many organisms of this species can be supported in this ecosystem)?

1. What is the carrying capacity for the lily pad population in your pond environment? Explain your reasoning (See your graph in Part I)

Part III: Population Crashes

If there is a sudden change that affects the amount of available resources (for instance, a drought or a frost), a population that is growing exponentially may experience a dramatic decrease in size. This is called a population crash.

Some populations experience cycles of exponential growth followed by crashes.

Questions:

1. What happens at point “A”? What happens at point “B”?

2. Look at your graph of the Lily Pad population, what are some density-dependent factors that

could cause your population to experience a “boom and crash” pattern of growth?

3. What are some density-independent factors that could cause your population to experience a

“boom and crash” pattern of growth?

4. What would happen if a population crashed all the way down to zero?

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