Biology I
Exponential Growth Lab
Name______
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 of birds, plants, or fish 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.
Exercise 1:
Lily pads can grow fast. One variety of lily pads that can double in number every day. It takes 10 days for them grow to cover a pond halfway.
1. How many more days will it take for the lily pads to completely cover this pond? (Circle one.)
a) About 10 more days
b) About 5 days
c) Tomorrow
d) Never, since the lily pads won't ever fill up the pond completely
2. Why do you think this?
Exercise 2:
Work in groups of 3 to 4 students. Imagine that your lab table or desk 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."
Get 64 index cards (3" X 5") from your teacher. Pretend that each card is a lily pad.
Lay one card in the corner of the "pond" to represent the first lily pad.
Double the population until half of the entire surface of the pond has been filled.
Use Table I below to keep track of your data.
Table I: Lily Pad Population
Generation (Number of Days) / Number of New Lily Pads / Number of Lily Pads in the Population1
2
3
4
5
6
7
8
3.How many lily pads does it take to fill half of your pond?
4. How many days passed before half of the pond was filled with lily pads?
5. How many more days will pass before the entire pond has been filled?
6. Graph your results
7. What is the shape of your curve? Interpret the growth rate.
There are several factors affecting the shape of the growth curve.
- Reproductive rate (number of births compared with number of deaths)
- limits on resources: food, water, living space
Resources and space are typically limited in ecosystems. Exponential growth generally occurs only when the population is very small relative to available resources or very aggressive in taking resources away from other populations.
8. What is the reproductive rate of the lily pads? (Hint: how many "offspring" does each lily pad produce per day?)
9. Is there a maximum number of lily pads that can grow in your pond?
10. How would you describe the growth of the human population today looking at the graph below?
11. What conditions do you think kept the human population lower and more under control before this century?
Figure 3 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".
12.What happens at point B (after about 13 generations)?
13. What is the carrying capacity of the population recorded in Figure 3 (about how many organisms of this species can be supported in this ecosystem)?
If there is a sudden change which affects the amount of available resources (for instance, a drought or a frost) a population which 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. This pattern is called boom and crash.
Sometimes a population drops to zero when it crashes. An example is the Hohokam tribe in central Arizona, which lived there more than 2,000 years ago. Experts believe that the population was at one time over 1 million people, but somehow the entire Hohokam culture vanished.
14.What was the carrying capacity of the earth for the human population prior to 1850 (see Figure 2)?
15. What is the carrying capacity of the earth for the human population today?
16. Can a population grow exponentially forever? Why or why not?
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