The Number Game: the Capture-Recapture Method
You’re probably wondering how
ecologists determine that a population grows or that a species has
become extinct. It’s
simple really. They count them. But not in the usual way.
Great amounts of time and effort would be required to count even
small populations. Imagine what it would be like to count millions of
invisible organisms like bacteria.
So, how do they measure the size of a population?
The most common method used to
measure animal populations is the capture-recapture method.
Here are the steps involved:
Some members of the population are
captured, usually by some sort of trap.
Ecologists then mark the captured
animal in a distinctive way (colorful band, dye, etc.).
These marked animals are released
back into the population and allowed to circulate for an extended period
Then, the ecologists return to
capture another sample of the population.
Finally, they use the number of
marked animals captured this time to estimate the population size.
|How do they estimate the total population
from the number of recaptured animals?
First, they assume that the marked individuals had time to
move randomly through the habitat upon release. After
recapture, they then calculate the ratio of the number of
recaptured animals that had marks to the number of animals that were originally
marked. Comparing this to
the total number of recaptured animals lets the ecologists form
an educated guess as to the total number of individuals in the
Do you think this would be an accurate approach
to estimating population size?
What is the point of recapturing?
Can you think of some drawbacks of this method?
A modern fruit-fly trap
For populations of birds and mammals, the
capture-recapture method has proven to be an effective tool for
studying ecosystems in terms of size and diversity.
However, how do we study invisible populations like those of
bacteria? Gaining knowledge
about these microscopic organisms requires more than just the ability to
see them. Ecologists take
samples of bacteria from various locations and then later attempt to
grow them in a laboratory environment.
But in the past, only a relatively small percentage of bacteria
have been successfully grown or cultured.
||In response to this problem, scientists have recently developed a
new approach that involves culturing the bacteria in an
environment similar to where they were found.
This brings up another question.
If we have only just begun to culture a variety of
bacteria, how do we know that one million species of bacteria
exist if we have only described a little under 5,000 in any
detail? We guess!