Population Balance - How We Find Out

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 of time.
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 population.

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!