How is the population for species calculated all over the world?

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I keep seeing and hearing that a particular species is 60% or some percent extinct. My question would be how was the total number of living species ever calculated. For example say dolphins. How would anyone ever know the number of dolphins alive at a particular time. At least for us we have a registered census but what is it for these species?

In: Earth Science

You estimate it from the numbers that you’re actually seeing. It’s a fairly common statistical technique to derive total populations from a limited sample set of it, and it’s fairly accurate. In addition, if you know that you’re seeing 60% fewer of a particular species than you were a few years ago, it’s reasonable to assume that the species as a whole has also declined by that sort of level–it’s extremely unlikely that 60% have decided to simultaneously go into hiding.

Well, not every member of a species is counted. Like all things in science, we guesstimate depending on sample results.

A method of population estimation is the capture-recapture method, where methods for trapping animals (e.g. pitfall traps) are employed to trap animals in a harmless way. Researchers then inspect the traps, count and tag any trapped animals and then release them back into the wild.

Some time later, same thing again. Capture and count. Then use this formula:

(Animals captured in first round x animals captured in second round) / Tagged animals captured in second round