How do scientists count populations of animals on earth?

Humans are the only beasts who use telephones or permanent addresses, or fill out census forms.

How are the other animals counted? How can anyone tell that the timber wolf and the California condor are rare and “endangered”? How do we know how many robins chirp every spring? The task of taking a census of wild animals is one of the most difficult in biology.

The methods of enumeration scientists use vary with the species; its size, behavior, and habitat make certain ways more practical than others.

The best way to count ducks, whistling swans, elephants, antelope, caribou, and timber wolves is to fly over them in a helicopter or bush plane and count them one by one, taking photographs to verify the number.

This is obviously not a good method for counting field mice; they are too small to be seen from the air, are too well camouflaged by the color of their fur, and spend too much time in their burrows. The only way to determine the number of mice living in a field is by “saturation trapping” catching every single mouse until no more are left and counting them.

Lizards are counted by the “capture, recapture” method. To find the population in a certain area, a herpetologist (one who studies reptiles and amphibians) might catch 50 lizards, mark them all with a harmless paint or metal tag, and set them free again.

After a few weeks, the marked lizards have dispersed back into the general population. The scientist then captures another 50 lizards and finds that some of this batch are creatures he marked in his first catch, and some are unmarked, meaning they were not in the first batch.

The herpetologist’s next step is to make assumptions for the purpose of his census. He assumes that the new batch of 50 marked and unmarked lizards is a representative sample, a microcosm, of the population as a whole. He assumes that after he marked the first 50 lizards and released them, they distributed themselves at random throughout the population.

Thus, when he catches the second 50 and finds that he earlier marked, say, 10 of them, or 20 percent, he assumes that 20 percent of the entire lizard population is marked. He knows that he originally marked 50 lizards; concluding from the second sample that 20 percent were marked, he assumes that 50 is 20 percent of the total population. Since 5 x 20% = 100%, 5 times 50 lizards is the whole population: 250 lizards.

Fish are counted a similar way. Experimenters put a knockout solution in the water, which does the fish no permanent harm but makes them float to the surface belly up. They then collect the fish, count them, mark them with dye or tags, and revive and release them. The same number are later recaptured, the marked ones counted, and the total figured as for lizards.

How about animals that are harder to grab, such as songbirds? Ornithologists often use a grid system in a wooded area to get an approximate number. They mark evenly spaced, parallel straight trails through the region that interests them. People carrying pads and pens walk down the trails in a phalanx, each member keeping another in sight to the left and right, counting every bird they see or hear. Each member only counts birds observed a certain distance to either side of him, so that two people don’t count the same bird. This ritual is performed several times and the results averaged.

How does one count things as small as the microscopic plankton that live in the ocean? A sample of ocean water is whipped around in a centrifuge, so that all the solids collect at one end, including the tiny plankton. This residue is slid under a microscope bit by bit and the plankton counted. That gives the plankton per unit volume of ocean water.

As you can see, different methods are needed to keep track of animals living in different niches or habitats. To get an idea of the total number of a species in an entire region or country (or planet), scientists determine the size of the habitat available to the species, instead of counting individuals, and multiply by the number of individuals that usually live in a given area of habitat. It is in the nature of living things that they fill any habitat with as many individuals as the food and space in the area will allow.

Knowing the number of acres of woodland, mountain, prairie, and city in the United States, we can arrive at a ballpark estimate of 6 billion land birds of all kinds in the country. By contrast, some water birds such as the whooping crane are not nearly so adaptable; whoopers can live only in certain areas of Texas marshland, where about 100 nest each year.

Other forms of life build on a minuscule scale and fit vast numbers of individuals into their ecological niches. Insects have adapted through evolution to live in an incredible variety of conditions. The world population of insects in their many habitats is estimated to total a billion billion, or 10 to the 18th power (1018), the number 1 followed by 18 zeroes.

That’s roughly a billion times the world’s human population; if the world insect population were represented by a bucketful of sand, the human population would be a single grain of sand in that bucket. More amazing still, if we look closely at the bodies of those insects, we find as many as a hundred thousand one celled animals called protozoa living in the digestive tract of each insect, eating what the insect is unable to digest. There are therefore about 10 of these digestive protozoa living in the world’s insects. That number is greater than the number of stars in the universe.

As Jonathan Swift wrote after the invention of the microscope, which revealed for the first time the existence of protozoa, animals smaller than the naked eye could see:

Big fleas have little fleas
Upon their backs to bite’ em;
And little fleas have lesser fleas, And so ad infinitum.