11.10 THIS IS HOW WE DO IT: How many species are there on earth?

11.10 THIS IS HOW WE DO IT: How many species are there on earth?

When contemplating a question in biology, the most useful approach often resembles a sort of “back-of-the-envelope” calculation. This typically requires gross simplification, but it can illuminate the information needed to come to a more accurate answer, while revealing assumptions that may have a large influence on the answer obtained.

As we consider the diversity of life on earth, one seemingly straightforward question comes to mind.

How many distinct species are there on our planet?

It might come as a surprise to learn that we don’t really know the answer. And it’s even a bit embarrassing that we aren’t very confident about making even a ballpark estimate. Some scientists put the number at around 3 million, while others believe it could be greater than 100 million.

In a famous paper published in 1982—a paper actually short enough to fit on the back of an envelope—the biologist Terry Erwin laid out a simple approach to estimating the answer. His strategy was two-pronged. It began by posing a question that he could answer with a high degree of accuracy.

How many species of beetles are there in one species of tree?

More specifically, Erwin set out to get a definitive count of all the beetle species living in the canopy of a single species of tree (Luehea seemannii) in a tropical forest in Panama. His methods were not elegant. Over the course of three seasons, he sprayed a fog of pesticide into the canopy of 19 trees of this species, then collected and identified all of the beetles that fell to the ground.

Here’s what he found: 1,143 different species of beetles fell from the trees. Because other researchers had reported collecting additional beetle species from the same species of tree, he rounded off the number to 1,200 species.

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The second prong of Erwin’s strategy was to estimate how this number was likely to translate into the numbers of other insect species living in the diverse tree species in the tropics. In essence, from data he had high confidence in, he made an extrapolation.

How is the number of beetle species in one tree species related to the number of all insect species in all types of trees?

This required several educated guesses (Erwin is, after all, an expert on beetles), along with a bit of speculation. His back-of-the-envelope calculations included estimates of the proportion of beetle species that are host specific (13.5%), the number of unique trees per hectare (~70), the proportion of all arthropod species that are beetles (40%), the proportion of arthropods that are in the forest canopy as opposed to elsewhere (67%), and the number of unique species of tropical trees (50,000). When he multiplied everything together, his estimates led him to conclude that there were almost 30 million species of arthropods in tropical forests. This number was remarkable because it was about 10 times higher than most previous estimates of the total number of species on earth!

Understanding the self-correcting and collaborative nature of scientific thinking, Erwin did not claim this was the true and final answer. He said, “I would hope someone will challenge these figures with more data.” Erwin had, after all, made numerous assumptions. (Which of these seem likely to have had a significant influence on his final answer?)

Taking Erwin’s ideas as a starting point, numerous researchers have noted (and tried to improve on) his numerous assumptions. In 2010, researchers using more data and more realistic assumptions came up with a better estimate. Rather than making precise estimates for each parameter, they used probability distributions (such as for the proportion of beetle species specific to each species of tree).

This enabled the researchers to come up with a whole range of estimates, each having a likelihood associated with it, and their estimates were much lower than Erwin’s 30 million. One of their models generated an estimate of the median number of tropical arthropod species as 3.7 million, with a 90% confidence interval that the true number was between 2.0 and 7.4 million.

These estimates and the process by which they’ve been generated give us greater confidence that we know the answer. Unfortunately, this isn’t an answer to our original question. It’s an answer to an easier question: how many arthropod species are there in the tropics?

The reason for limiting the scope of the question is that it is much easier to define and identify beetle or other arthropod species. But does our answer bring us closer to knowing how many species, in total, there are on earth? That is difficult to know. Consider another question.

What types of species are the most difficult to estimate (or count)?

Given the impossibility of applying the biological species concept to asexual species, for example, how should we define and identify species of prokaryotes and viruses? And, are the numbers of marine species proportional to the numbers of arthropod species.

The back-of-the-envelope approach described here can be a useful starting point for these new calculations as well.

TAKE-HOME MESSAGE 11.10

Determining how many distinct species there on earth has long been a challenge for biologists. By counting the exact number of beetle species in one species of tropical tree and estimating several parameters that can be used to extrapolate this number to the number of species of all arthropods across the entire tropics, it is possible to make headway on this challenge. Initial estimates using this method suggest there may be many more species on earth than previously believed.

Describe how Terry Erwin approached the challenge of determining how many distinct species there are on earth.

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