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Global Ideas

Estimating global biodiversity a tricky task

Whether it’s species that fly, grow, swim, crawl or walk, scientists have long tried to puzzle out just how much diversity our planet is home to. But it’s far from easy. Global Ideas gives an overview.

photo: A poison dart frog

A poison dart frog on the Falkland Islands. During expeditions in the rainforest, researchers often find new species.

How many animal species are there on our planet? The short answer is – no one knows exactly. According to an Australian estimate, scientists have named and cataloged 1.9 million species on Earth, out of which 1.4 million are animals.

But calculating species remains more or less a guessing game. It’s unlikely that anybody will start counting and come up with a precise figure on global biodiversity in his or her lifetime. The last time that happened was 250 years ago when Swedish naturalist Carl von Linne, considered the godfather of systematic classification of species, created the catalog “Systema naturae”. It consists of 4,400 animal species, complete with names and distinguishing features as is common practice today.

Just a decade and numerous revised editions later, Linnes contemporary, Johan Samuel Schroeters wrote a somewhat desperate essay whose title is still relevant today. “Can we hope for a complete system of nature?” it read, with the subhead “And if so, through which method can we hope to get there?”

Ever since, there’s been plenty of puzzling and head-scratching in the scientific community about the exact number of species on the planet as well as how many are still left to find. Many researchers have developed methods in which they build on existing figures and estimates.

For instance, in 1988, British evolutionary biologist Robert May reasoned that we probably have found most of the species of big animals, like mammals and birds, so he used their diversity to calculate the diversity of smaller animals. That resulted in a linear connection between physical size and number of species – the bigger the body, the fewer the species. This link however did not apply to animals that are smaller than one millimeter, thus raising questions about May’s estimate.

Fogging beetles to estimate species

In reality, much of the estimates and guesses on the number of species seem to depend on the judgment and discretion of the author. One good example of quantitative efforts to determine the number of species was provided by American entomologist Terry Erwin.

He decided to take samples of insects in tropical forests which appeared to contain vast unexplored areas of biodiversity. An expert on beetles, Erwin fogged the canopy of several trees of the species “Lubea seemannii” with a pesticide and then counted the insects that fell into the nets he’d spread out. It was a laborious task because the tropics are home to a bewildering range of animal species and also because beetles, Erwin’s insect of choice, are considered the most diverse among insect species.

Illustration: A man sprays something (Illustration: Klaus Esterluß)

US entomologist Terry Erwin’s experiment.

Erwin first estimated that 160 of those beetle species were specialized to the canopy of that particular species of tree. He then went on to extrapolate his results, calculating that the 50,000 species of trees in the tropics would host some 20 million insect species. But then his own experiment led him to realize that fogging up the canopy of a tree couldn’t begin to cover the whole diversity of insect species and he upped his estimate by a third.

In an essay in 1982, Erwin wrote that he wanted to provide an estimate that was as close to reality as possible but said he had been overwhelmed by his own results.

Erwin’s calculations finally took him to the number 30 million species – far removed from the 1 to 1.5 million insect species he’d expected. It’s a high number which could in reality be smaller or even bigger.

By that measure, if one were to assume that each plant and each of these insects were to carry a parasite that were specialized to their species, one could easily come to a figure of 100 million.

In 2010, another research group looked at Erwin’s extrapolation of data, adjusted it for other factors, and came to the conclusion that there must be between 2.5 to 3.7 million insect species, of which at least 70 percent are still undiscovered. The work could drag on for hundreds of years if it continues at the current pace.

18,000 new species each year and counting

Each year, on an average researchers report more than 18,000 new animal species. It’s a figure that works with a different calculation model and builds on it in order to estimate global biodiversity.

But in 2012, a group of researchers led by Australian marine ecologist Mark Costello wrote that in the past scientists used to discover many more animal species each year. They concluded that the fact that ever fewer species are described today means that it’s becoming more difficult to find new ones because they have already been discovered.

And the numbers seem to support that. Estimates suggest that by 2050, only two new species each in birds and mammals are expected to be discovered. That means the workload of evolutionary biologists and scientists could ease up and be completed quicker than believed and the entire number of animal species could be tagged at around 1.8 to 2 million.

However, in 2011, researchers led by marine ecologist Camilo Mora at the University of Hawaii came to a completely different result. They estimated there are 8.7 million species on the planet, plus or minus 1.3 million. For the new estimate, the scientists came up with a method of their own, based on how taxonomists classify species. Each species belongs to a larger group called a genus, which belongs to a larger group called a family, and so on. We humans, for example, belong to the class of mammals, along with about 5,500 other species.

But whether we can hope to document and understand the Earth’s diversity in its entirety remains hugely questionable. What we do know is that the question of just how many species are on the planet leaves plenty of room for calculation models and guessing games of all kinds.