Opinion by William Laurance

One of the most heated debates in biology today revolves around how many species share our planet with us. Discounting microbes, plausible estimates range from around 2 million to more than 50 million species of animals, plants, and fungi. Given that scientists have been studying the natural world for centuries, why is there still so much uncertainty about this number?

The biggest reason, I’d argue, is that a lot of biodiversity is surprisingly hard to find or identify. This has profound implications for nature conservation and for our understanding of life on Earth.

We are finding new species every day, but the organisms we are identifying now are often quite different from those discovered in the past.

Not surprisingly, the very first species that scientists described were typically the largest and most conspicuous. The earliest naturalists to visit Africa, for instance, could hardly fail to discover zebras, giraffes, and elephants.

Many of the species we’re finding today have been overlooked simply because they’re harder to see. For instance, new lizard species that scientists discover now are generally nocturnal and smaller than those described formerly. The tiniest lizard known to science—a thumbnail-sized chameleon from Madagascar—was discovered just a few years ago.

Other species have remained undiscovered because they’re hard to capture. A biologist friend of mine discovered one such species. While visiting family in tropical Queensland his mother-in-law’s cat strolled in with an odd-looking animal in its mouth. His interest piqued, my friend wrestled the cat’s dinner away and eventually discovered that it was a mammal species, the prehensile-tailed rat, that was completely unknown to science prior to that moment.

Now known to be quite common in north Queensland rainforests, this tree-dwelling rat had gone unnoticed because it hadn’t been caught in a conventional wildlife trap. We can thank my buddy’s mother-in-law’s cat for the discovery.

Other species live in poorly explored places, such as the deep sea, underground, or hidden in caves. After spending some 1,100 hours digging holes in the ground, biologists in India stumbled upon the first species of caecilian—a primitive, snake-like burrowing amphibian—ever seen on the subcontinent. And on a far-flung beach in Alaska, a dead animal that washed ashore just last year turned out to be a completely new species of beaked whale.

In terms of places where unknown species almost certainly reside, high on everyone’s list is the rainforest canopy. In the early 1980s a Smithsonian Institution ecologist, Terry Erwin, used an insecticidal fog to sample insects in the rainforest canopy in Panama. He was stunned by his findings, because most of the species that fell to the ground were entirely new to science. Based on quick calculations he estimated that there could be 30 million species of insects living in the canopies of the world’s rainforests.

Erwin’s conclusions went viral, for in one fell swoop he had increased estimates of global species biodiversity at least tenfold. Today, most biologists consider his original estimate of rainforest insect diversity rather high. Some think he overestimated by a factor of just two. Others think it was far greater. Regardless of the magnitude of the overestimate, biologists agree that myriad unknown species reside within the roof of the rainforest.

Beyond species that are difficult to find or catch, there’s a lot of unknown biodiversity staring us right in the face—we simply can’t see it. For these species, new discoveries are coming from rapid advances in molecular genetics.

In recent years, for example, researchers have discovered that Africa has not just one species of elephant, but two: the savannah elephant and forest elephant. Formerly considered separate subspecies, genetic analyses have revealed that they’re as dissimilar to one another as the Asian elephant is to the extinct wooly mammoth.

Genetic studies have also uncovered hidden variation among Africa’s giraffes. Just last year, scientists announced that what was considered a single species of giraffe is actually four. And in Costa Rica, what was assumed to be one species of butterfly turned out to be at least 10 different species.

For biologists, molecular genetics is shaking the earth beneath our feet. Organisms we used to think were only distantly related, such as antelopes and whales, are practically kissing cousins in evolutionary terms. And genetics is revealing a hidden Eden of biodiversity. Some 60 percent of all new organisms described today are so-called “cryptic species” that are nearly indistinguishable from one another based on appearance alone.

There’s yet one more reason why so many species have yet to be discovered: They live in small, isolated habitats. Known as “restricted endemics,” such species are geographically concentrated in certain areas, such as tropical mountains, islands, and climatically unique environments.

Most of Earth’s restricted endemics reside in “biodiversity hotspots”, which are defined by having more than 1,500 locally endemic plant species (i.e. species that live nowhere else in the world) and less than 30 percent of their original habitat remaining. Of 35 currently recognized hotspots, half are in the species-rich tropics with the remainder divided among heavily fragmented Mediterranean habitats, temperate islands, and other isolated ecosystems. Some of the best-known biodiversity hotspots are Madagascar, the Brazilian Atlantic forest, the rainforests of West Africa, and the coastal shrublands of California.

Today, the bulk of new species are being discovered in biodiversity hotspots. But here’s what’s scary: Our recent analyses published in Biological Conservation show that more than half of all hotspots have already lost more than 90 percent of their intact habitat. Furthermore, most hotspots occur in poorer nations with rapidly growing populations and escalating social and economic challenges, creating even greater pressures on their already-beleaguered ecosystems and species. In total, intact habitats in biodiversity hotspots cover just 2.3 percent of the world’s land surface.

Taken together, these studies suggest three important points: that there’s an enormous wealth of biodiversity on Earth that we have so far failed to discover; that much of this biodiversity is imperiled; and that many unknown species differ from the life forms that have been scientifically described to date.

Exactly what is being lost or threatened is anyone’s guess. Not only is it unclear how many species there are on Earth, but evolution has had billions of years to create biologically active compounds that can combat human diseases, generate genetic diversity that could save our food crops from disastrous pathogens, and spawn ecological innovations that can inspire marvelous new inventions. This is untapped potential that we may lose without ever knowing it existed.

The notion that much of Earth’s biodiversity is undiscovered and often restricted to specific places has huge implications for conservation. Globally, we are losing tens of millions of acres of natural habitats every year, and given limited resources, we clearly need strategies to prioritize our species-conservation efforts.

To limit extinctions, we must play particular attention to the biodiversity hotspots. These ecosystems have already been savaged, but the difference between losing 95 percent versus 99 or 100 percent of the last remnants of a species-rich habitat is enormous. In that sense, there is no such thing as an unimportant habitat remnant. Even tiny tufts of surviving vegetation could have value for rare, endemic species. In the hotspots, we must fight to protect every last shred of surviving habitat and actively restore vegetation that has already been damaged

Climatically unique environments are also critical. For example, isolated mountaintops are often teeming with locally endemic species, many of which are specialized for cool, wet, cloudy conditions. As the global thermometer rises and killer heat waves become more common, many mountaintop endemics will have nowhere to go except heaven. This provides yet another compelling reason to combat harmful climate change.

Finally, isolated ecosystems, such as oceanic islands or geologically unusual sites, are often rich in vulnerable endemics, and therefore should be protected. For example, patches of serpentine soil, which present a range of chemical challenges for the plants that live there, harbor many unique species that have adapted to these bizarre conditions. Isolated granite crests and jagged limestone outcrops known as karsts also support many distinctive plants and animals.

This is just scratching the surface, of course. So many of our planet’s life forms are unique in some way or another, and many remain a complete mystery. What a tragedy it would be to lose this unknown biodiversity before we ever have a chance to discover or learn from it.