For years now, conservationists have warned that Earth is in the middle of the “sixth great extinction,” with dozens of species going extinct every day owing to habitat loss, pollution, climate change, and other factors.

But here’s even worse news: That may be just the tip of the iceberg.

According to new research, previous estimates may seriously underestimate the number of species that we’re losing. A paper published in the Proceedings of the National Academy of Sciences suggests that we may have already lost 130,000 species, or a staggering 7 percent of the world’s total biodiversity.

How could we have lost so many species without noticing? It’s simple: The authors say most of these extinctions are not big, noticeable creatures such as rhinos and tigers. Instead they’re tiny insects and other invertebrates that don’t get much attention. These species tend to have very small ranges with specific habitat needs and aren’t often well studied. Of the estimated 1.4 million invertebrates worldwide, fewer than 16,000 have been evaluated for their extinction risk by the International Union for the Conservation of Nature Red List. Of that number, nearly a third are listed as “data deficient,” meaning we don’t know enough about them to say if they’re at risk.

By contrast, every single known bird species appears on the Red List. Of the more than 15,000 birds and mammals listed, fewer than 6 percent are ranked as data deficient.

So, Why You Should Care? The authors of the new paper say that focusing on larger, more easily studied species means we’re not getting a true picture of the extent of the sixth great extinction. Thus we may be losing species that could be important to human health even before we discover them. Snails and other invertebrates form an important part of the food web for all manner of animals, so their extinctions can have a cascading effect on biodiversity.

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To close that knowledge gap, the researchers took a random sampling of 200 land snail species around the world and then looked at the scientific record to see what we know about them. They didn’t find much. Snails haven’t been studied in some parts of the globe for decades, and some entire genus groups have never been studied at all. The researchers write that a full 84.5 percent of their 200 random species would be considered “data deficient” if they were added to the Red List today.

Even that doesn’t tell the full picture. Of the 200 species examined, 79 had not been observed in more than five decades. One example was a Hawaiian species known as the Amastrid land snail. Amateur snail-shell collectors have wiped out many of Hawaii’s snails, probably including this one. However, because no formal scientific studies have gone looking for it, it can’t yet be officially declared extinct.

Another example came from Mexico, where a species known only as Eucalodium moussonianum hasn’t been seen since 1872. Is it extinct? The original scientific description doesn’t mention the snail’s distribution or habitat, and no one has looked for it since.

Chances are high that these two species—and many others—are gone. “Mollusks are the species group most affected by extinction,” said the study’s lead author, Claire Régnier of Université Pierre et Marie Curie in Paris. The most common factors for their extinction include habitat loss, water pollution, and invasive species.

We do know of many other mollusk species extinctions because the animals leave their shells behind when they die. That’s not the case for most other invertebrates, Régnier said. The paper extrapolates this mollusk extinction risk to all other invertebrates because, as Régnier said, “they share the same characteristics as mollusks regarding their causes of extinction: size, restricted range, rarity, and specific habitat requirements.”

Régnier said the purpose of the paper is not to criticize the current ways that we look at endangered species and extinction risk but to “suggest an alternative methodology which would give us a more global idea of our current losses.”

That might not be enough to stop the sixth great extinction anytime soon, but it might help us to understand it and start to take action.