The truth about spiders: If they don’t eat, we don’t eat. (Cathy Keifer/iStockphoto)

Most ecologists talk about saving the world’s wildlife from the animals’ perspective, which is a public relations mistake. Humans — one of the most wildly successful species of the Cenozoic era , which began about 65 million years ago — have trouble empathizing with polar bears, tropical frogs and dolphins as those animals sink toward extinction. A better way is to appeal to a human’s unstoppable desire to forward his own self-interest. This is how Norman Platnick talks about spider conservation.

“If spiders disappeared, we would face famine,” says Platnick, who studies arachnids at New York’s American Museum of Natural History, where a live spider exhibit debuted this month. “Spiders are primary controllers of insects. Without spiders, all of our crops would be consumed by those pests.”

Using the word “famine” is a good way to get an otherwise indifferent person’s attention. Although the magnitude of the insect apocalypse that would occur without spiders is not clear, the importance of spiders to agriculture certainly is. Predation and chemical control are the only ways to limit herbivorous pests, because there’s so much food available to them in our amber waves of grain.

Spiders are excellent at this task. And there are many of them prepared to apply themselves. A 1990 study found 614 species of spiders in U.S. croplands, representing 19 percent of the spider species in North America. Spiders are particularly crucial in organic farming, which relies heavily on biological pest control.

There’s more to spider conservation than crop protection, though. How much more? No one really knows.

You rarely see former Secretary of Defense Donald Rumsfeld quoted in a column about conservation, but one of his more famous lines is relevant here. “There are known knowns; there are things that we know that we know,” Rumsfeld said in 2002. “We also know there are known unknowns; that is to say, we know there are some things we do not know. But there are also unknown unknowns, the ones we don’t know we don’t know.” This is why you should care about spiders, in a nutshell. (Go with me on this.)

Crop protection is the known known. We know that spiders are important, and we know how and why. Now for the known unknowns: A spider’s venom contains hundreds, or even thousands, of different chemical compounds. We know they exist, but we don’t know if they might be useful to humans. Some of them may be medically active.

Researchers at several institutions are testing many of these chemicals. Scientists at Yale, for example, are examining whether chemicals in the venom of the Australian funnel-web spider could be used to improve pain-control medications.

This is just one of the possibilities. A physiologist at the University of Buffalo is trying to heal muscular dystrophy patients with a compound in the venom of a South American spider. Venom from scorpions — which are related to spiders — could help identify brain tumors. A Seattle scientist named Jim Olson is pursuing that project, as reported elsewhere this week in the Health & Science section. The list grows every year.

There are more known unknowns. In addition to the chemicals in their venom, spiders produce compounds in their silks that might have important applications. Spider silk has a higher strength-to-density ratio than steel. Its components could one day be put to use in such varied products as airplanes, bulletproof vests, surgical threads and prostheses.

That brings us to the unknown unknowns: the thousands upon thousands of undiscovered chemicals in the venom of currently unidentified spider species.

“Scientists have identified almost 45,000 different spider species,” says Platnick, “and that’s at best one-half of what actually exists. When we lose a spider species, we may lose a compound that could have cured epilepsy. We may lose a silk that could have produced a strong and lightweight material.”

Although some spiders have been captured to near-extinction by hobbyists, habitat loss and fragmentation of habitat are by far the greatest threat to spiders. Most serious conservation scientists have backed away from precise estimates of how many species disappear from the Earth every hour, day or year, but if the number were known, it would almost certainly shock you. In 2011, an expedition led by the California Academy of Sciences identified 300 new species, including many spiders, on Luzon Island in the Philippines in just 42 days. There is incredible biodiversity in underexplored places. Fragmentation of habitat compounds the problem, stranding small numbers of spiders and leaving each population more vulnerable to existential threats.

Platnick likens our destruction of spider habitat to tinkering with a plane’s engine while in flight. I’d suggest it’s even worse than that: We don’t fully understand how the engine works, and there are parts we haven’t even seen yet.