One of the southern legs of the Oak Grove Loop Trail extends southeast from the picnic area (the northernmost southern leg, if that makes sense: at about 32.818693, -117.053897). There my wife, son, and I saw many hundreds or thousands of empty snail shells on the north side of the trail. On the southern side, there were almost none.
It looked like a snail apocalypse, except when we looked closer we could see that many were bleached white by the sun and weather, as if they’d been there for years, although a few still showed their stripes and coloring.
Why are there so many empty shells there? What happened to the snails? Were they drowned? Eaten? Poisoned? Killed off by disease?
I asked the folks at Mission Trails Regional Park and got an email response from Education Coordinator and Ranger Heidi Gutknecht:
Those shells are from the common garden snail, which has a large variety of natural predators, including ground beetles and parasites. If not killed and eaten by a predator, they are usually eaten by ants or just dry and shrivel up in their shell after dying. If you saw a whole bunch of empty shells in one area, it’s probably due to the recent rains washing them out from beneath the shrubs.
Ranger Gutknecht pointed me to videos hosted by Bill Howell, a trail guide instructor at MTRP. The first is a short introduction to the common garden snail, Helix vulgaris, and the second is about white garden snails, Theba pisana. Both species can be found in the park.
The brown garden snail, Helix aspersa, considered a pest, probably also accounted for a lot of the shells we saw. Some also could have been Rumina decollata, which eats brown garden snails, though I don’t recall seeing any of their distinctive elongated, conical shells.
Another snail predator has the fantastic name of the devil’s coach horse, Ocypus olens.
As you can see by Alan King’s photos of snails in the park, some snail shells look white even before sun-bleaching, so shells might have been more recent than they appeared.
But why so many snail shells?
Well, besides being prolific breeders (laying up to 80 eggs at a time — up to 2,000 over a lifetime, according to this 2001 Union-Tribune article), snail shells take years to disintegrate.
One academic study reports that the half-life of snail shells is about 7.5 years, meaning the shells will degrade by about half during that time. It would take another 7.5 years for half of the remaining shell to disintegrate, and so on. They lie in the underbrush until swept together by freshets and caught against natural barriers.
More information about snails and their natural predators, including pictures, can be found on a brochure and web page from the University of California Statewide Integrated Pest Management Program at UC Davis.
You can also view all of my pictures of the snail shells in high resolution at Flickr.
Correction: As pointed out in a comment below, the original description of a snail shell’s half-life was incorrect. It has been updated to accurately indicate how long a snail’s shell would take to decompose.