DEAR JOAN: Decades ago, garden snails were ubiquitous. They would leave tiny silver trails of slime on the walkways early in the morning, but I haven’t seen a snail in years.

The folklore I heard was that they were imported during the gold rush by a French forty-niner, and escaped from his food sack. They were an invasive species, regarded as a pest by gardeners.

So what happened to them?  Have they been eradicated by the same pesticides that afflict amphibians and other creatures?

— Amelia Marshall, Oakland

DEAR AMELIA: Although many of my gardener friends have a different experience when it comes to snails in the garden, worldwide there is a decline in snail populations primarily because of loss of habitat. About half of all native snail populations have disappeared.

The loss is mostly in native species; invasive snails are holding their own. In the Bay Area, our most common — and non-native — snail is the European brown snail.

The European brown snail didn’t just escape out of a food sack. They were deliberately brought here as a familiar food source for French miners and as a commodity to sell.

In the San Jose area, a man named A. Delmas introduced the snails into the Santa Clara Valley in the 1850s. In the 1960s, scientists released the decollate snail to combat the spread of the brown snail, but the brown appears to have mostly won out.

If you’ve seen a decline of snails in your yard, it’s likely because of the use of pesticides and an increase in natural predators. Populations tend to rise and fall through the years.

DEAR JOAN: My wife asked me to write to you over a crow infestation.

The crows seem to be driving the cute little birds out of our yard, where they would come and enjoy our birdbath.

Every solution I have seen drives away all the birds, not just the bullies.

Do you have any ideas on the subject?

— Robert Blackwood, Oakland

DEAR ROBERT: Trying to curate a guest list for wildlife in your yard is not the easiest thing to do. We might not like the attitude of some birds — people really seem to dislike crows — but what scares one bird will likely scare all the others.

However, there is a reliable method for keeping crows out of your yard and letting the others live in peace. First, you need to purchase a fake lifelike crow. They are easier to find closer to Halloween, but check Amazon or a wild bird store near you.

Once you’ve got the fake crow, wait until dark and then hang the crow upside down somewhere conspicuous. It’s very important that the crows don’t see you doing this because they will then tag you as a murderer and they will make your life miserable.

In the morning, the crows will see the birds. They often will gather around as if they’re having a funeral, and after a little bit, they will leave. Other birds won’t be bothered, but the crows will stay away for a long time.

The Animal Life column runs on Mondays. Contact Joan Morris at AskJoanMorris@gmail.com.