Yes and no. Yes, they almost certainly did drive this plant to extinction. No, it probably wasn't that good a method of birth control. Learn how silphim, a long-dead relative of fennel, powered an entire economy, before it disappeared forever.
Top image: Roman Mosaic from Centocelle.
The long-ago Romans had a problem. They were big fans of sex — and with all that gym time they logged, who could blame them? — but they weren't into big families. Famously, the Romans had condoms, but considering most people didn't have the money to buy disposable condoms, and some people didn't even have the soap to wash the condoms out with, this was not an attractive option.
So the Romans cast about for other methods of birth control — until they found a little weed that was first discovered by ancient Greek settlers in North Africa. Silphium, or silphion, was a fennel-like herb that grew along the coasts. The Romans learned that a bunch of its leaves, when ground up and put into a resin, reduced the likelihood of pregnancy. Word got around fairly fast, and soon both the Cerenean settlers, and the island they had left, Thera, were dishing out the herb to the Romans — and anyone else who could buy it. They weren't bashful about explaining what the herb supposedly did. Their coins showed pictures of silphium on one side, and a woman gesturing indelicately to her crotch on the other. And they had plenty of coins. Towards the end, silphium was worth its own weight in silver.
The high prices were good in the short-term for the vendors, but they reflected a long-term problem. Silphium couldn't be cultivated. Like the caper bush or the brazil nut tree today, it grew wild on the coast, or it didn't grow at all. Because coastlines are tough to guard, this meant that the strict limits on the yearly harvest did no good. Smugglers would come in and grab as much of the herb as they could. Although some people say that the weed has sprung up again on the coastlines of Libya, the most reliable sources claim the last pill was swallowed by the Emperor Nero. The Romans almost certainly caused the extinction of this plant.
But what, exactly, did they cause the extinction of?
Remember, these people also believed that a person could cure a headache by tying a fox's genitals to their foreheads, and that eating the heart of a donkey could cure epilepsy. Faking birth control had to be easier than faking a cure for epilepsy. Today people point to the fact that silphium is related to plants that are believed to be modern-day abortifacients — which makes Nero's gesture selfish and pointless. Understandably there are few studies to evaluate the efficacy of these modern herbs. Reputable medical websites refuse to even list them — again, understandably. The fact that any number of plants could be related to the extensive list means that supposing silphium to be an actual birth control agent is a taking a big leap, based on a big hypothetical.
Then again, the Romans were a practical lot, and they had to have talked among themselves. How likely is it that they'd shell out for something with no value at all when it came to something as relatively predictable as pregnancy prevention? Perhaps, in the end, the fact that silphium wasn't widely available worked in its favor. If only a relative few people could get their hands on any quantity, this would make a fraud, or a negligible effect, harder to detect than if anyone could try it for themselves. The only thing we can be sure of now is, we're lucky to have other, better options.
Plant image: USDA