Happy early St. Patrick’s Day! I admit, I had some trouble coming up with the appropriate theme for today’s post. The chemistry of green beer? The many negative health effects of corned beef? The genetics of the four-leafed clover? All of these were tempting, but I decided to pay homage to the saint himself. What is Patrick famous for? Well, converting Ireland, duh, but one of the most pervasive associations is snakes. According to legend, St. Patrick beat a drum, and drove all the snakes out of Ireland. While oral tradition is rife with somewhat unlikely feats performed by saints, it must be said that Ireland is notably snake free to this day. So, what exactly happened there?
Ireland, as most of you know, is an island. The ecology and biogeography of islands are necessarily a little different from those of continents, as islands are cut off by water. Plants and animals on the mainland don’t always make it over to islands. Particularly remote islands, like many in the Pacific share few, if any, native species with the continents. This is the usual progression. An island is created (typically through volcanism or some other tectonic event). A handful of organisms that travel particularly well (birds, floating seeds, etc.) reach the island. There, they slowly diversify through evolution until the island boasts a number of distinctive species. Ok, so Ireland is an island, and while it’s near the European continent, it’s surrounded by cold, stormy waters. Snakes are cold-blooded. Maybe they just never got there?
There’s just one problem. Ireland isn’t the only island off the European continent. For better or for worse, Ireland and the island of Great Britain are neighbors, and Great Britain has snakes. The lack of snakes in Ireland was long wondered at by the inhabitants of Great Britain. In fact, the title for today’s post comes from Shakespeare’s Richard II (II.i.159), where Ireland is referred to as a land where there is no venom (Great Britain may be no hotbed of Serpentes diversity, but it does boast one venomous native species). So, why does one island have snakes, but the other doesn’t?
Let’s do a quick review of the geology of Ireland. Unlike other islands, this isn’t a straightforward story of volcano spewing out enough matter to create a landmass above water. Ireland formed over time through plate tectonics - sometimes part of one continent, sometimes separated over two (Great Britain has a similar history). Ireland reached it’s present configuration after the break-up of the last supercontinent. However, while its position has remained relatively constant over the last few hundred thousand years, its island status has not. Enter glaciers, stage left. One consequence of Ice Ages is that, with all that water locked up in ice, sea levels dramatically fall. At various points in time, Ireland was connected to the European mainland. Fossil evidence is lacking, but it is possible that snakes might even have reached Ireland at certain points in the land bridge cycle.
The thing is, as often as you could walk from Dublin to London, half the time you’d need crampons and an ice axe to do so. Ireland was completely covered by ice time and time again - events which exterminated the island of all life. Even at other points in time, when the island had some ice free land, that ice-free land was tundra, and again, not hospitable to snakes. Or much else.
Check those maps out. Ireland has a level of biodiversity rivaled only by the far north of Scandinavia, in spite of having what most would agree to be a milder climate. Great Britain isn’t overly well endowed with fauna, either. Interestingly, marine mammal diversity off Ireland is quite high, but, then, the marine ecosystem as a whole is more connected to the rest of the world. Also, the oceanography of Ireland is fascinating in its own right, and may get covered here more thoroughly in the future. On land, though, what populates Ireland are the comparative handful of species that managed to make it over from the European continent. Given that speciation takes places over millions of years, and it’s only been in the tens of thousands of years since Ireland last emerged from the glaciers, these “pioneer” species simply haven’t had time to fill all the available niches.
It’s more a quirk of fate that Ireland has no snakes. Ireland does have one species of newt - if things were a little different, St. Patrick might be credited with driving the newts out! Ok, that’s one thing that we haven’t answered yet. We know why Ireland has no snakes, but how the heck did St. Patrick wind up taking the credit? For the answer to this one, we need to leave science and delve into comparative religion. St. Patrick’s two most-credited achievements are converting the Irish to Christianity, and the snake thing. These are actually connected. Before Patrick, the Irish followed Celtic religion, which ascribed ritual significance to snakes (unlike the harbinger of evil role ascribed to snakes in Christianity). Like everything else on the island, the Irish had, in the distant past, migrated in from a part of Europe where there were snakes aplenty.
So, to any Irish-American readers, go enjoy some corned beef and cabbage! To any Irish readers, go enjoy some bacon and cabbage. To everyone else, enjoy your celebration, but for the love of all that is lacking-in-snakes, don’t drink the green beer. You really don’t want to know how they get it that color.
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