Like a Noah in reverse St Patrick kicked snakes off the rain-drenched ark of Ireland. So complete was his mystical sterilization of the land that seven hundred years later in his Topographia Hibernica (1187) Gerald of Wales could write: “There are neither snakes nor adders, toads nor scorpions nor dragons… It does appear wonderful that, when anything venomous is brought there from foreign lands, it never could exist in Ireland.” Indeed, even as late as the 1950s the Irish naturalist Robert Lloyd Praeger wrote, “The belief that “venomous” animals – which term included toad, frogs, lizards, slow worms and harmless as well as poisonous snakes – did not and could not flourish in Ireland, owing to St Patrick’s ban, long held sway, and possibly is not yet extinct.” (Natural History of Ireland (1950))
Snakes, however, are not the only species that can be found in Britain or continental Europe while being entirely absent from Ireland. Moles, several species of bats, many bird species, including the Tawny Owl, several titmouse species, and woodpeckers, innumerable insects species, many plants, and so on, might be added to the roster of St Patrick bio-vandalism. Of course, biogeographers have long known that the impoverished nature of the Irish biota is attributable to a number of factors unrelated to St Patrick.
Firstly, Ireland is a relatively small island with an area of 84,421 km² compared to Great Britain which is almost three times the size (229,848 km²). The European land area is considerable larger still being over one hundred times that of Ireland’s (at 10.18 million km²). Now, one of ecology’s more robust laws posits a relationship between area and species diversity. The more land, the more species. A consideration of the relatively restricted latitudinal range of Ireland in comparison to Europe intuitively suggests why Ireland must have fewer species. For example, since Ireland does not have a considerable southern stretch it has no Mediterranean zone, though it does have an enigmatic “Lusitanian flora” found disjunctly in Ireland and in North Spain and Portugal. This includes a saxifrage commonly known as St Patrick's Cabbage, but, the component to Irish vegetation is rare indeed. Nor does Ireland have tundra habit, though, of course, it can be get chilly there at times.
Secondly, the present day biota of Ireland was assembled largely after the the glaciers of the Last Ice Age retreated. Although there may be some relicts of those formerly icy time, for example the Irish Arctic char, an apparently delicious trout-like fish, which is found in some Irish upland lakes, most Irish wildlife migrated there over the past several thousands of years.