The Red Notebook

An unexpected Helix aspersa last Thursday

It isn’t every Thursday morning that one makes a paradigm-destroying observation while rushing, umbrella-in-hand, to one’s car. But that is exactly what happened to me last Thursday. It was pouring with rain, I was running six minutes late for work, I was wet and cross about it, when suddenly, there it was, as bold as brass and as bright wet as day, in front of my very eyes: a common (but, until that point, not at all garden) snail, Helix aspersa, sliming its way across my driveway.

You have no idea how ridiculously happy this made me feel. It is six years this month since I moved to my home in the Yorkshire Pennines, and, in all that time (with one very minor exception), I have seen (if you’ll forgive the inappropriate cliché) neither hide nor hair of a snail. And, believe me, I’ve looked.

I first wrote about the lack of snails in my garden in 2002, in an essay entitled …So Let’s All Be Scientists! It was my contribution to Darwin Day Collection One: The Single Best Idea Ever [ISBN: 0972384405, Amazon.com], a collection of articles, reviews and cartoons in celebration of Darwin and science. In the essay, I suggested a number of hypotheses—some of them more serious than others—why there were no snails in my garden.

Until 06:16 last Thursday, the acidic soil hypothesis (i.e. the acidity of the soil preventing snails from forming shells) was my favourite explanation for the dearth of snails, but that has had to go by the wayside. I am now beginning to favour the out-competed-by-slugs hypothesis. For the first four years that I lived in this house, the garden was literally plagued by slugs: thousands and thousands of slugs. But, for the last two years, the number of slugs has dropped considerably. I’m not sure why this should be—a combination of an unusually dry summer last year, and more dilligent weeding by yours truly is my best guess—but maybe the marked drop in slugs has let the snails get a (literally) single foot in the door. A Darwinian mollusc war in my own garden: who’d have thought it?

I will continue to monitor the situation with renewed interest.

[T]he great tragedy of Science—the slaying of a beautiful hypothesis by an ugly fact…

Thomas Henry Huxley
Biogenesis and Abiogenesis (1870)

Back in 2002, I wrote an essay entitled (in tribute to Huxley) …So Let’s All Be Scientists! It was my contribution to Darwin Day Collection One: The Single Best Idea Ever [ISBN: 0972384405, Amazon.com], a collection of articles, reviews and cartoons in celebration of Darwin and science. In the essay, I observed:

…for all the thousands of slugs I have found, I have never come across a single snail in my garden. Why is that? Is it too cold (I live in the Pennines)? Do the slugs eat them […], or out-compete them? Am I just not looking hard enough? Or is there simply not enough calcium in the area to allow snails to make shells (possibly because the soil is too acidic)?

Well, it would appear that the not looking hard enough hypothesis had some merit. The other weekend, I was moving a rockery in my garden (don’t ask), when I came across this:

A very small snail!

OK, as snails go, it wasn’t exactly the biggest (or, indeed, the alivest), but a snail is a snail, and an ugly fact (no matter how small) is an ugly fact: my no snails in my garden hypothesis is well and truly falsified. So now I have had to modify it slightly:

There are no big snails in my garden.

Actually, I have been keeping an active look-out for snails ever since I wrote my essay, and I can confidently say that I believe my modified hypothesis is correct. Which is odd, because I have observed large snails a couple of hundred yards away from my garden, albeit at a significantly lower altitude (I live on a very steep hill).

But, despite this stark evidence to the contrary, I still like my acidic soil explanation for the (near) absence of snails in my garden. In fact, until last week, I was growing increasingly confident that it was the correct explanation, having first confirmed that the soil in my garden is indeed acidic, and having recently come across the following in the New Scientist subscribers’ archive:

Acid attack

Acid rain has progressively thinned the shells of eggs laid by British thrushes over the past 150 years, a new study suggests. Ornithologists fear that the trend could make thrush eggs less likely to hatch…

[Rhys Green of the Royal Society for the Protection of Birds] thinks that acid rain, caused by sulphur emissions from the burning of fossil fuels, is the most likely cause. This would reduce both the calcium content of leaf litter consumed by worms and the abundance of snails, which together make up a large part of the birds’ diets.

So maybe there is still enough calcium in my garden for some very small snails, but not enough for any snails bigger than a couple of millimetres across.

I will get to the bottom of this one! Eventually.

Postscript [18-Jun-2007]: Hypothesis well and truly falsified!