Of otters and scurvygrass

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A dense patch of scurvygrass (Cochlearia officinalis agg.) in an area much frequented by otters based on the clear trails, spraint and crushed mollusc shells.

On a sunny February weekend I made a trip out to Bere Island (Oiléan Béarra), a short ferry-ride off the southwestern tip of Ireland, to investigate a peculiar botanical phenomenon. An old friend had called on me recently and enquired what the connection was between otters and scurvygrass (Cochlearia officinalis L.*). I confessed to knowing nothing about it and was slightly sceptical, but he was adamant that there was such an association, and wanted to show me in person. Any excuse for a field trip is welcomed so off I went.

Once there on the ground there’s no doubt about it. The otter haul-outs are very obvious, and invariably marked with scurvygrass. The plant does grow elsewhere, unsurprisingly, but never in such densities and with such luxuriant foliage as where they occur alongside otter signs. Whatever the otters are doing, the scurvygrass is enjoying it. A representative sample of rocky outcrops suggests that if you spot a patch of scurvygrass it’s clear indication that once you get up close there will be matching evidence of otters. Neighbouring outcrops lacking the plant are also devoid of otter signs.

It’s not only at the shoreline where otters leave the sea, and where their spraint or the remnants of smashed shells indicates favoured spots to hang out. Their tracks continue inland. Often after coming onshore the track leads directly to a freshwater pool, suggesting that they like to wash the salt water off their fur, then out the other side. They then wend their way through the grass and heather, apparently choosing to cross the island on foot rather than swim their way round.

This is where it gets even more interesting: in the midst of these fields, scurvygrass is only found on the otter trails. Elsewhere the sward is higher and there is no sign of the plant. Otters are clearly carrying scurvygrass inland and encouraging it to grow in places where it otherwise would not. This effect is only seen in fields which don’t contain livestock; cows and sheep have a tendency to share the same trails with otters and perhaps browse or beat down the scurvygrass. But on the old military firing range, where otters have the land to themselves, every one of their trails is peppered with patches of the plant.

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An inland otter track through a field with a dense grass sward. Scurvygrass can only be found along this trail.

On returning to the office I consulted the usual books to see whether I could find any record of this particular association, and drew a blank. Web searches, whether in the scientific literature or the internet at large, have also come up with nothing. So what is going on? I have a few hypotheses:

  1. It’s a coincidence. Any ecologist has to keep the null hypothesis in the back of their minds. Maybe this is pure chance, or else some unknown independent environmental factor dictates the combined presence of otters and scurvygrass. I haven’t done a randomised sampling design to demonstrate a statistical association, but often the evidence of your eyes is enough to tell you that there’s no need.
  2. Otter disturbance. Scurvygrass thrives in patches with frequent disturbance, and the constant to-and-fro of otters might open up denser vegetation in such a way that they allow it to enter. Perhaps scurvygrass is more tolerant of this kind of disturbance than other plants.
  3. Dispersal by otters. The seeds of scurvygrass don’t look like they are adapted for sticking to the sides of animals, which would be one mechanism. Do otters eat scurvygrass, and thereby carry the seeds with them, defacating them in freshly-disturbed areas that aid its germination? This is currently my favoured explanation, but I don’t know enough about the diet of otters to be sure.
  4. Saline environments. Perhaps all the salt water clinging to the fur of otters changes the soil at their haul-outs and along their trails, favouring the growth of a halophyte such as scurvygrass. This is possible, but not entirely plausible given the presence of patches quite a way inland and even after they’ve taken a freshwater bath. Soil samples might resolve this. There are also no other halophytes which show the same pattern.
  5. Otters go where the scurvygrass is. Maybe they like the feel of it on their paws? It’s unlikely to be that they’re feeding on it, otherwise you would expect to see less scurvygrass in the places they use most frequently, while the opposite appears to be true.

Have I missed anything? Is this a known phenomenon that my friend has independently discovered? I’d be interested to hear from anyone who knows more about this in the comments. For now it’s only a small mystery, but also an intriguing natural history observation, and a reminder that people who walk outdoors and watch the world around them carefully often spot patterns that professional ecologists in their offices would never find.

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Credit to Bere Island resident Maurice Neligan for spotting this interesting pattern.


* This is as close as I can get to an exact species identification. Stace notes (in the 3rd edition; I don’t have the latest one just yet) that C. officinalis is highly variable, an aggregate of several likely species, and also freely hybridises with C. anglica and C. danica, especially in Ireland.

UPDATES!

As ever, Twitter provides a wealth of insights from other botanists. Here are the pick of the suggestions:

The first can be summarised as ‘disturbance + fertiliser’, the second with the twist of extra salinity, which might not be important given observations elsewhere. But scurvygrass isn’t only found where spraint occurs. It’s also along trails, some of which are quite steep, and any spraint is likely to roll or blow away pretty quickly. Otters may be marking trails with urine which could have a similar effect. Scurvygrass is also found right at the point at which otters emerge from the sea. I can’t help thinking that there must be a dispersal element to the story as well. This seems to be supported by another sighting of scurvygrass associated with birds:

There’s only one thing for it — we need to do some experiments! This may not be the most important project on my list but it makes for an enjoyable distraction.

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