Tag Archives: natural history

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.

That’s not a jungle

Last night I watched the episode of the BBC’s Planet Earth II on jungles, narrated by Sir David Attenborough, and it’s provoked me into a rant. Now I’m well aware that any criticism, even indirect, of Sir David is likely to stir a backlash, so I’ll get the disclaimers in early. TV nature documentaries serve a number of functions, of which the most important is to entertain. In this regard the series is an undoubted success. The spectacular footage of the natural world is dazzling, and will inspire a new generation of naturalists, ecologists and taxonomists.

Nevertheless, there is another function, which is to inform and educate. The balance between the two is difficult to strike; the dry tones of an academic lecture would hardly boost viewing figures, and this is no place to be showing data. It is still important, however, to convey the correct impression, and in this the choice of terminology and manner of presentation are crucial. Hence my great discomfort at the use of the word ‘jungle’. At the end I’ll explain why this matters to those of us who care about forests.

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The Jungle Book is another example of conflation of distinct biotas. Ostensibly set in India, but there is nowhere on earth (outside a zoo) where you will find this combination of species. Also, look how sparse the canopy cover is.

What is a jungle? There is no accepted vegetation type known as ‘jungle’, and you won’t find it used in the scientific literature. The whole episode of Planet Earth II was in some doubt about what the term ought to mean. Segments switched from tropical rain forests — and Sir David frequently talked as if this was the accepted definition of ‘jungle’ — to dry forests, igapo*, and subtropical forests. By the end it was clear that the producers felt the word ‘jungle’ to be defined in popular imagination as ‘place with lots of big trees’.

Perhaps that is what most people have in mind when you say ‘jungle’, and it’s consistent with the dictionary definition, although the word also applies to such disparate entities as a musical genre and the former refugee camp in Calais. The irony is that the original derivation was quite different. The Hindi word jangal could be applied to any uncultivated ground or wasteland, encompassing everything from forests to deserts. Going further back, the Sakskrit jangala refers to an arid area with sparse trees. Of course the meanings of words drift through time and with their transfer between cultures, but this only reinforces my point that the word jungle can mean many things to different people. This leads onto my second gripe.

There is no such thing as ‘the jungle’, in the same way as there is no single thing called ‘the’ tropical rain forest. Every tropical forest is as different from one another as they are from any temperate forest. This point is the main message of Corlett & Primack’s excellent and strongly-recommended book Tropical Rain Forests, which itself only reinforces the lessons of earlier books by the late Tim Whitmore and Peter Richards, and I could go back further. We’ve known this for centuries.

Now in fairness to Sir David, he does use the plural ‘jungles’, but many of the segments failed to even mention the locations where filming had taken place. This serves to obfuscate and trick the unwary viewer into believing that all these species can be found together in some common, unitary habitat. The three photos below come from forests in Africa, Australia and Malaysia. Though they are all recognisably forests (call them jungles if you like), the similarity is superficial, and there is unlikely to be any single species of plant or animal in common among them.

Why does this matter? Perhaps at this point you’re thinking that I’m an academic pedant, preciously guarding the intellectual high-ground against any incursions from enjoyable, popular culture. You’d be right. But there’s a serious motive behind my rant, which is that the conflation of so many habitats and biomes around the world diminishes the importance of their diversity, variety and local particularity. As part of the segment on indri, Sir David noted the rapid rate of deforestation in Madagascar. But one rain forest is not the same as another. The loss of a hectare of rain forest in the Philippines will lead to the loss of a completely different set of species than one in the Western Ghats or the Brazilian coastal forest. Each biome has its own distinct composition and threats. By blurring forests into a composite, we lose the appreciation of the value that any single one has in particular.

The audience of this series includes viewers in countries around the world. The real work of conservation takes place on the ground, in the places that host all this diversity. One of the challenges of environmental education and outreach is to get people to care about the diversity on their doorstep. By making forests more abstract, they become more distant and less relevant, even whilst appearing in your living room. It matters to say where particular species are found, because they can provoke interest and pride in the host nations whose citizens have the greatest power to ensure their ongoing survival. It’s not just any forest — it’s your forest.

What should we do? If you’ve been inspired by Planet Earth II — and I’m sure that many have — then take it as an entry to learning more about the enormous diversity within and between forests around the world, and what makes the forests in your own area so special. If you’re an educator, at whatever level, then use the brilliant BBC materials as a starting point. Then tell your students about how much more diverse, ingenious and spectacular nature is than even the most high-definition TV screen can ever convey, and to go out and see it for themselves.


* Thanks to fellow forest ecologist Sophie Fauset, who corrected my initial post, in which I’d called it varzea. Extra pedant points to her!