How giant earthworms changed the landscape of Rum Island

<span rang=The summit of Hallival is 723m above sea level. Kevin Richard Butt, CC BY-ND” src=”–/YXBwaWQ9aGlnaGxhbmRlcjt3PTk2MDtoPTcyMA–/″ data- src=”–/YXBwaWQ9aGlnaGxhbmRlcjt3PTk2MDtoPTcyMA–/″/>
The summit of Hallival is 723m above sea level. Kevin Richard ButtCC BY-ND

The Isle of Rum, part of Scotland’s Inner Hebrides, is known as a field laboratory for scientific research, renowned for its thriving and complex study population of around 900 red deer. But the earthworms on Rum are just as impressive. These invertebrates act as “ecosystem engineers”, actively shaping the landscape, often after humans have left their mark on this remote island.

My investigations over 30 years have revealed how humans have affected the current fragmented and uneven distribution, diversity and abundance of earthworms on this national nature reserve.

While taking my geography students on field trips to Rum in the mid-1990s, I realized that there was scope to research earthworm ecology. One of my PhD students was studying soil development here and she quickly pointed out to me the differences in the number of earthworms found under different species of trees planted in the late 1950s. There were more worms living under birch and oak trees than under pine trees or on unplanted heath. This discovery spurred me into action.

Rum’s human history goes back 9,000 years. Early humans came here to collect bloodstone, a flint-like mineral used to make arrowheads and other hunting or cutting tools. Early humans deforested the island and the wet climate (with more than 2m of rain per year) resulted in leaching of soil nutrients. The resulting poor quality acidic soil supported heath plants and low numbers of three earthworm species.

If nothing else happened to the soils of Rum, this would be a very exciting place to do earthworm research.

But later human inhabitants improved the soil enough to make a living as tenant farmers at several places around the coast. They used kelp seaweed to benefit the cultivated land and to enrich the soil quality. Then, around 200 years ago, these industrious people were removed from their settlements on Rum (and much of Scotland) in the “Freith Highlands”.

At sites on Rum such as Harris, Dibidil and Kilmore, the landscape is still marked by distinct ridges and hollows known as “lichens”. These show where the land was painstakingly dug by hand to grow potatoes and other crops. The pits allowed drainage and the crops were grown on raised ridges. Two hundred years since the last cultivation, these soils are even more fertile than the surrounding areas, and continue to support more earthworms.

At Papadil, another abandoned settlement, rarely visited these days, brown forest soil developed beneath stands of trees planted a century ago. Inside these trees, my colleagues and I found large earthworm holes about 1cm in diameter. On an island with no badgers and no moles, a good supply of leaf litter for food and little human disturbance, we found the biggest in the UK. Lumbricus terrestris never reported in the wild.

man's hand open with large earthworm on palm, green grass in background

man’s hand open with large earthworm on palm, green grass in background

At over 13g, about three times the normal weight for this species, these earthworms could be up to ten years old. This was truly an exciting discovery. We put the worms back into the soil – hopefully they have multiplied.

The rich owners of Rum treated this island as a shooting and fishing estate for over a century and kept most people away from the so-called “Forbidden Island” from the late 19th century to the early 20th century .

When textile tycoon George Bullough built Kenloch castle in 1897, his wife, Lady Monica wanted to grow roses in the garden. To facilitate this and generally improve the landscape, Bullough imported 250,000 tonnes of good quality Ayrshire soil to spread around their new homes. They lived in this castle for six weeks every year, but this human opulence greatly changed the underground ecosystem.

rugged Scottish landscape, large building on left with green treesrugged Scottish landscape, large building on left with green trees

rugged Scottish landscape, large building on left with green trees

There were earthworms in the imported soil and this invertebrate community flourished around the castle in Kinloch. Now, 12 species of earthworms – ones that prefer neutral pH soils – are present in high abundance (200 worms per square meter). Colleagues and I sampled at 50m intervals from here (at sea level) up to the summit of a steep rocky peak called Hallival. Our research has shown that the richness and abundance of this earthworm species comes suddenly at the wall around the estate – the border of the imported soil.

Natural soil modeling

In addition to human impacts, natural processes can affect soil properties. On the slopes of the Rum peaks, many patches of bright green vegetation can be found among the rocks at an altitude of 500-800m. The so-called “freshwater springs” are the fruit of Manx pine nests.

a ridged green grass hillside with gray rocks and a clear sky in the backgrounda ridged green grass hillside with gray rocks and a clear sky in the background

green grassy ridge hillside with gray rocks and clear sky in the background

A pair of these black and white seabirds visit the hillside to pick a single cherry each year, before beginning their long migration to South America. The faeces of the mature birds fertilize the freshwater meadows from above before they fly out for small fish such as herring and salt to feed their chicks.

The chickens in the underground burrow also produce more nutrient-rich faeces from the digested fish, so the soil enrichment is a marine source. This supports the growth of grass and more earthworms – the same three species found on the moor, but in much greater numbers.

On a low moor, deer are kept out by hedgerows planted in the 1950s and 1960s, just after Rum became a national nature reserve. Now, these sheltered trees provide roosts for songbirds, and the soil below is abundant with earthworms as the tree’s leaf litter adds nutrients to the soil. These plots inspired a small-scale reforestation project that could change the landscape of this island, its soils and its multitude of earthworms.

Rum has yielded significant results of earthworms, often associated with human activities or dynamic natural processes. As earthworms engineer this ecosystem and add naturally derived nutrients, soils change. Long-term monitoring of Rum could help us better understand landscape transformations and soil health, here and elsewhere.

This article from The Conversation is republished under a Creative Commons license. Read the original article.

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Kevin Richard Butt does not work for any company or organization that would benefit from this article, does not consult with, shares in a company or organization that would benefit from this article, and has not disclosed any relevant affiliations beyond his academic appointment.

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