Reader in Ecology, University of Central Lancashire
Kevin Richard Butt does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
University of Central Lancashire provides funding as a member of The Conversation UK.
Renowned for a thriving and intricately studied population of around 900 red deer, the Isle of Rum, part of Scotland’s Inner Hebrides, is often considered an outdoor laboratory for scientific research. The earthworms on Rum, however, are equally remarkable. These creatures act as “ecosystem engineers,” actively molding the landscape, often in areas previously impacted by human activity on this secluded island.
Over the past 30 years, my research has revealed the impact of human activities on the current fragmented and uneven distribution, diversity, and abundance of earthworms at this national nature reserve.
During geography field trips with my students to Rum in the mid-1990s, the opportunity for research on earthworm ecology became apparent. A PhD student of mine, studying soil development on the island, was quick to notice variations in the number of earthworms beneath different tree species planted back in the late 1950s. Significantly more worms were found under birch and oak trees compared to pine trees or the unplanted moorland, prompting further investigation.
The history of human presence on Rum spans 9,000 years. Early inhabitants mined the island for bloodstone, a flint-like mineral used in fabricating arrowheads and other tools. Early deforestation and the island’s wet climate, with over 2 meters of rain annually, caused nutrient leaching from the soil. The resultant nutrient-poor acidic soil fostered moorland plants and supported only a few species of earthworms.
If nothing else had occurred, Rum would have been a dull site for researching earthworms.
However, past human interventions have significantly improved the soil, allowing for agricultural subsistence by tenant farmers in coastal areas. These farmers utilized kelp seaweeds to enhance the soil. Approximately 200 years ago, these settlers were expelled during the “Highland clearances”.
In places like Harris, Dibidil, and Kilmory on Rum, remnants of “lazybeds” can still be seen. These are distinctive ridges and furrows formed by manual labor for potato and crop cultivation, with furrows providing necessary drainage. Even two centuries after their last use, these soils are richer and support a larger population of earthworms compared to adjacent areas.
In the isolated former settlement of Papadil, under a canopy of century-old trees, a brown forest soil has emerged. Within these forests, my colleagues and I discovered substantial earthworm burrows approximately 1cm in diameter. Despite the absence of badgers and moles on the island, the abundant leaf litter and minimal human disruption have allowed the discovery of the UK’s largest Lumbricus terrestris ever reported in the wild.
At over 13g, some three times the normal weight for this species, these earthworms may have been up to ten years old. This was truly an intriguing discovery. After studying them, we returned the worms to the soil – hoping they continue to thrive.
The affluent owners of Rum regarded the island as a hunting and fishing retreat for over a century, effectively keeping visitors off what was dubbed the “Forbidden Isle” from the late 1800s to the early 1900s.
Constructed by George Bullough, a magnate in the textile industry, in 1897, Kinloch Castle was a testament to wealth. His wife, Lady Monica, wished to cultivate roses in the gardens. To support this endeavor and enhance the scenery overall, Bullough had 250,000 tonnes of high-grade Ayrshire soil transported to their estate. They resided in their castle for only six weeks per year, yet this act of human luxury markedly altered the subterranean life forms.
The introduced soil brought along earthworms, which prospered in the landscaped areas around the castle at Kinloch. Currently, there are 12 species of earthworms demonstrating a high population density (200 worms per square metre), which prefer soils with a neutral pH. Together with colleagues, I sampled at 50m intervals in elevation from the seashore up to the summit of the rugged peak known as Hallival. Our studies indicated that the rich diversity and numbers of earthworms sharply decline at the boundary of the area where the soil was imported, marked by the wall surrounding the estate.
As well as human influences, natural processes can affect soil properties. On the slopes of Rum’s peaks, many patches of bright s green vegetation can be found among the rocks at elevations from 500-800m. These so-called “shearwater greens” are the result of nesting Manx shearwaters.
Pairs of these black and white seabirds burrow into the hillside to raise one chick each year, before beginning their long-distance migration towards South America. The verdant shearwater greens are fertilised from above by the faeces of the adult birds before they fly off to forage for small fish such as herring and sprat to feed their chicks.
More nutrient-rich faeces from the digested fish are also produced by the chicks in the burrow below ground, so soil enrichment is from a marine source. This supports grass growth and more earthworms – the same three species found on the moorland, but in much greater numbers.
On low-lying moorland, fenced plots keep deer away from trees that were planted in the 1950s and 1960s, just after Rum became a national nature reserve. Now, these protected trees provide roosts for songbirds, and the soil beneath them is rich with earthworms as the tree leaf litter adds nutrients to the soil. These plots have triggered a small-scale reafforestation project which could change this island landscape, its soils and its many earthworms.
Rum has been the site of significant earthworm discoveries, frequently associated with human interventions or active natural events. These earthworms, as ecosystem architects, contribute to soil transformations as they incorporate organic materials. Continuous observation in Rum may provide deeper insights into the evolution of landscapes and the condition of soils both locally and beyond.
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