June 9, 2024
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by Kevin Richard Butt, The Conversation
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. But the earthworms on Rum are equally remarkable. 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 uncovered how people have influenced 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 there was scope for research on earthworm ecology. One of my Ph.D. students was studying soil development here and she quickly alerted me to differences in earthworm numbers found below different species of trees planted in the late 1950s. More worms lived below birch and oak trees than beneath pine trees or on unplanted moorland. 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. The island was deforested by early humans and the wet climate (with more than 2m of rain per year) led to the leaching of soil nutrients. The resulting poor-quality acidic soil supported moorland plants and low numbers of just three earthworm species.
If nothing else had happened to Rum soils, then this would be a very unexciting place to undertake research on earthworms.
Later settlers enhanced the soil on the island to sustain themselves as tenant farmers along certain coastal regions. They utilized kelp seaweed to improve the soil fertility. Around 200 years ago, these industrious individuals were evicted from their homes on Rum (and extensively across Scotland) during the events known as the “Highland clearances”.
In areas of Rum such as Harris, Dibidil, and Kilmory, one can still see the legacy of their agricultural efforts. Known as “lazybeds,” these patterns of ridges and furrows mark where the land was manually tilled to cultivate potatoes and other crops, with the furrows enabling drainage. Even two centuries after the abandonment, these soils remain more fertile than the nearby lands, supporting a higher number of earthworms.
In a deserted area known as Papadil, a brown forest soil has developed beneath forests planted a hundred years ago. Within these woods, my colleagues and I discovered sizable earthworm burrows measuring about 1cm in diameter. On this island, devoid of badgers and moles and with an ample supply of leaf litter as food and minimal human interference, we encountered the largest Lumbricus terrestris ever reported in the UK. Weighing over 13g, roughly three times the typical weight for this species, these earthworms could be up to ten years old. Discovering these was truly thrilling, and we made sure to return them to their soil, where they hopefully continue to thrive.
For over a hundred years, the elite owners utilized Rum as an exclusive hunting and angling preserve, effectively rendering it the “Forbidden Isle” during the late 19th to early 20th centuries.
The construction of Kinloch Castle in 1897 by the textile magnate George Bullough, at the behest of his wife, Lady Monica, who desired to cultivate roses, marked a significant alteration to the landscape. To support the garden and enhance the surroundings, they brought in 250,000 tonnes of high-quality soil from Ayrshire. Though they resided in the castle merely for six weeks annually, their actions profoundly impacted the local underground ecosystem.
This newly introduced soil brought earthworms to the area surrounding Kinloch Castle. Presently, the region supports 12 species of earthworms that prefer soils with neutral pH, flourishing at significant levels (200 worms per square meter). Our studies conducted by sampling at 50-meter elevation intervals from the sea level up to Hallival’s peak illustrate that the richness and abundance of these worms conclude sharply at the estate’s boundary where the added soil ends.
Aside from human intervention, natural activities also influence soil characteristics. On the higher slopes of Rum, between 500-800 meters, patches of vibrant green vegetation are visible amidst the rocks. These areas, known as “shearwater greens,” emerge due to the nesting activities of 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 fertilized from above by the feces 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 feces 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 yielded some remarkable earthworm findings, often linked to human activities or dynamic natural processes. As earthworms engineer this ecosystem and naturally derived nutrients get added, soils change. Longterm monitoring on Rum could help us better understand landscape transformations and soil health, here and elsewhere.
Provided by The Conversation
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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