An oasis of fertility on a barren island: earthworms at Papadil, Isle of Rum
The Isle of Rum, Inner Hebrides, has an impoverished earthworm fauna as the soils are generally acidic and nutrient-poor. Species associated with human habitation are found around deserted crofting settlements subjected to “clearances” in the mid-19th century and at Kinloch, where a large volume of fertile soil was imported from the mainland around 1900. Earthworms, and the dew worm Lumbricus terrestris L. in particular, were investigated at Papadil, an abandoned settlement and one of the few locations on Rum where a naturally developed brown earth soil is present. The small (1.5 ha), fertile location is isolated, so was also suitable for field experimentation. Visits over six years allowed dew worm distribution to be assessed within low lying grassland and woodland and also within an adjacent sloping broadleaved woodland. The factors limiting dew worm distribution at the site were investigated with associated translocation to adjacent uninhabited areas. Small scale spatial dynamics were studied with density manipulation and containment experiments where Visual Implant Elastomer marking of individuals was utilised. Translocations from streamside woodland to adjacent grassland was successful over a short period (5 months), but the colonies did not persist over a longer term (5-6 years). Field trials with earthworm tagging were successful, but highest tag recovery rate was 25%. Where adults/sub-adults were removed, recruitment of juveniles was notable. Exceptionally large (>12 g live mass) individuals were found in soils of terraces on wooded slopes, suggesting that dew worms may be long lived at this location, where food is abundant and relatively few terrestrial predators are present.