Ronay Island: A First Complete Picture of a Remote Island’s Biodiversity

eDNA gave Ronay Island its first complete biodiversity picture in a single sampling trip.

The Client

Agri Natural Capital works with landowners to develop practical strategies for nature restoration, helping translate conservation ambition into measurable, evidence-based action. Ronay Island in the Outer Hebrides is one such project; a community-based regeneration initiative aimed at restoring biodiversity across this remote and ecologically significant island. To guide conservation strategy and track the impact of restoration over time, the team first needed to understand what was already there.

The Problem

Existing surveys on the island had focused almost entirely on terrestrial habitats, leaving the island’s lochs largely uncharacterised despite their importance to local biodiversity. Much of what was known about the aquatic environment came from anecdote; stories from local residents of brown trout caught in the waterways and otters moving in from the coast. Valuable as these accounts were, they couldn’t provide the standardised, repeatable data needed to engage statutory and conservation bodies, or to measure change over time.

Our Solution

A full aquatic biodiversity survey of 12 separate lochs using traditional methods would have meant multiple visits, targeting different species groups one at a time (slow, expensive, and disruptive to a sensitive remote environment). eDNA made it possible to do all of that in a single trip.

Working with Agri Natural Capital and the Ronay Island team, we collected eDNA water samples across all 12 lochs. Back in our laboratories, we applied 13 genetic markers across 32 taxonomic groups, producing a comprehensive biodiversity profile of the island’s freshwater systems from one sampling visit.

“The collection was remarkably simple,” the field team noted, “if not wet at times!”

What We Found

The survey revealed 1,316 unique taxa across the lochs sampled, including 185 plant species, 121 insects, 27 fish, 14 mammals, and 9 birds.

The results confirmed what local residents had long reported: brown trout and Atlantic salmon were present in the waterways, and otters were detected in 4 of the 12 lochs, a finding that underlines the ecological connections between the island’s terrestrial and aquatic systems. The strong detection of European eel (Anguilla anguilla), one of the UK’s most threatened species, provided an immediate and concrete basis for engagement with statutory conservation bodies.

Connectivity shaped the pattern of diversity clearly: the five lochs connected to the sea supported the greatest biodiversity, while more isolated lochs were less diverse. One finding stood out as particularly striking. A loch that had been cut off from the sea by a dam was found to contain both freshwater and marine species simultaneously, including thicklip mullet, conger eel, and poor cod, suggesting the barrier had not fully severed the ecological connection, or that these species had established themselves before isolation.

Significant from a conservation management perspective was the detection of black rat (Rattus rattus). On an island actively undergoing biodiversity restoration, the presence of this potentially invasive predator is an important finding, and one that would likely have gone undetected without eDNA, and that now demands attention in the island’s management planning.

“Everyone involved within the project was blown away by the detail in the results,” the Ronay team commented.

The Impact

The survey has established a robust ecological baseline for Ronay Island, against which all future monitoring can be measured. That baseline is already shaping conservation priorities: the European eel data has prompted active engagement with statutory and conservation bodies to explore protective measures for this important species.

More broadly, the project demonstrated what eDNA makes possible in remote and ecologically sensitive settings; comprehensive, minimally disruptive, and genuinely revelatory biodiversity assessment in a single visit. The Ronay Island team can now make evidence-based decisions about habitat protection and land management, with the data to back them up.