What the fate of the scoter duck tells us about our changing environment

We have never had a better sense of the timescales of Earth, of the ebb and flow of life across the planet, the rise and fall of species (not least, in eventual prospect, our own). Measured against the long millennia of change, worry about immediate threats to this or that habitat, this or that creature, can sometimes seem a little academic, especially where risk is marginal to its total presence in the world.

Take a little black duck called Melanitta nigra , the black scoter or common scoter. The drake is the only duck with plumage so black as to seem shoe-polished, with a smidgen of orange on his bill; the female is –modest dark brown. It is common, indeed, at this moment in the Baltic Sea, where up to three-quarters of the world's population – about 1.6 million – are bobbing about in rafts and diving after mussels and cockles in the shallows. (They swallow them whole and grind the shells up in their gizzards.)

David Cabot has described watching upwards of 6,500 scoters in large rafts in Dingle Bay, in Co Kerry, where beds of blue mussels draw some of our largest winter congregations. They moved around, he wrote (in his New Naturalist book, Wildfowl , from 2009), "like large flocks of starlings, with groups of birds constantly lifting up off the water to join other flocks, leapfrogging other flocks in the process".

Haunt the bays
These winter birds, like those that haunt the bays of the Irish Sea, sometimes in huge numbers, are nearly all migrants from the Baltic and points farther east. Their movements are still deeply mysterious, but they are triggered by the need to moult safely on water after breeding ashore on lake islands.

The scoter breeding range is the “low Arctic” belt, stretching round the world at the level of Arctic Russia. Well south of its lowest latitude are the lakes of Connacht, where the nesting of common scoter is not common at all. In fact, after sharp decline, Ireland’s breeding population in 2012 was down to about 39 pairs – 28 at Lough Corrib, five at Lough Ree, five at Lough Arrow and one at Lough Conn and Lough Cullin. That is half the figure in 1999, when Lough Ree alone had 32 pairs. Like similarly dwindling numbers in Scotland, concludes Cabot, the Irish scoter is “clinging on by its proverbial eyelashes at its southernmost European breeding point, way outside its main nesting range”.

As climate change is moving so many species northwards (some trees, for example, and ocean plankton and fish), shouldn't we just shrug apologetically and move on? If changing climate was, indeed, the only and provable cause of the scoter's fortunes, perhaps we could. But there are other possibilities, and all of them a threat to many more species than Melanitta nigra .

Dramatic extinction
They are listed in a new report on the scoter's breeding status, surveyed last year for the National Parks and Wildlife Service. Perhaps the most urgent of them had already emerged from the dramatic extinction of the ducks at Lough Erne, in Co Fermanagh. Here, the scoters breeding on the islands, beginning in 1905, had increased steadily to 152 pairs in the 1960s. Then came the feral American mink, escaping Ulster's fur farms and slaughtering female scoters as they sat on their eggs.

By the 1990s, all the birds had gone. Breeding moved to other lakes, farther south in Connacht, where the feral mink were just arriving. They have savaged so many native waterbirds – moorhens, coots, grebes, the summer lake colonies of black-headed gulls – that they must be prime suspect in the scoters’ decline.

Pending an absolute link, the NPWS is already trapping mink at scoter lakes, and the report suggests mounting cameras at the ducks' nests "to clarify this issue further".

Predators
What else, then? Water pollution, for one thing: scoter decline at Loughs Erne, Conn and Cullin all coincided with peak eutrophication.

Invasion by alien roach has brought fishy competition for the ducks’ summer food, such as insect larvae and fish eggs. Scoters prefer to nest at the fringe of lake islands where grazing has thinned out the vegetation, but grazing on those in Lough Ree has declined.

And mink are not the only predators: hooded crows and large gulls take eggs and ducklings. Pike eat ducklings, too, and big pike are increasing as their numbers are boosted to attract angling tourism. The floods of drenching summers rise to drown the scoters’ nests, with little chance of them starting to breed again.

Such a menu of menacing questions (it even adds alien zebra mussels and curly pondweed, and too much disturbance from people and boats) is highly instructive, not least of what ecology is about.

Melanitta nigra may well follow the Arctic's little red-necked phalarope in final desertion from Connacht, but we'll have learned a lot more about the ways in which we're driving them out.