NATURE Ingenious secrets of a machair ecosystem

Tell-tale snail shells


Marine life
John Paul Tiernan

Around a small blue rock which jutted out of the sand and buffeted from the beach by a cottage-sized sand dune, a collection of broken snail shells lay damaged and useless like cars piled irregularly on a scrap heap. The protagonists in this intriguing scene were terrestrial (land) snails, but the background to everything found there was essentially marine in origin.
Firstly, the sand dunes. It’s less science, and more mechanics to explain their existence. Sand-dune formation needs strong and consistent winds from a variety of angles to keep sand supplied and sculpted. As the sand accumulates at the unstable front end of the system, marram grass helps stabilise the dune. In those areas exposed to the strongest winds, the system moves far inland, blown flat to create a stable ‘machair’, or in our language ‘duach’, ecosystem.
These systems are very species-diverse and grazing, which for centuries they have been used for, maintains this diversity. Co Mayo, and in particular the Mullet peninsula, has some of the finest examples of machair ecosystems in Ireland. The east of the country in comparison, where the wind blows away from shore most of the time, offers smaller, narrower sand dune systems.
Unsurprisingly, the machair ecosystems are host to lots of bird species. When waders such as the ringed plover, dunlin, and oystercatcher are not at the waterline feeding, they nest here. Choughs and crows can also be found in the dune system, and in winter-time, snow buntings.
So what of the scrap heap of snails? The sand that makes up the dune is composed of, among other things, the remains of calcifying marine organisms (shellfish).
This can make the sand dune ecosystem high in calcium and thus home to quite a lot of snails who readily recycle this calcium for their own shell-building purposes.
A bird not usually associated with the shore, the common song thrush, knows there are plenty of snails to be found here and employs a similarly astute method of extracting the fleshy snail from its shell. Holding the ‘lip’ of the shell in its beak, it returns time and time again to a chosen rock known as the ‘anvil’ to break open the shell through repeatedly smashing it against the rock.
The thrush’s housekeeping is not as good, and over time the shells pile up around the ‘anvil’ to leave an impressive, if a little untidy, trace of ingenuity – to match a most elaborate and equally impressive coastal ecosystem.

John Paul Tiernan, Louisburgh, runs www.irishmarinelife.com, a website dedicated to the creation of knowledge of our marine ecosystems. He is currently studying for an MSc in Marine Science.