Mike Dilger’s wildlife watching
Life among the sand dunes
Dunes are a world of constant change and movement, and must surely be the most mobile of all the UK’s land habitats. Peppered along stretches of our coastline, with a distinct westerly and northerly bias, they must also represent one of our least modified and therefore most ‘natural’ vegetation types.
Typically, dunes are characterised by ridges or hillocks of sand beyond the reach of the highest tides, and usually run in bands parallel to the sea. Among them are low-lying depressions, called slacks. Dunes also tend to become progressively taller and more vegetated the further from the sea you go.
Britain’s oldest dune systems date back 9,000 years, but all coastal dunes form the same way. Sand is blown onto the beach before becoming trapped by either debris or plant material just above the high-water mark. The steadily accumulating sand then begins providing habitat for a limited cast of tough, trailblazing plants, such as sea saltwort, sea sandwort and sand couch. As these hardy, specialised pioneers become ever more engulfed by sand, they in turn give way to classic dune specialists such as marram grass, which grows quickly to ensure it doesn’t become swamped, while also stabilising the dunes with its strong, stable roots.
As we move inland, further along the ‘dune construction conveyor belt’, we start to see older, more stable dunes, known as fixed dunes. Here, the slow build-up of organic matter from dead plant material, and a corresponding drop in salinity, results in finer grasses, herbs and even bushes.
To further complicate things, ‘blow-outs’ sporadically occur. A blow-out forms when the wind is able to exploit weak points in the dune system, scouring out sometimes quite large areas, often right down to the water table. Ultimately, this can result in the formation of lowlying slacks with a diverse
wetland vegetation. Ponds in these slacks are the main spawning habitat for one of Britain’s rarest native amphibians – the natterjack toad.
Peaks and troughs
Compared to the flatter, wetter, greener slacks, the surrounding dunes are more undulating, which leads to a far greater range of aspects. The subtly different microclimates warm up at different rates, and this creates innumerable nooks and crannies that can provide respite from the near relentless wind. These coastal conditions frequently suit many plants that tend to be poor competitors elsewhere, as well as an array of bumblebees, digger bees, burrowing wasps and sand lizards.
Due to the ‘get up and go’ nature of all sand dune systems, they are challenging to conserve in the UK’s crowded landscape, where there’s little free space for nature reserves on the move. Many of the best sites have become hemmed in by golf courses, roads or coastal conurbations. The ecological value of dune systems has also been chronically underestimated in the past, with sites being covered by conifer plantations.
Sand dunes act as important natural barriers that keep the power of the waves at bay. Yet the erection of hard sea defences and tidal barrage systems poses a considerable threat, by cutting off that all-important supply of sand. It’s also worth bearing in mind that despite many sand dunes appearing in rude health, they can be surprisingly vulnerable to high visitor numbers, with the dune-stabilising marram grass particularly sensitive to trampling.
So do your bit – when coronavirus is behind us and we can once again enjoy visiting beaches across the country – make sure you stick to the clearly marked boardwalks or established footpaths, to help preserve precious sand dune habitat.
Sand dunes can be surprisingly vulnerable to high visitor numbers, with marram grass sensitive to trampling.