An insect’s take on coastal squeeze

There’s no denying it: our coastlines are changing more rapidly than ever. But crumbling cliffs and receding or stabilising dunes also have a profound effect on coastal wildlife. Matthew Oates, one of the National Trust’s nature experts and a keen butterfly enthusiast, tells us what coastal squeeze means for the Glanville Fritillary butterfly on the Isle of Wight, and why learning about insects is key to their conservation.

Many of our rarest terrestrial plants and animals are restricted to the coastline. A number occur there because they are genuine coastal species – they’re salt-loving plants or their associated invertebrates, or they’re strongly dependent on the sea’s influence in other ways. Others are species which have effectively been evicted from inland Britain, and are staging last ditch stands along our coastline, out of the way of modern agriculture and urban development.  There are also quite a few for whom our southern coasts form the northern limit of their European range – they’ve got a toehold here and could spread if our climate becomes warmer. 

A number of these are highly specialised. There are specialists of brackish mud, the diverse types of salt marsh, acidic or calcareous sand dunes, ‘hard’ cliffs on diverse firm geologies, and ‘soft rock’ cliff specialists – plus their parasites and predators.  Then there are of course various birds, including summer and winter migrants, which move in to feed on coastal vegetation, like geese, or which probe for invertebrates, like waders.  Then there are the true marine species, animal and plant, and those associated with the inter-tidal reaches.

Compton Bay, Isle of Wight: Paradise for soft rock cliff invertebrates. ©Matthew Oates.

Compton Bay, Isle of Wight: Paradise for soft rock cliff invertebrates.
©Matthew Oates.

The soft rock cliff invertebrates are an interesting group. They consist mainly of mining bees and digger wasps, ground beetles, assorted flies, and some Lepidoptera (butterflies and moths). For many of these, our soft rock coasts form the northern limit of their European ranges, especially the coastline from Dorset to the Isle of Wight and around East Anglia. A specialist report on this fauna by the invertebrate charity Buglife identifies 29 invertebrates which are wholly restricted to these habitats, like the rare Cliff Tiger Beetle, plus at least another 75 with a strong affinity. These beasties include creatures of cliff pools, seepages and runnels. There’s even a daddy-long-legs that hangs out under cliff-side waterfalls. 

Looking at rockpool creatures at South Milton Sands, Devon. ©National Trust Images/Ben Selway.

Looking at rockpool creatures at South Milton Sands, Devon.
©National Trust Images/Ben Selway.

The threats to them are coastal protection works, sea level rise, adverse climate change, ignorance and even conservation disinterest. Sadly, we do not know as much about many of these species as is necessary for their effective conservation. 

But perhaps the best known of these coastal invertebrates is the Glanville Fritillary, named after Eleanor Glanville. She was the first lady of entomology, and her will was overturned on the grounds of insanity due to her interest in bugs. She discovered the butterfly in Lincolnshire a little before 1703, whilst hunting an errant son. Since then, her fritillary has become all but restricted to the Isle of Wight, where its headquarters are the crumbling cliffs of the island’s south-west coast. The butterfly periodically breaks out from there, to breed on the south-facing slopes of the island’s whaleback downs and even forming short-lived colonies on the Hampshire coast. It’s a boom or bust species, expanding wondrously in good summers but contracting to its most favoured cliffs during wet summers (so, it’s bust at present). In other words, it’s hugely affected by weather.  

Glanville Fritillary, male. ©Matthew Oates.

Glanville Fritillary, male. ©Matthew Oates.

Any significant change in climate will alter the status and distribution of the Glanville Fritillary radically. Increased storminess could make the cliffs too unstable for the simple vegetation it depends on, and poorer summers could knock it out altogether as it occurs here towards the northern limit of its European range. But if our summers actually become drier (whatever happened to ‘global warming’ in the UK?) it could quite quickly recolonise the mainland. After all, it’s an insect, which means it seeks nothing less than world domination.

Are you a fan of the Glanville Fritillary? Or just want to share your insect stories with us? Feel free to comment at the end of this post.

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