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Nature Notes - October 2016

Nature Notes

I’m back on the Island of Great Britain now.  Incidentally, I discovered recently that it was the Ptolemy (AD 100 – 170), the Greco-Egyptian writer that first referred to Great Britain as the larger of the two main islands with Little Britain (Ireland) of the British Isles referred to by the Celts and Romans as the islands off north-west Europe.

The natural history of these islands is extremely complex and stems back to their geology which itself is complex, and is a bizarre mixture of volcanic, sedimentary and metamorphic rock.  The latter term refers mainly to sedimentary rock that has been transformed by heat and pressure from one rock type, say chalk into another such as limestone or marble.  Here in Sussex all our substrates are either sedimentary such as clay, chalk or sandstone with a little metamorphic bits like Sussex Marble.  Horsham stone is another very hard sand-rock that has been transformed from sediments into a hard rock commonly used for roofing, including the roof of Framfield Church.

The wide variety of geological substrates in Sussex has resulted in a huge biodiversity, because chalk tends to be alkaline and sand tends to be acidic with the clay and other rocks somewhere in between.  Many plants are either calcicoles (calcium loving) or calcifuges (refugees from calcium) and we have both in Sussex leading to a very large flora.  Consequently, the small animals that feed specifically on particular plants are also present in high variety.  For example, snails have to make shells for which they use calcium from which chalk and limestone are made, so there are billions of snails of a wide variety of species on the chalk downs, feeding on the wide variety of plants rich in calcium.  However, there are small animals that are only found on acidic heathland such as several species of moth and butterfly which become common on heathy places such as Ashdown Forest.  However, there are not many snails to be found on heathland due to the lack of calcium.

Finally, there are large animals that eat the small ones and search them out in these habitats.  It is not well known, for example, that the sheep of the South Downs love to eat snails while they are grazing on the multitude of herbs that grow there.  For this reason, the meat of South Downs sheep is supposed to be particularly flavoursome.

Dr.Martyn Stenning