Biological and geological Site of Special Scientific Interest

A Site of Special Scientific Interest (SSSI) in Great Britain or an Area of Special Scientific Interest (ASSI) in the Isle of Man and Northern Ireland is a conservation designation denoting a protected area in the United Kingdom and Isle of Man. SSSI/ASSIs are the basic building block of site-based nature conservation legislation and most other legal nature/geological conservation designations in the United Kingdom are based upon them, including national nature reserves, Ramsar sites, Special Protection Areas, and Special Areas of Conservation. The acronym "SSSI" is often pronounced "triple-S I".

Sites notified for their biological interest are known as Biological SSSI/ASSIs, and those notified for geological or physiographic interest are Geological SSSI/ASSIs. SSSI/ASSIs may be divided into management units, with some areas including units that are noted for both biological and geological interest.

Biological SSSI/ASSIs may be selected for various reasons, which for Great Britain is governed by published SSSI Selection Guidelines.

Within each area, a representative series of the best examples of each significant natural habitat may be notified, and for rarer habitats all examples may be included.

Sites of particular significance for various taxonomic groups may be selected (for example birds, dragonflies, butterflies, reptiles, amphibians, etc.)—each of these groups has its own set of selection guidelines.

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