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<li>This account presents information on all aspects of the biology of <em>Coincya monensis</em> (L.) Greuter & Burdet that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the <em>Biological Flora of the British Isles</em>: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history and conservation.</li>
<li><em>Coincya monensis</em> is represented in the British flora by two subspecies; ssp. <em>monensis</em> is a diploid, endemic taxon largely confined to open, coastal, sand dune habitats along the west coast of Britain; ssp. <em>cheiranthos</em> is a tetraploid neophyte (alien) of ruderal and coastal habitats.</li>
<li>Both subspecies occur as short-lived perennials in outbreeding, insect-pollinated populations in open, early successional communities. Individuals may perennate as hemicryptophytes with buds in basal rosettes or as chamaephytes with buds in aerial shoot rosettes, but adequate seed production is essential for the long-term maintenance of populations. Seed production by the much larger and more floriferous tetraploid alien populations is much greater than that of the native endemic populations.</li>
<li>Neither subspecies is able to compete effectively in late successional, grassland communities. <em>Coincya monensis</em> ssp. <em>monensis</em> benefits from moderate disturbance by trampling, which helps to maintain open, sandy, semi-fixed dune communities, such as those dominated by Marram Grass (<em>Ammophila arenaria</em>).</li>
<li><em>Coincya monensis</em> ssp. <em>monensis</em> has been designated a scarce plant in the British flora and is a priority species in the UK Biodiversity Action Plan. The main threats to its conservation are loss of habitat through natural successional processes and urban development. <em>Coincya monensis</em> ssp. <em>cheiranthos</em> is well-established in South Wales where it appears to be undergoing a range expansion. At present there are no known mixed populations of the two subspecies but this remains a possibility if the alien subspecies continues to expand its range. Competition and hybridisation between subspecies in mixed populations will present additional problems in the conservation of endemic biodiversity in the British Flora.</li>
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