In the British Isles, the marsh fritillary is usually found in damp, heathy grasslands that are called "rhos pastures," from the Welsh word "" meaning heath. Small populations can be an important element of the ecology because they produce many mobile individuals which can then found other populations. The marsh fritillary is protected under UK law, listed under Schedule 5 of the Wildlife and Countryside Act, and the EU Habitats and Species Directive (Annex II). The Devon Wildlife Trust owns a number of sites at which it monitors this species. Examples include Stowford Moor (near Holsworthy, Devon), Dunsdon nature reserve (near Bude), Mambury Moor (near Great Torrington), Vealand Farm nature reserve (near Holsworthy), and Volehouse Nature Reserve (near Holsworthy). In 2009, population counts had significantly increased from years 2007 and 2008. In 2018, a breeding population was found in Carmarthenshire, Wales, after an absence of 50 years.