There will be a very large butterfly population at Rhos Llawrcwrt which will be viable in the long term. Because the marsh fritillary is parasitised by a wasp, the number of butterflies in the population will vary over a cycle of several years, but, during the peak years, a visitor taking a walk through the site on a sunny day in June will see several hundred adult butterflies. In these years, the caterpillars, feeding communally in silken webs on their food-plant, devil's bit scabious, will be found in their thousands throughout large areas of Llawrcwrt and Cors y Clettwr.

Rosettes of the food-plant will be both very numerous and widespread throughout the cattle-grazed rhos pasture, growing amongst a short turf of grasses, sedges and flowering herbs, with scattered tussocks of purple moor grass and rushes providing shelter for the caterpillars in wet weather. This colourful wet grassland mosaic will extend throughout Llawrcwrt, Cors y Clettwr and the fields which were drained and reseeded for agriculture in the 1980s but have reverted back to rhos. Dense mixed hedges of hawthorn, hazel, mountain ash and other locally native species grow around the boundaries and between fields and offer vital shelter to the breeding adult butterflies during poor weather in what is otherwise a very exposed landscape with little shelter.

There are a number of smaller breeding populations of marsh fritillary on rhos pasture sites within 5 km of the National Nature Reserve. Butterflies from Rhos Llawrcwrt will occasionally visit and breed on these sites, and butterflies from the smaller populations will visit Rhos Llawrcwrt. This exchange of butterflies will help to keep all populations in a healthy condition.