This dataset are records from comparison surveys of invertebrate species from Newborough Warren where sand dune restoration occurred, and Morfa Dyffryn and Morfa Harlech where natural sand dune habitat occurs. Following management works on Newborough Warren National Nature Reserve in 2013 to create pioneer dune slacks and other early-successional habitats, invertebrate surveys had demonstrated a rapid dispersal/colonisation by seven of sixteen beetle species which Natural Resources Wales regards as requiring these conditions. Additional excavations in March 2015 provided an opportunity to find how quickly this colonisation occurred. Within two months, three pioneer dune slack beetles - Bembidion pallidipenne, Bledius subniger and Dyschirius politus - were recorded in pitfall traps set in this area, all of which had previously been difficult to locate on the dune system. As it was suspected that the colonising fauna of the excavated slacks had its origin in the estuaries to the north and south of Newborough Warren, surveys for Bledius beetles were undertaken in the Cefni and Braint estuaries by mapping the casts thrown up by their activities. Similar surveys were undertaken along the Afon Artro at Morfa Dyffryn and the Glaslyn and Dwyryd estuaries north of Morfa Harlech. It revealed that the Bledius colonies occurred in limited areas and seemed to be associated with sand newly deposited by wind or water and of a low organic content. Diatoms were found abundantly in the intestines of the Bledius beetles. Pioneer slack habitat is at a premium on all Welsh dune systems other than Morfa Dyffryn where mobile dunes with drifting sand help to create pioneer slack as waves of sand move in front of the prevailing wind. This 2015 survey compared the invertebrate faunas of these naturally-occurring slacks with the artificially-engineered slacks on Newborough Warren. The opportunity was also taken to sample two slacks on Morfa Harlech, a well-vegetated slack on Morfa Dyffryn and two unexcavated slacks on Newborough Warren. The survey was made at 20 sites with five pitfall traps at each and the total data set consisted of 100,205 individual invertebrates spread across 505 species. The taxa used in this survey consisted of beetles, spiders, harvesters, millipedes, centipedes, woodlice and a few orthopteroid insects. This study has divided a previous definition of pioneer slack on Morfa Dyffryn into pioneer slack (damp, almost bare, yellow sand at the foot of an eroding dune) and post-pioneer slack (upwind of pioneer slacks and more vegetated, with bare sand having a black crust). Whilst there are biases with pitfall trapping and some limitations on drawing conclusions from analyses, pitfall data was examined by Principal Components Analysis (PCA) after splitting the data into 13 sub-sets based on taxonomic groupings. The ground cover of the vegetation was assessed in the three slacks sampled on Morfa Dyffryn to determine the differences between pioneer and post-pioneer slacks, with PCA separating them into two distinct clusters. The analysis also groups the excavated slacks on Newborough Warren close to the Morfa Dyffryn pioneer slack. This is particularly so where the data set is presented in the form of histograms of the numbers of selected species across the 20 sites. An analysis of the sixteen beetle species regarded by NRW as being associated with pioneer slacks, using information from this study and other sources (published literature, species records in the NBN Atlas) suggests that several are often if not exclusively associated with the post-pioneer slack habitat. The pioneer slack beetle fauna is more properly restricted to four species - Bembidion pallidipenne, Bledius fergussoni, Bledius subniger and Dyschirius politus - which are also found in the estuarine habitat at the edge of the salt marsh. A small survey of the tiger beetle Cicindela maritima found it breeding in numbers in the damp sand of the pioneer slack ...