Data for: Trends in abundance and occupancy of the protected waterbeetle Graphoderus bilineatus in the Netherlands

The European Habitats Directive prescribes Member States to report on trends in abundance and distribution of protected species. One of these species is Graphoderus bilineatus (De Geer, 1774), a middle-sized predaceous aquatic beetle, listed in Annex II and IV of the Habitats Directive. In the Netherlands, a monitoring scheme for this species has been set up to assess the national trend as well as the trend in the national Natura2000 sites. In this scheme, a selection of 1 km2 squares is surveyed in cycles of six years using a standardized field method by professional fieldworkers. In each selected square, five locations were sampled per annum (September-October) by two different observers using hand netting. The data of the first two rounds (2011-2016 and 2017-2022) have been analysed using both an occupancy model and a Poisson GLMM. We found evidence of a declining trend in occupancy as well as in population size. The decreases were stronger outside than within Natura2000 sites and al..., From the 85 km2 squares in which Graphoderus bilineatus is known, 68 squares were chosen, covering 80% of the known distribution at the time, including all Natura2000 sites (41 squares). The remaining 27 squares were chosen at random from the pool of 44 squares outside Natura2000 sites, plus 12 squares with a high probability. Most selected squares were in the western and northern peat areas, with a few squares in the higher sandy soil region where the species still occurred.
The 1 km2 squares have been used to evenly distribute the sample locations across the entire range of the species. Within each 1 km2 square, the first observer selected five sampling locations which were visually judged to be potentially suitable for G. bilineatus. This species mainly occurs in dead ends of waterways, 90° angle bends or inlets in the shoreline, particularly between floating organic matter. The preference for these places is probably due to the accumulation of animal remnants on which the adults fee..., , # Data\_Graphoderus\_bilineatus\_the\_Netherlands

[https://doi.org/10.5061/dryad.p5hqbzkwm](https://doi.org/10.5061/dryad.p5hqbzkwm)

Graphoderus_bilineatus_the_Netherlands.csv is in csv-format and should be processed in R.

## Description of the data and file structure

* Site code;
* Species name;
* Count1 and Count2: number of specimens found for species in Species name. When a site was only visited once, there was no second visit to the site, the value of Count2 is NA;
* Date1 and Date2: dates of observation. When there was no second visit to a site within the season, the value of Date2 is NA;
* Observer1 and 2: Observer code 1-4. When there was no second visit to a site within the season, the value of Observer2 is NA;
* N2000: site is within (1) or outside (0) a N2000 site.