DOI: 10.5061/dryad.p5hqbzkwm
title: Data for: Trends in abundance and occupancy of the protected waterbeetle Graphoderus bilineatus in the Netherlands

description: 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
 also stronger than those of three other beetle species that are often
 found together with G. bilineatus. In addition, considerable differences
 between observers were detected in the data, despite the application of a
 standardized field method.
description: From the 85 km<sup>2 </sup>squares in which
 <em>Graphoderus bilineatus </em>is known, 68 squares were
 chosen, covering 80% of the known distribution at the time, including all
 Natura2000 sites (41<sup> </sup>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 km<sup>2</sup> squares have been used to evenly
 distribute the sample locations across the entire range of the species.
 Within each 1 km<sup>2</sup> square, the first observer
 selected five sampling locations which were visually judged to be
 potentially suitable for <em>G. bilineatus</em>. 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 feed and the availability of shelter while resupplying
 oxygen. Furthermore, vegetation composition, such as the presence of
 <em>Stratiotes aloides</em>, <em>Hydrocharis
 morsus-ranae,</em> and <em>Lemna trisulca</em>, the
 presence of open water, water clarity, water depth, and natural banks for
 pupation, determine the suitability of a location for <em>G.
 bilineatus</em>. After sampling, coordinates and a description of
 the sampling location were shared with the second observer who surveyed
 the same location 5-8 days after the first visit. Six years later, the
 same locations were re-examined twice again. Each year,
 12 squares, representing 60 sampling locations, have been examined.
 Currently, data are available for 68 squares with 337 locations, each
 surveyed twice (2011-2016 vs 2017-2022), resulting in a total of 1336
 field surveys.
description: # 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.