DOI: 10.5061/dryad.0rxwdbs4f
title: Dataset for: Designing a surveillance program for early detection of alien plants and insects in Norway

description: Naturalized species of alien plants and animals comprise &lt; 3% of
 biodiversity recorded in Norway but have had major impacts on natural
 ecosystems through displacement of native species. Encroachment of alien
 species has been especially problematic for coastal sites close to
 transport facilities and urban areas with high-density housing. The goal
 of our field project was to design and test a surveillance program for
 early detection of alien species of vascular plants and terrestrial
 insects at the first phase of establishment in natural areas. In our
 3-year project (2018–2020), we sampled 60 study plots in three counties in
 the Oslofjord region of southern Norway. Study plots (6.25 ha) were
 selected by two criteria: manual selection based on expert opinion (27
 plots) or by random selection based on weights from a hotspot analysis of
 occurrence of alien species (33 plots). Vascular plants were surveyed by
 two experienced botanists who found a total of 239 alien species of
 vascular plants in 95 rounds of surveys. Insects and other invertebrates
 were captured with a single Malaise trap per site, with 3-4 rounds of
 repeated sampling. We used DNA-metabarcoding to identify invertebrates
 based on DNA extractions from crushed insects or from the preservative
 media. Over 3,500 invertebrate taxa were detected in 255 rounds of
 sampling. We recorded 20 alien species of known risk and 115 species that
 were new to Norway, including several ‘doorknocker’ species identified by
 previous risk assessments. We modeled the probabilities of occupancy (y)
 and detection (p) with occupancy models with repeated visits by multiple
 observers (vascular plants) or multiple rounds of sampling (insects). The
 two probabilities covaried with risk category for alien organisms and both
 were low for species categorized as no known or low risk (range = 0.052 to
 0.326) but were higher for species categorized as severe risk (range =
 0.318 to 0.651). Selecting sites at random or manually did not improve the
 probability of finding novel alien species, but occupancy had a weak
 positive relationship with housing density for some categories of alien
 plants and insects. We used our empirical estimates to test alternative
 sampling designs that would minimize the combined variance of occupancy
 and detection (A-optimality criterion). Sampling designs with 8–10 visits
 per site were best for surveillance of new alien species if the
 probabilities of occupancy and detection were both low, and provided low
 conditional probabilities of site occupancy (psi-hat(cond) £ 0.032) and a
 high probabilities of cumulative detection (p-hat(star) ³
 0.943).  Our field results demonstrate that early detection is
 feasible as a key component of a national surveillance program based on
 early detection and rapid response (EDRR). 
description: We used two criteria for
 selection of our 6.25 ha study plots: manual selection based on expert
 opinion for the probability of finding new invasive species and random
 selection based on a set of predefined criteria: i) a housing
 density of at least 8-12 detached houses, ii) a population density between
 30–125 residents, iii) site locations within 100 meters of a natural
 forest area, and iv) a probability of selection that was weighted by the
 predicted proportion of alien vascular plants from a recent ‘hotspot’
 analysis by Olsen et al. (2017).   Two rounds of plant surveys were conducted in
 the 1-month period between mid-August and mid-September by two experienced
 botanists who were familiar with the flora of the Oslofjord region (A.
 Often and H. Hegre). We used a double-observer approach and the two
 observers conducted independent plant surveys at the study plots.
 In 2018, the study plots
 were subdivided into four transects offset at 45° angles. Observers were asked to search a 10 m
 strip along each transect, then spend ca. 30 minutes searching the
 remainder of the study plot, with a maximum time limit of two hours per
 plot. Fixed transects proved to be impractical in residential and
 industrial areas, and search time was dependent on the habitat complexity
 of the study plot. In 2019 and 2020, observers used a random walk to
 search the entire plot, with a maximum time limit of five hours per
 plot. Insects and other invertebrates were
 sampled with a single Malaise trap at each study site in the 1.5-month
 period between mid-June and late August. In 2018, Malaise traps were set
 up on the study plots in the last week of August (week 35), and we
 completed three rounds of weekly sampling in September (weeks 36 to 39).
 In 2018, the collection bottles were filled with a mixture of water,
 ethanol, and propylene glycol in a 1:1:3 ratio. In 2019 and 2020, Malaise
 traps were set up on plots in mid-June (week 25), and we completed four
 rounds of biweekly sampling between mid-July and late August (weeks 29 to
 35). In 2019 and 2020, the collection bottles were filled with ca. 400 ml
 of 96% ethanol.  After emptying the traps, the preservative fluid was
 removed with a sieve, and insects were stored in 96% ethanol in the
 freezer for subsequent analyses.  
description: Data files are stored as comma-delimited CSV files and can be
 opened in Notepad, Program R, or Excel. Analyses were
 conducted using base functions and packages of Program R.