Naturalized species of alien plants and animals comprise < 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). 

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.  

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.