Dataset for: Designing a surveillance program for early detection of alien plants and insects in Norway

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).