Abstract
AbstractAimBiological invasions at the intracontinental scale are poorly studied, and intracontinental invasions often remain cryptic. Here, we investigate the recent range expansion of scotch broom (Cytisus scoparius) into Norway and clarify whether the genetic patterns indicate natural spread or human introduction. Furthermore, we investigate whether plants were moved within the native range and how this influences invasion success. We also infer the level and structuring of genetic diversity within and between the putative native and introduced range.LocationEurope.MethodsWe analysed the chloroplast sequence variation in 267 scotch broom samples from its northern expansion front and from its native range across Europe, including herbarium samples dating back to 1835. For 37 populations, we analysed variation in nuclear single‐nucleotide polymorphic markers to study gene flow and genetic diversity.ResultsWe identified 20 different haplotypes, which lacked spatial and temporal distribution patterns in the recent expansion range in Norway. They also mostly lacked patterns across the native European range of scotch broom. The genetic diversity of nuclear genomic SNP markers across populations in the introduced range was similar to that of populations in the native range, with limited differentiation among populations.Main conclusionsScotch broom is alien to Norway and was introduced by humans on multiple occasions from diverse origins over a long period of time. High propagule pressure has probably maintained the high genetic diversity in the novel range through a combination of genetically diverse source populations and high gene flow among them. Within the native European range, our results suggest the presence of cryptic intraspecific admixture, most likely mediated by humans moving genotypes among the regions occupied by distinct native genotypes. Intracontinental invasions may easily go unnoticed and revealing these invasions and the factors driving them may be of great importance for the management of alien species.
Highlights
Despite more than 50 years of research on the biological invasion of alien species across continents, studies at the intracontinental scale are rare, because it is difficult to prove whether a species is native or alien (Hulme et al, 2016; Webber & Scott, 2012)
Our study suggests that the recent range expansion of scotch broom was mainly driven by human introductions; the species is alien to Norway, and its cryptogenic status is resolved
The rapid northern range expansion of scotch broom in the recent century was driven by human introductions, scotch broom can be classified as alien in Norway
Summary
Despite more than 50 years of research on the biological invasion of alien species across continents, studies at the intracontinental scale are rare, because it is difficult to prove whether a species is native or alien (Hulme et al, 2016; Webber & Scott, 2012). The genetic admixtures resulting from the crossing of plants from geographically distant origins produces novel genotypes that may have stronger invasive potential (Bock et al, 2015; Ellstrand & Schierenbeck, 2000; Morais & Reichard, 2018) and increased ability to adapt to the new environment. Since its first record in southern Norway in 1876 (Blytt, 1876), it has spread northwards (Figure 1, see Appendix S1) and has increased in abundance in threatened habitats, such as endangered coastal heathlands (Lindgaard & Henriksen, 2011) This species was included on the Norwegian Black List in 2007 but was removed in the 2012 revised list, considering that it might be native (Gederaas, Salvesen, & Viken, 2007; Gederaas, Moen, Skjelseth, & Lar sen, 2012). We aim to (a) establish whether this range expansion is natural or is caused by human introductions, (b) determine whether the expansion is the result of one or multiple introductions (prop‐ agule pressure) and pinpoint the origin of these introductions (propagule origin) to establish the role of propagule pressure and origin in invasion success and (c) quantify the level and geograph‐ ical structure of genetic diversity to investigate its possible role in invasion success
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