Introduction history mediates naturalization and invasiveness of cultivated plants

Aim: Species characteristics and cultivation are both associated with alien plant naturalization and invasiveness. Particular species characteristics are favored for cultivation, obscuring the relationship between traits and naturalization success. We sought to better understand the drivers of naturalization and invasiveness by analyzing relationships with species characteristics and cultivation and by disentangling the direct effects of characteristics from the indirect effects mediated by cultivation.

Location: Great Britain

Time period: c. 1000–present

Major taxa studied: Seed plants

Methods: We used a comprehensive dataset of 17,396 alien plant taxa introduced to Great Britain before 1850, a country with one of the most well-documented histories of plant introductions. We integrated this with cultivation data from historical and modern records of botanic gardens and commercial nurseries and with trait data. Accounting for time since introduction, we quantified the influences of cultivation and species characteristics on present-day naturalization and invasiveness in Great Britain.

Results: Larger native range size, earlier flowering, long-lived herbaceous growth form, and outdoor cultivated habitat were all associated with naturalization. However, these relationships between characteristics and naturalization largely reflected cultivation patterns. The indirect, mediating influence of cultivation on naturalization varied among species characteristics, and was relatively strong for growth form and weak for native range size. Cultivation variables, particularly availability in present-day nurseries, best explained invasiveness, while species characteristics had weaker associations.

Main conclusions: Human influence on species introduction and cultivation is associated with increased probability of naturalization and invasiveness, and it has measurable indirect effects by biasing the distribution of species characteristics in the pool of introduced species. Accounting for human cultivation preferences is necessary to make ecological interpretations of the effects of species characteristics on invasion.

Methods
The main dataset used in this manuscript is provided as a CSV file (CultivationGB_Dataset.csv). Each row describes a seed plant taxon introduced to Great Britain that was listed in J.C. Loudon's (1850) Hortus Britannicus. Associated data describing the introduction year, species characteristics, and cutivation variables are also provided for each taxon. The sources used to compile this dataset are lengthy and the methods complex, and all has been described in detail in the main manuscript and the accompanying Supporting Information. All columns in the main dataset are described in the README file. The R code files used for analysis are provided in a ZIP file (CultivationGB_Analysis.zip), and each R file is described in detail in the README file. Last, the phylogenetic tree of the taxa in this study is included as a TRE file (CultivationGB_Phylogeny.tre).

Loudon, J. C. (1850). Hortus Britannicus: A Catalogue of All the Plants Indigenous, Cultivated in, or Introduced to Britain (J. W. Loudon, Ed.; 4th ed.). Longman, Brown, Green, and Longmans.