Data collection
I obtained a list of all non-native flowering plants recorded as
naturalised in Britain from Stace & Crawley (2015), which is the most
up-to-date published list currently available. I excluded taxa of
uncertain origin and taxa listed as casual or survivors, retaining only
taxa classed as naturalised, which Stace & Crawley (2015) defined as
self-reproducing in the wild in Britain after 1987 (i.e., the list
excludes species recorded as naturalised before 1987 that no longer
appear to be self-reproducing in the wild). I obtained a list of the
naturalised non-flowering plants (gymnosperms and ferns) from Stace
(2010) and Hill et al . (2005). Both lists included hybrids,
cultivars, and subspecies. For convenience, I term each taxon a species.
For each species in the combined list, I searched for information on the
year the species was first recorded as grown in cultivation in Britain
(which I term year of introduction), and the year it was first recorded
as growing outside cultivation in Britain (which I term year of
naturalisation), using Hill et al . (2005), Stace & Crawley
(2015) and The Online Atlas of the British and Irish Flora
(https://www.brc.ac.uk/plantatlas/ accessed in December 2020). In
Hill et al . (2005), the year of introduction was sometimes given
as a range when it could not be determined precisely. In those cases, I
used the earliest year of introduction, and I used the earliest recorded
year of introduction and naturalisation when sources differed.
For the analyses in this paper, I included only naturalised species with
data for both the year of first introduction and naturalisation, and I
excluded species introduced before 1500 AD because their introduction
dates were less certain. The final list comprised 708 naturalised
species in 99 plant families. The year in which species were recorded as
first introduced ranged from 1500 to 1960, while the year in which
species were recorded as first naturalised ranged from 1597 to 2000. I
calculated the lag time (in years) as the year of first recorded
naturalisation minus the year of first recorded introduction.
I classified each species into one of three life-form categories:
tree/shrub (213 species), perennial herb (398 species) or
annual/biennial herb (97 species). I did this because we expect that, on
average, lag times should differ among these life-form categories, with
longer-lived, slower-growing species taking longer to reach reproductive
maturity and hence taking longer to produce propagules that could
establish wild populations relative to faster-growing, shorter-lived
species (Wangen & Webster 2006; Caley et al. 2008). The
life-form classifications were determined primarily from information in
The Online Atlas of the British and Irish Flora
(https://www.brc.ac.uk/plantatlas/).