Non-native plants add to the British flora without negative consequences for native diversity

Abstract

Plants are commonly listed as invasive species, presuming that they cause harm at both global and regional scales. Approximately 40% of all species listed as invasive within Britain are plants. However, invasive plants are rarely linked to the national or global extinction of native plant species. The possible explanation is that competitive exclusion takes place slowly and that invasive plants will eventually eliminate native species (the "time-to-exclusion hypothesis"). Using the extensive British Countryside Survey Data, we find that changes to plant occurrence and cover between 1990 and 2007 at 479 British sites do not differ between native and non-native plant species. More than 80% of the plant species that are widespread enough to be sampled are native species; hence, total cover changes have been dominated by native species (total cover increases by native species are more than nine times greater than those by non-native species). This implies that factors other than plant "invasions" are the key drivers of vegetation change. We also find that the diversity of native species is increasing in locations where the diversity of non-native species is increasing, suggesting that high diversities of native and non-native plant species are compatible with one another. We reject the time-to-exclusion hypothesis as the reason why extinctions have not been observed and suggest that non-native plant species are not a threat to floral diversity in Britain. Further research is needed in island-like environments, but we question whether it is appropriate that more than three-quarters of taxa listed globally as invasive species are plants.

Keywords: Anthropocene; biodiversity; botany; ecology; invasive species.

Fig. 1.

The number of sites (A) and mean percentage cover per site (B) of the most widespread (A) and most abundant (B) native species (white polygon with black outline), archaeophytes (gray bars), and neophytes (black bars) recorded during the Countryside Survey in 2007. In A, 250 species (native = 221, archaeophytes = 21, neophytes = 8) are shown. In B, 100 species (native = 92, archaeophytes = 2, neophytes = 6) are shown. Note that the x axes have been truncated. In A, a further 427 species (native = 332, archaeophytes = 47, neophytes = 48) were recorded in Countryside Survey sites in 2007. These species were all recorded in ≤11 sites. In B, a further 171 species (native = 101, archaeophytes = 14, neophytes = 2) recorded in at least 10 sites had mean cover of more than 0% in Countryside Survey sites in 2007. The mean cover of each of these species, per site, was ≤0.088%.

Fig. 2.

The frequency of occurrence (A) and mean percentage cover per site (B) of native species, archaeophytes, and neophytes in 1990 (gray boxes, left-hand box of each species group) and 2007 (white boxes, right-hand box of each species group). Only species recorded in at least 10 sites in each survey year are included in each panel. Sample sizes (numbers of species) are provided at the top of each box. Medians are represented by the horizontal black lines; the top and bottom of each box are the 75th and 25th percentiles, respectively; outliers are represented by hollow dots; and whiskers represent data within 1.5 × interquartile range of the upper and lower quartiles.

Fig. 3.

Changes in the frequency of occurrence (A) and mean percentage cover per site (B) of native species, archaeophytes, and neophytes in 1990 (gray boxes, left-hand box of each species group) and 2007 (white boxes, right-hand box of each species group). Only species recorded in at least 10 sites in both survey years are included in each panel. Sample sizes (numbers of species) are provided at the top of each box. Medians are represented by the horizontal black lines; the top and bottom of each box are the 75th and 25th percentiles, respectively; outliers are represented by hollow dots; and whiskers represent data within 1.5*interquartile range of the upper and lower quartiles.

Fig. 4.

Changes in numbers of native plant species as a function of changes in the number of non-native plants species (comprised of neophytes plus archaeophytes) in Countryside Survey plots between 1990 and 2007. Each point represents a site (n = 479 sites). There was a significant positive relationship (line ± 95% confidence interval) between changes in the diversity of native and non-native species (y = −0.58 + 1.28x; R² = 0.14; P < 0.0001).