How weeds emerge: a taxonomic and trait-based examination using United States data

Abstract

Weeds can cause great economic and ecological harm to ecosystems. Despite their importance, comparisons of the taxonomy and traits of successful weeds often focus on a few specific comparisons - for example, introduced versus native weeds. We used publicly available inventories of US plant species to make comprehensive comparisons of the factors that underlie weediness. We quantitatively examined taxonomy to determine if certain genera are overrepresented by introduced, weedy or herbicide-resistant species, and we compared phenotypic traits of weeds to those of nonweeds, whether introduced or native. We uncovered genera that have more weeds and introduced species than expected by chance and plant families that have more herbicide-resistant species than expected by chance. Certain traits, generally related to fast reproduction, were more likely to be associated with weedy plants regardless of species' origins. We also found stress tolerance traits associated with either native or introduced weeds compared with native or introduced nonweeds. Weeds and introduced species have significantly smaller genomes than nonweeds and native species. These results support trends for weedy plants reported from other floras, suggest that native and introduced weeds have different stress adaptations, and provide a comprehensive survey of trends across weeds within the USA.

Keywords: herbicide resistance; introduced plant; taxonomic selectivity; weed.

Figure 1

A simplified version of the problematic species continuum, adapted from Williamson (1996) and applied to (a) introduced and (b) native species (the sizes of the circles in (a) and (b) are not comparable). In (a), the circle represents the proportion of species that become naturalized, invasive, and herbicide resistant, and dashed and dotted lines depict the introduction of species that were herbicide resistant and invasive/weedy in their native area, respectively. In both (a) and (b), diamonds represent environmental filters as envisioned by Williamson (1996).

Figure 2

The percentage of species from our data set of all US (n = 19 180) and herbicide-resistant (n = 67) species categorized as introduced weeds, introduced nonweeds, native weeds, and native nonweeds.

Figure 3

Comparison of traits related to growth, reproduction and tolerance of edaphic factors between (a) weeds and nonweeds and (b) introduced and native species. The common log-odds ratios and 95% confidence intervals (CIs) are shown. The height of each bar indicates the relative frequency of each trait, with values > 0 indicating that the frequency is higher among weeds in (a) and introduced species in (b).

Figure 4

Comparison of traits related to growth, reproduction and tolerance of edaphic factors between (a) native weeds and native nonweeds, (b) introduced weeds and introduced nonweeds, and (c) native weeds and introduced weeds. The common log-odds ratios and 95% confidence intervals (CIs) are shown. The height of each bar indicates the frequency of each trait, with values > 0 indicating that the frequency is higher among native weeds versus native nonweeds in (a), introduced weeds versus introduced nonweeds in (b), and introduced weeds versus native weeds in (c).

Figure 5

Genome size and introduction date for US plant species. (a) Average genome size estimates (Mbp ± SE) for introduced and native species that are nonweeds (triangle) and weeds (circle). C-values were obtained from the Kew database (http://www.kew.org). (b). Average year of introduction (± SE) (using average first year of herbarium record) for 100 randomly selected introduced nonweed and weed species. Introduction dates were obtained from the Global Information Biodiversity Information facility (http://data.gbif.org/).