Rapid alignment of functional trait variation with locality across the invaded range of Sahara mustard (Brassica tournefortii)

Abstract

Our results identify key functional traits of this invasive species that showed significant variation among introduced populations across a broad geographic and climatic range. Further, trait variation among populations was strongly related to key climatic variables, which suggests that population divergence in these traits may explain the successful colonization of Sahara mustard across its invaded US range.

Multiple introductions and population structure during the rapid expansion of the invasive Sahara mustard (Brassica tournefortii)

“…For example, Spartina alterniflora are cross‐pollination limited in the San Francisco Bay area of California and individuals that have high selfing rates also produce high viable seed sets compared to nonselfing individuals (Daehler, ). This is similar to patterns we observed in our current study and matches previous work showing Sahara mustard aligns its reproductive efforts based on ecological site factors (Winkler et al, ).…”

“…Self‐fertilization coupled with high propagule pressures can further reduce the potential negative impacts of inbreeding by increasing chances of establishment (Hargreaves & Eckert, ; Levin, ). A single Sahara mustard plant invests heavily in reproductive structures, can produce over 16,000 seeds, and disperses across relatively large distances via animals, wind, water, and roadways (Bangle et al, ; Berry, Gowan, Miller, & Brooks, ; Sánchez‐Flores, ; Trader et al, ; Winkler et al, ). It is likely that even if a small number of Sahara mustard were initially introduced into the United States, the species’ huge reproductive investments in offspring (Winkler et al, ) enable it to maintain adequate population sizes to overcome bottlenecks and establish itself at least locally (Lockwood, Cassey, & Blackburn, ).…”

“…A single Sahara mustard plant invests heavily in reproductive structures, can produce over 16,000 seeds, and disperses across relatively large distances via animals, wind, water, and roadways (Bangle et al, ; Berry, Gowan, Miller, & Brooks, ; Sánchez‐Flores, ; Trader et al, ; Winkler et al, ). It is likely that even if a small number of Sahara mustard were initially introduced into the United States, the species’ huge reproductive investments in offspring (Winkler et al, ) enable it to maintain adequate population sizes to overcome bottlenecks and establish itself at least locally (Lockwood, Cassey, & Blackburn, ). We observed 87%–91% selfing rates which are similar to results obtained in studies on other invasive plants (Kleunen, Fischer, & Johnson, ; Lott et al, ).…”

“…Since its presumed introduction in the 1920s, this invader has become increasingly common in semi‐arid regions, including all counties in Southern California (Sanders & Minnich, ) and throughout 500,000 km 2 ha in Southwest United States and Northwest Mexico. Although a few ecological studies have examined the species’ performance and impacts in a few invaded areas (Barrows et al, ; Li et al, ; Marushia et al, ; Marushia, Cadotte, & Holt, ; VanTassel et al, ; Winkler, Gremer, Chapin, Kao, & Huxman, ), no research has been conducted to examine the genetic structure of this invasive. Identifying introduction sites and understanding how the species has, and is, spreading via population genomics is a critical first step to elucidating the mechanisms by which species invasions can rapidly occur over large distances.…”

Phenotypic variation of life‐history traits in native, invasive, and landrace populations of Brassica tournefortii

“…The clinal patterns we determined indicate that aridity is a likely agent of selection for B. tournefortii , which means it may have affected genetic differentiation among populations and among ranges. Thus, we expected to find patterns suggesting adaptive or maladaptive phenotypic differentiation for ecologically important traits, as Winkler, Gremer, Chapin, Kao, & Huxman, have identified in other populations of B. tournefortii . This was true for one composite trait, phenology PC1, which had a defined cline for the native populations, but relatively neutral or flat clines for invasive and landrace ranges.…”