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Assessing global variation in phenotypic traits and linking that variation to colonization and range expansion is notably rare in invasion biology. Here, we studied variation in seed size in Centaurea solstitialis, a weed with worldwide distribution. Additionally, we explored how seed size variation affects seedling survival of C. solstitialis under favorable precipitation conditions in Anatolia, an ancestral region, and unfavorable precipitation conditions in Argentina, a non-native region. To that end, we conducted seed collections following dispersal pathways of C. solstitialis in ancestral, expanded, and non-native ranges. Locally, collections followed elevation gradients. Also, we performed a greenhouse experiment with C. solstitialis populations varying in seed size and water additions simulating precipitation patterns in Anatolia and Argentina. Seeds from ancestral populations at low elevation were smaller than those from the rest of study populations. Also, seed size in populations at high elevation in the expanded range, the main source of non-native populations, was similar to that in all, but one non-native population, where seeds exhibited further increase. Increments in seed size thus track range expansion in C. solstitialis. Locally, seed size increased with elevation in all three ranges, suggesting convergent responses to that gradient. Seedlings from larger seeds displayed greater survival than those from smaller seeds only under Argentinean conditions. Consequently, populations with large seeds may have been instrumental for colonizing that non-native region. Our findings suggest that ancient and recent dispersal of large-seeded populations contribute to explain the reported global pattern of seed size divergence and worldwide distribution of C. solstitialis.

Lepidium draba (Brassicaceae) is a major concern for agriculture and biodiversity in the western United States. As current control methods do not provide long-term, sustainable solutions, research has been conducted to find biological control agents. Ceutorhynchus assimilis is one of the currently investigated candidates. Known as oligophagous in the literature, a specialist clade of this root-galling weevil exists in southern Europe. This raised the question of its ability to survive in colder climates in the target range. We investigated the phenology of C. assimilis in the field in southern France (specialist clade) and Romania (generalist clade) and measured various temperature-dependent parameters in the laboratory. In both ranges, weevils were univoltine. Oviposition in autumn started later in France compared to Romania, while mature larvae exited galls (to pupate in the soil) earlier the following year. On average, 25% and 32% of galls from France and Romania were completely below the soil surface, respectively, and this appeared to depend on soil substrate. Weevils transported from France to Romania were able to develop, but at a much lower rate than Romanian weevils. Mortality of overwintering larvae of both clades increased with decreasing temperature and exposure time. At −5°C, lethal times Lt50 and Lt95 were 15 and 42 days for the specialist clade and 26 and 72 days for the generalist clade. A higher proportion of third instar larvae compared to first and second instar larvae survived. Pupation time at different temperatures did not differ between weevils from France or Romania. A climate match model (comparing winter temperatures) indicated that the specialist clade of C. assimilis from France has the potential to establish in some parts of the target range (e.g. Washington, Oregon, California). However, temperature extremes and winters without snow cover will likely limit its establishment unless rapid adaptive evolution takes place.

Biogeographic experiments that test how multiple interacting factors influence exotic plant abundance in their home and recipient communities are remarkably rare. We examined the effects of soil fungi, disturbance and propagule pressure on seed germination, seedling recruitment and adult plant establishment of the invasive Centaurea stoebe in its native European and non-native North American ranges. Centaurea stoebe can establish virtual monocultures in parts of its non-native range, but occurs at far lower abundances where it is native. We conducted parallel experiments at four European and four Montana (USA) grassland sites with all factorial combinations of ± suppression of soil fungi, ±disturbance and low versus high knapweed propagule pressure [100 or 300 knapweed seeds per 0.3 m × 0.3 m plot (1000 or 3000 per m2)]. We also measured germination in buried bags containing locally collected knapweed seeds that were either treated or not with fungicide. Disturbance and propagule pressure increased knapweed recruitment and establishment, but did so similarly in both ranges. Treating plots with fungicides had no effect on recruitment or establishment in either range. However, we found: (i) greater seedling recruitment and plant establishment in undisturbed plots in Montana compared to undisturbed plots in Europe and (ii) substantially greater germination of seeds in bags buried in Montana compared to Europe. Also, across all treatments, total plant establishment was greater in Montana than in Europe. Synthesis. Our results highlight the importance of simultaneously examining processes that could influence invasion in both ranges. They indicate that under ‘background’ undisturbed conditions, knapweed recruits and establishes at greater abundance in Montana than in Europe. However, our results do not support the importance of soil fungi or local disturbances as mechanisms for knapweed's differential success in North America versus Europe.