Identifying sources of genetic variation and reconstructing invasion routes for non-native introduced species is central to understanding the circumstances under which they may evolve increased invasiveness. In this study, we used genome-wide single nucleotide polymorphisms to study the colonization history of Centaurea solstitialis in its native range in Eurasia and invasions into the Americas. We leveraged this information to pinpoint key evolutionary shifts in plant size, a focal trait associated with invasiveness in this species. Our analyses revealed clear population genomic structure of potential source populations in Eurasia, including deep differentiation of a lineage found in the southern Apennine and Balkan Peninsulas and divergence among populations in Asia, eastern Europe, and western Europe. We found strongest support for an evolutionary scenario in which western European populations were derived from an ancient admixture event between populations from eastern Europe and Asia, and subsequently served as the main genetic ‘bridgehead’ for introductions to the Americas. Introductions to California appear to be from a single source region, and multiple, independent introductions of divergent genotypes likely occurred into the Pacific Northwest. Plant size has evolved significantly at three points during range expansion, including a large size increase in the lineage responsible for the aggressive invasion of California’s interior. These results reveal a long history of colonization, admixture, and trait evolution in C. solstitialis, and suggest routes for improving evidence-based management decisions for one of the most ecologically and economically damaging invasive species in the western United States.

The early phases of biological invasions are poorly understood. In particular, during the introduction, establishment, and possible lag phases, it is unclear to what extent evolution must take place for an introduced species to transition from established to expanding. In this study, we highlight three disparate data sources that can provide insights into evolutionary processes associated with invasion success: biological control organisms, horticultural introductions, and natural history collections. All three data sources potentially provide introduction dates, information about source populations, and genetic and morphological samples at different time points along the invasion trajectory that can be used to investigate preadaptation and evolution during the invasion process, including immediately after introduction and before invasive expansion. For all three data sources, we explore where the data are held, their quality, and their accessibility. We argue that these sources could find widespread use with a few additional pieces of data, such as voucher specimens collected at certain critical time points during biocontrol agent quarantine, rearing, and release and also for horticultural imports, neither of which are currently done consistently. In addition, public access to collected information must become available on centralized databases to increase its utility in ecological and evolutionary research.

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Question: How do the diversity, size structure, and spatial pattern of woody species in a temperate (Mediterranean climate) forest compare to temperate and tropical forests? Location: Mixed evergreen coastal forest in the Santa Cruz Mountains, California, USA. Methods: We mapped, tagged, identified, and measured all woody stems (≥1 cm diameter) in a 6‐ha forest plot, following Center for Tropical Forest Science protocols. We compared patterns to those found in 14 tropical and 12 temperate forest plots. Results: The forest is dominated by Douglas‐fir (Pseudotsuga menziesii) and three species of Fagaceae (Quercus agrifolia, Q. parvula var. shrevei, and Lithocarpus densiflorus), and includes 31 woody species and 8180 individuals. Much of the diversity was in small‐diameter shrubs, treelets, and vines that have not been included in most other temperate forest plots because stems <5‐cm diameter had been excluded from study. The density of woody stems (1363 stems ha−1) was lower than that in all but one tropical plot. The density of large trees (diameter ≥30 cm) and basal area were higher than in any tropical plot. Stem density and basal area were similar to most other temperate plots, but were less than in low‐diversity conifer forests. Rare species were strongly aggregated, with the degree of aggregation decreasing with abundance so that the most common species were significantly more regular than random. Conclusions: The patterns raise questions about differences in structure and dynamics between tropical and temperate forests; these need to be confirmed with additional temperate zone mapped plots that include small‐diameter individuals.

In some plant populations, the availability of seeds strongly regulates recruitment. However, a scarcity of germination microsites, granivory or density-dependent mortality can reduce the number of plants that germinate or survive to flower. The relative strengths of these controls are unknown for most plant populations and for exotic invaders in particular. We conducted a seed addition experiment with a granivore exclusion treatment in a field setting to explore how these factors interact to regulate populations of the widespread invader Centaurea solstitialis (yellow starthistle) at three study sites across the plant's range in California. We coupled the experimental approach with observational studies within established C. solstitialis populations to estimate seed rain, recruitment and mortality at natural densities. Seed limitation occurred in both experimental and observational plots in all populations. Although vertebrate granivores were active at each site, they had no effect on C. solstitialis recruitment. Density increased mortality, but the effect was variable and weak relative to its effect on fecundity. The seed limitation that was evident at the seedling stage persisted to flowering. Seed-limited populations such as these ought to be highly sensitive to losses to seed predators, and many biological control agents, including those established for C. solstitialis, are seed predators. However, flowering plant density was decoupled from seed production by a strong compensatory response in the surviving plants; seed production was nearly constant in plots across all seed addition levels. Thus, flowering plant density is reduced by the established biocontrol agents, but seed production compensates to replace the population every generation, and no long-term decline is predicted.