Few studies specifically address the evolutionary processes involved in newly invasive species that are still quite limited in range, yet actively expanding. We chose false brome (Brachypodium sylvaticum) as a model species to investigate morphological and genetic patterns expected during colonization and range expansion. False brome is a newly-invasive species that is under high levels of scrutiny by local managers because of its aggressive displacement of native species.

Our project had three main objectives: 1) We sampled the native and invaded range to determine putative source regions and the number of independent invasion events; 2) We examined morphological and genetic changes incurred during initial colonization and among newly colonized populations in the invaded range; and, 3) We tested whether expansion processes involved diffusion dispersal or long-distance dispersal. For genetic analyses, we used one polymorphic cpDNA region, and 11 polymorphic microsatellite markers. We sampled from 10-25 individuals from each of 44 populations in Oregon, two in California, and 25 populations from throughout the native range in Europe, North Africa and the Middle East.

Results/Conclusions

The false brome invasion is characterized by at least two independent invasion foci in the US, each consisting of genotypes from multiple source regions. Overall, introduced populations are genetically similar to at least two different areas in Western Europe. Individuals from source populations in Oregon are admixed and have high growth rates, suggesting that ongoing hybridization fueling the invasion.

We found strong signatures of restrictive founder events in newly colonized populations in the invaded range. Such populations have low diversity and may suffer from inbreeding depression, as they have lower plant density and individuals have lower growth rates than high diversity populations. While inbreeding in peripheral populations may effectively slow the rate of range expansion, this situation could change if these populations can successfully purge genetic load, in which case range expansion could increase exponentially.

Dispersal does not follow a pattern of isolation by distance even among sample sites separated by 1.5km, strongly suggesting that long-distance dispersal is the rule for this invasion. This indicates that aggressive management at an early stage will be necessary to inhibit further invasion by this species.

In our efforts track expansion and collect data on population dynamics we are assisted by both undergraduate and graduate students, and volunteers from institutions and organizations from across the Willamette Valley of Oregon.