Biological invasions are ultimately interactions between native and non-native populations, therefore knowledge of population structure and geneflow are vital to understanding, controlling, and predicting the spread of invasive species. We examine the population structure of two contrasting species; Ailanthus altissima, an extremely successful high impact invasive tree, and Schismus arabicus, an invasive Eurasian annual grass that severely alters fire regimes in the American Southwest. These two species were chosen for the severity of their ecological impact, and for contrasting life histories.
Ailanthus altissima samples were collected from six sites in the northern New Jersey area, with equal representation of urban, suburban, and exurban sites. Schismus arabicus samples were collected from the Mojave and Sonoran deserts in the American southwest, sites were isolated by a distance of approximately 200 miles. The genetic structure of Ailanthus altissima was determined using short tandem repeat (STR) regions, while the structure of Schismus arabicus was estimated through the use of the Internal Transcribed Spacer (ITS) region. The resulting data was used to estimate population parameters, haplotype networks, effective population size and geneflow.
Results/Conclusions
Ailanthus altissima was found to have high levels of geneflow between all sites. Despite a reputation for highly aggressive growth from root suckers, we found no evidence of clonal reproduction at the population level. From this we concluded that root sucker growth is primarily beneficial for individual survival, but has little to no effect on the genetic structure of Ailanthus populations. Pairwise comparisons of Ailanthus sites indicates that the population structure is not dictated by geographic distance, nor by the level of human development. The level of geneflow between sites was high, indicating continuous dispersal of propagules. No isolation was observed between any of the six sites, indicating a highly fluid metapopulation.
Schismus arabicus was found to have high isolation between the Sonoran and Mojave populations. A small unidirectional geneflow was observed from the Mojave site to the Sonoran, which was determined to be anthropogenic in origin. Within sites, Schismus was largely panmictic. The site structure was reflected in our haplotype network, which showed clear delineation between the two sites. The spread of Schismus appears to be dispersal limited.
The difference in population dynamics of these two species illustrates the importance of dispersal mechanisms, and their role in biological invasions.