This post is part of our series covering the large number of urban evolution manuscripts that were published in August 2018 in the special issue “The Evolution of City Life” in the Proceedings of the Royal Society B.
Urbanization can create new pathways of introduction for invasive species. The recently introduced invasive grass Brachypodium sylvaticum has been found near Portland, Oregon, USA in the Clackamas Watershed. Tina Arredondo, a recently graduated Master’s student at Portland State University, and co-authors investigated the role that urbanization plays in the recent range expansion of the invasive grass.
In their publication “Evidence for human-mediated range expansion and gene flow in an invasive grass“, Arredondo and her coauthors sampled leaf tissue from 22 locations in the Clackamas Watershed area. The Clackamas Watershed is the leading edge of the range that has recently expanded and includes both urban and non-urban land types.
Levels of genetic diversity were measured and matched expectations for populations in a recently colonized area. First, they found very low genetic diversity in some of the populations. This suggests that these populations are the newest since they have not received additional alleles from other populations nor have they generated new alleles. Next, they found that some populations had an excess of genetic diversity. This suggests that these are not only the oldest populations, but also possible points of introduction from other locations.
When they ran landscape models to identify which land types impact gene flow the most, they found non-significant impacts from development, roads, canopy cover, and agriculture. However, rivers were a significant facilitator for gene flow. Interestingly, gene flow along the rivers was found to be bi-directional, which means that seeds cannot be moving with the flow of the river!
If rivers are the only land type that was found to significantly impact gene flow, but the movement is not with the flow of the river, what could really be driving the patterns they see? This river-mediated gene flow is most likely to be related to human movement along the river. In particular, Arredondo suggests that the barbed awn on the grass seeds get caught in socks of hikers and their animal’s fur. Instead of human-mediated gene flow, it is really hiker-and-their-dogs-mediated gene flow!
Read the full paper here:
I am interested in urban ecology and evolution. Urbanization can have drastic impacts on local ecosystems, both negative and positive, and creates a new environment for organisms to experience. To understand the impacts that urbanization plays on organisms, I look through the lense of population genetics and molecular evolution.
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