I presented the results of my environmental niche modeling work at the 2016 Weed Science Society of America Meeting in San Juan, Puerto Rico. I attended the meeting with my advisor, Jacob Barney, and Dan Tekiela, a graduate student in Jacob’s lab.

The weather was lovely and we had an excellent time escaping winter’s grip to enjoy some sunshine and sea air! We also had a fantastic conference, meeting new people and learning about the exciting work that WSSA members are doing.

Congratulations to Jacob for winning the Outstanding Early Career Weed Scientist award!

An exotic invasive plant selects for increased competitive tolerance, but not competitive suppression, in a native grass

Fletcher, Callaway & Atwater (pdf)


At each of four sites, bluebunch from knapweed invaded plots (I) had much better ability to tolerate knapweed than those from uninvaded plots (U). However they were not better at suppressing knapweed.

Many congratulations to Rebecca Fletcher, who just had a paper accepted into Oecologia. Her paper shows that spotted knapweed, an invasive plant, selects for bluebunch wheatgrass plants that are tolerant of knapweed competition, but not for ones that are better at competitively suppressing knapweed.

This is the first direct test of the hypothesis that neighbor suppression does not provide fitness benefits to competing plants. Her finding is important because it challenges long-held notions of what makes a plant a good competitor. Becky performed this research as an undergraduate student at the University of Montana, on a grant she wrote for the Montana Integrative Research Experience for Students (MILES) program.



Propagule pressure alone cannot always overcome biotic resistance: The role of density-dependent establishment in four invasive species

Barney, Ho & Atwater (link)
I am very pleased to announce that we have just had a paper accepted for publication into Weed Research. This paper documents Master’s Student Matt Ho’s research into the role of propagule pressure in invasion. He found that negative density-dependent germination probability causes diminishing returns for species that broadcast a large number of seeds. His results suggest that site conditions and species interactions play an important role in determining invasion probability even when propagule pressure is intense.


Invasion probability increases as the number of seeds (aka propagule size) increases. When germination is density dependent (solid line), invasion probability increases much more gradually than when it is not (dashed line). This means that site conditions play a very important role in determining invasion probability, even when propagule pressure is high.

Pictured, left to right: Big Dan, Morgan, Little Dan (top); Jacob and Benjamin Franklin (bottom)

I recently presented the results of my niche modeling work at the 2016 Northeastern Plant Pest and Soils Conference in Philadelphia. I went the the conference along with most of the rest of my postdoc advisor’s (Jacob Barney’s) lab and we had a great time. I met a lot of people, including the First American, Benjamin Franklin, who looked great despite being only a week away from his 310th birthday.

At the conference, I presented the results of our study of climatic niche shifts in introduced species. Jacob and I have found that species experience major niche shifts as they cross continents. This work is in preparation for submission.

Also, congratulations are in order for “Little” Dan Tekiela, who won the Robert D. Sweet Outstanding Graduate Student Award and placed first in the Student Paper Contest. Virginia Tech also won the NEPPSC quiz bowl, besting teams from Cornell, Penn State, UMass, Maine, NC State, and Delaware, as well as the EB-ESA Linnaean Games. Graduate students Morgan Franke and Dan Tekiela both gave excellent presentations of their research.

Reconstructing changes in the genotype, phenotype, and climatic niche of an introduced species

Atwater, Sezen, Goff, Kong, Paterson & Barney (link)


The climate occupied by initial invaders (black, dashed) resembled the home climate (black, solid). As the invasion progressed, Johnsongrass moved into cooler habitats in the US (grey, solid & dashed). Noteably, these are not climates Johnsongrass occupies in its native range in Eurasia, although they are available. How invasive species change their climatic niches in their introduced ranges is a mystery.

Invasive species must deal with enormous environmental variation in their introduced ranges. Some evolve rapidly, and others tolerate a wide variety of conditions. We examined how one invader, Johnsongrass, has responded to environmental variation in North America. This devastating agricultural weed is ever-present in the fields, roadsides and railways of the United States.

Almost 500 individual Johnsongrass plants collected from 70 locations in the United States show enormous variation in their size and shape. Plants from cool, wet climates grow especially large, growing 10 feet tall and gaining almost 5 pounds of dry weight in a single year. Plants from agricultural habitats grew much larger and taller than those from roadside habitats and meadows, and responded differently to growing conditions.

These results paint the clearest picture to date of how an introduced species changes genetically and phenotypically as it encounters habitat variation in their introduced range. Local adaptation and phenotypic plasticity play important roles in the ability of Johnsongrass to invade the United States.

The 2015 Hokie Half Marathon is officially vanquished! I set a PR (1:38:22) and had a lot of fun.

The weather was absolutely beautiful and there over 1200 runners. It was a great time. Thank you to my cheer squad for keeping me going!

On Thursday at the 2015 Ecological Society of America meeting, I presented preliminary data from an analysis of the global distributions of 1135 introduced species. We find that species experience massive niche shifts as they cross continents. The magnitude of these niche shifts depends upon the methods used to remove sampling bias.

These data have significant implications for our ability to use native-range data to predict future distributions in the introduced range — a critical issue in the development of invasive species risk assessments. Fortunately, I was able to finish my talk just before a fire alarm cleared out the convention center!

Big ups to Dan and Morgan, the students in my post-doc lab, for giving great talks. Dan presented his study of non-additive effects of dual invaders, and Morgan presented her work on restoring autumn olive-invaded surface mine sites. Surface mining in Appalachia involves complete mountaintop removal. The topology, plant communities, and “soil” of these sites are 100% artificially reconstructed and have to be seen to be believed. Jacob, my advisor, presented our work on evaluating genetic and phenotypic variation in 70 US populations of Johnsongrass, a devastating invader.

Testing the mechanisms of diversity-dependent overyielding in a grass species

Atwater & Callaway (2016)(link)
007-boxplotsEDI’m very pleased to announce that Ray Callaway and I have just had a paper published in Ecology. In this paper, we find that genetically diverse populations of bluebunch wheatgrass (Pseudoroegneria spicata) yield about 50% more biomass than populations with low diversity.

This diversity-dependent “overyielding” is thought to occur because diverse plant communities function more efficiently than communities with low diversity. There is thought to be less demand for shared resources in diverse communities and less disease (e.g. by soil fungi). As a result, overyielding in species-diverse communities is often associated with changes in soil nutrients, and the yield of low-diversity communities can be “rescued” by killing soil pathogens.

However, we find that diversity-dependent overyielding in bluebunch wheatgrass populations was not related to changes in soil nutrients, and was unaffected by fungicide applied to the soil. We conclude that overyielding in genetically diverse populations may not be caused by the same processes that cause overyielding of species diverse communities.


Root contact responses and the positive relationship between intraspecific diversity and ecosystem productivity

Yang, Callaway & Atwater (2015) (link)

When grown next to a familiar neighbor, Johnsongrass root growth was initially rapid, but slowed when root contact occured. The opposite was true for plants grown next to an unfamiliar neighbor.

I’m very pleased to announced that Lixue’s manuscript has just been accepted by Annals of Botany: Plants.

In this paper, we find that bluebunch wheatgrass (Pseudorogneria spicata) plants are able to recognize the identity of their neighbors even before their roots touch. The rate of their growth depends on the familiarity of their neighbor.

These exciting results suggest that plants can sense and respond to their neighbors both before and after root contact has occurred, and that neighbor recognition may play a very important role in determining the outcome of competition.


Two phases of invasion: Initial invasion (red) appears to have been in warmer climates. Later in invasion (blue) Johnsongrass has expanded to cooler climates.

On Thursday, at the Weed Science Society of America meeting in Lexington, I presented the results of a large scale survey of phenotypic differentiation in 499 accessions of Johnsongrass (Sorghum halepense) from 70 populations throughout the United States. Jacob Barney — my postdoctoral advisor — was initially scheduled to present, but I took his place when a scheduling conflict prevented his attendance.

I had a great meeting and met a lot of wonderful people. Special congratulations to Kate Venner, Sandeep Rana, and John Brewer from the lab upstairs for their recognitions and accomplishments at the meeting.