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.

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.

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.