A New Invader: "Villainous" Ventenata and the Invasion of a Native Ecosystem

Wispy inflorescence of Ventenata dubia in foreground

In an earlier post I discussed how a group of winter annual grasses are spreading like wildfire in the vast Sagebrush ecosystem in the western part of the USA. 

This armageddon is destroying not only the pristine sagebrush shrubs which are critical to numerous animals like the sage grouse, but it also is significantly lowering the forage value of the land as palatable perennial grasses are replaced by the mostly unpalatable invasives.

In addition to earlier invasives like Bromus tectorum (cheatgrass) and Taeniatherum caput-medusae (medusahead grass), a newer species has been creating problems for land managers.

Distribution of V.dubia (EDDMaps, 2022)

Ventenata dubia was first discovered in Washington State in the early 1950s.  It is native to Southern Europe and North Africa, and it has rapidly spread at the rate of 1.2 million hectares per year (Native Invasive Plant Council, 2001), which means an area three times the size of Rhode Island is being invaded every year! It now is found in 11 states and 5 Canadian provinces, and land managers are frantically trying to slow its spread while it is still possible to do so.

Invasion by Ventenata dubia (photo credit: Novak et al)

Like other winter annual grasses that have caused havoc in the local ecosystem, V. dubia tends to create a so-called grass-fire cycle, where its presence increases the probability of fires, which in turn removes its competitors and allows it to overran the area. Another way that winter annuals can outcompete other plants is by germinating in fall or early winter, and senescing by mid-summer. This gives it a head start when it comes to taking advantage of the early spring moisture.

The grass-fire cycle of T. caput-medusae

This species has only recently arrived in Montana, and while botanizing in a local park in the city of Bozeman, I stumbled on a small cluster hidden amongst the other grasses.

On first look, the grass does not exactly inspire dread. Unlike T. caput-medusae with its alien looking inflorescence, or B. tectorum with its multitudes of sharp spear-like awns, V. dubia looks like a puny weakling that would roll over with the smallest puff of wind.

The sharp spear-like awns on B.tectorum is more intimidating than the diffused panicles of V. dubia

The inflorescence of this species is a spread out panicle, and its thin culm ends at the base with very shallow roots. But its outward appearance belies the heart of a bully. Some studies have shown that invasion by V. dubia in hay fields can result in losses of up to 50% (Prather and Steele, 2009), and like other hegemonic invaders, the diversity of native species suffers when it starts taking hold in an area (Jones et al, 2020).

Ventenata dubia whole plant. Notice weak shallow roots

Even more frightening is the fact that this new invader seems to be able to successfully dominate habitats that earlier invasives like T. caput-medusae and B. tectorum cannot. In one study researchers found that it had invaded dry forest, woodland, and forest scablands ranging from 1250-1665 m of elevation, places which previously had been relatively immune to winter annual grass invasions. This is important to note, because it means that the spread of this species has increased the overall footprint of invasive annual grasses in the already battered Western USA (Applestein et al, 2022; Tortorelli et al, 2020).

Very long ligule of V. dubia and the remnant dark red color of a node

I must admit I was lucky in finding the small cluster in that park in Bozeman, MT. The small size of V. dubia and its open diffuse panicles made it hard to distinguish from the masses of other dried grasses that dominated the landscape. 

But the specimens I found had panicles with horizontal branches that almost diverged at 90 degree angles, and the spikelets (the structure that holds the grass flowers) on the panicles were restricted to the ends of these branches, both of which are characteristics of the species. A quick check of the ligules (the membrane between the top part of a blade and the culm) also pointed to them as being V. dubia: the species has relatively long and large ligules. The very shallow roots of one specimen that I carefully uprooted is another characteristic of V. dubia, as is the dark nodes that were red earlier in its life cycle.

Dried spikelets of V. dubia with ribbed glumes and lemma with long awns.

Magnification also offered other clues as to their identity. The dried spikelets had no seeds, but each spikelet had a conspicuous lemma (a bract like  structure enclosing a flower) with a protruding awn, and the glumes (the two bracts enclosing the spikelet) were distinctly ribbed along the long axis.

The small infestation that I found may not remain so small, given the nature of V. dubia. But perhaps the much larger grasses around it will contain its spread, or perhaps it will be eradicated after its presence has been reported. Only time will tell.

As a last note, the spread of V. dubia is another major problem in an endangered ecosystem that many people don't even know about. But there is nothing "evil" or even "villainous" about the species, notwithstanding the title of the post. We tend to anthropomorphize other organisms when in the heat of emotion, but this species (like all the other "weedy" plants) is simply doing what it needs to do  in order to survive and prosper, with no malice in whatever green heart it has.

Literature Cited

Applestein, C., Germino, M.J. Patterns of post-fire invasion of semiarid shrub-steppe reveals a diversity of invasion niches within an exotic annual grass community. Biol Invasions 24, 741–759 (2022). https://doi.org/10.1007/s10530-021-02669-3

EDDMapS. 2022. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/; last accessed August 26, 2022.

Jones, L., Davis, C., & Prather, T. (2020). Consequences of Ventenata dubia 30 years postinvasion to bunchgrass communities in the Pacific Northwest. Invasive Plant Science and Management, 13(4), 226-238. doi:10.1017/inp.2020.29

Stephen J. Novak, Massimo Cristofaro, Dorothy Maguire1, and Rene F.H. Sforza. The Invasive Grass Ventenata (Ventenata dubia): A New Threat for Nevada

Prather, T. S., and V. Steele. 2009. Ventenata control strategies found for forage producers.

Tortorelli, C, Krawchuk, M, Kerns, B. Expanding the invasion footprint: Ventenata dubia and relationships to wildfire, environment, and plant communities in the Blue Mountains of the Inland Northwest, USA. Appl Veg Sci. 2020; 23: 562– 574. https://doi.org/10.1111/avsc.12511

What Strange Beasts: The Hidden Armageddon in Sagebrush Country

What strange beasts slither across the hidden landscapes?

There is a war going on. 

It is a war with devastating consequences and monumental importance, but one which slides below the attention of most people who otherwise care about the natural state of the world. It is a war of a scope that beggars the mind, where an entire ecosystem that has existed for millennia is rapidly being destroyed.

Sagebrush covers a huge swath of the American West

The sagebrush ecosystem in the Great Basin of the Western USA is characterized as having a shrub overstory of Artemisia spp, with sagebrush steppe areas having a large component of perennial grasses and forbs, and sagebrush shrublands dominated by larger sage species with fewer grasses and forbs. It spreads over 11 western states and occupies 62 million hectares, and it is home to a multitude of plant and animal species, some of which are threatened or endangered.

Artemisia tridentata (Big Sagebrush). From Wikipedia. 
By Peemus - Own work, CC BY-SA 3.0 

It is in this gargantuan stage that a fight to the death has been ongoing for decades, although mostly hidden from the eyes of people. Hordes of invasive annual grasses are sweeping through the landscape like a scythe, rapidly converting sagebrush into a sea of low lying grasses. One study estimates that more than 200,000 ha of sagebrush are destroyed and replaced by exotic annual grasses every year (Smith et al, 2021). This is a mind boggling number, an area that is twice the size of Los Angeles and nearly triple the land area of New York City!

Intact sagebrush habitat. Wikipedia. By Famartin

Conversion to a sea of exotic annual grasses

Yet typical invasive species groups in social media almost never mention the rapid destruction of these expansive ecosystems, and perhaps this is understandable. People tend to focus on invasives that are closer to home, and those that blight their day to day lives or their recreational activities. The sparsely inhabited and pristine sagebrush areas are thus a blind spot when it comes to most. The most ferocious winter annual grasses that are swallowing up the Great Basin are relative unknowns, and include species such as Bromus tectorum (cheatgrass), Taeniatherum caput-medusae (medusahead grass), and Ventenata dubia. 

Taeniatherum caput-medusae (medusahead grass) 

Yet the consequences of our blindness may just be as dangerous as the spread of more well known invasives in the suburbs and rural areas. Not only does the sagebrush ecosystems support major endangered species like the Sage Grouse, but this annual grass invasion results in major economic losses as rangeland is lost. 

 

Bromus tectorum (cheatgrass) sprawls along hillside in Bandelier National Monument in New Mexico

In addition, the annual winter grasses use fire as a way to increase and maintain their stranglehold on the land by increasing the frequency and intensity of wildfires (see diagram below). This makes their spread dangerous for human lives and habitation, and in fact these grasses contribute to the huge wildfires that are engulfing the American West and destroying not only our homes but irreplaceable national treasures as well, such as the iconic and ancient Sequoias trees. 

How one winter annual invasive achieves dominance

It is far too late to eradicate these annual winter invasives, and efforts to reclaim land dominated by them have failed to yield fruitful results on a large scale, so land managers are now exploring various other options to mitigate their spread by focusing only on areas that have so far escaped this armageddon  (Maestas et al, 2022). 

We can only hope that it is not too late.

Bromus tectorum

Aegilops truncialis

Maturing Taeniatherum caput-medusae

Literature Cited

Jeremy D. Maestas, Mark Porter, Matt Cahill, Dirac Twidwell (2022). Defend the core: Maintaining intact rangelands by reducing vulnerability to invasive annual grasses, Rangelands, ISSN 0190-0528, https://doi.org/10.1016/j.rala.2021.12.008.

Smith, J.T., Allred, B.W., Boyd, C.S., Davies, K.W., Jones, M.O., Maestas, J.D., Morford, S. L., Naugle, D.E., 2021. The elevation ascent and spread of exotic annual grasslands in the Great Basin, USA. bioRxiv 2021, 425458