The Nature of Invasion

by Jacoba Poppleton and Anna McEntire

USU grad student explores why grass is mightier than the sword.

It’s a brute force mechanism of entry, and, to the detriment of many hosts, it works.

Invasion occurs when a foreign entity encroaches on a native one, like when a tumor attacks healthy tissue, a country conquers its neighbor or a computer virus corrupts an operating system.

And, as the tone of the word indicates, invasion most often inflicts negative–and sometimes devastating–effects on the native system.

“Invasion is a huge but often overlooked environmental problem of today,” said Joanna Hsu, a graduate student researcher at Utah State. “In the Intermountain West, one of the biggest and most successful invaders in cheatgrass, a plant native to Eurasia.”

When cheatgrass invades, it takes over the space between sagebrush and bunchgrasses–the dominant native plants in the region. Unlike the native vegetation, cheatgrass has a lifespan of less than one year. For a cheatgrass population to increase from one year to the next, all individual plants must be replaced by new plants, and then some. Certainly, cheatgrass has been successful at this task: in the United States, this invader has taken over at least 31.5 million acres, an area about the size of North Carolina.

“The question I will study isn’t really about cheatgrass itself–ecologists already know a lot about cheatgrass life history and economy,” said Hsu.  “I’m interested in cheatgrass populations, and how they manage to sustain their large, unwelcome presence  in the Intermountain West. the biological and economic costs of plant invasion make it a question well worth answering.”

A master’s degree candidate in ecology at USU, Hsu has been awarded a prestigious graduate fellowship to study the problem. She is one of four USU students honored with the National Science Foundation’s Graduate Research Fellowship, and she’ll receive support for three years of graduate study at any accredited institution, as well as a one-time international travel allowance.

“I’m very excited about receiving the NSF fellowship, which will give me the opportunity to pursue the research tht interests me most,” said Hsu.

And that research has urgent implications: cheatgrass has the potential to wreak havoc on native ecosystems.

“In areas invaded by cheatgrass, ground that used to be bare is now occupied by a continuous, highly flammable fuel bed,” said Hsu. “Because of this, these ecosystems are burning a lot more frequently–every three to five years, instead of every 30 to 100 years. Post-fire, cheatgrass usually does much better than its native counterparts.”

“I’m interested in cheatgrass populations, and how they manage to sustain their large, unwelcome presence in the Intermountain West.”

With such frequent fires, Hsu says sagebrush and bunchgrasses may soon disappear from the ecosystem, leaving a one-species culture of cheatgrass. The foreign invaders win; the native ecosystems lose.

Hsu’s proposed research focuses on how climate change could affect cheatgrass invasion.

“Some ecologists think that plant invaders in arid ecosystems are more successful in years of high rainfall, when they don’t have to compete for water with native species as much,” said Hsu. “If this is true, then climate change could have implications for cheatgrass invasion because it is leading to more very wet and very dry years.”

Will climate change promote cheatgrass invasion? An increase in the frequency of wet years should help cheatgrass invade, but an increase in the frequency of dry years should hold the invader back. According to Hsu, the answer hinges on how the growth rate of cheatgrass populations varied based on soil moisture levels. In her NSF fellowship application, Hsu proposed to measure the growth rates of cheatgrass populations in soils with different moisture content.

The climate change, cheatgrass project is Hsu’s latest research idea, but Hsu is already studying how climate change could impact ecosystems. For her master’s thesis research, she uses long-term data sets from around the globe to study how changes in precipitation patterns could impact primary production, the biomass plants produce through photosynthesis.

“Primary production–how much green stuff plants are making–is very important,” said Hsu. “As the base of the food chain, it supports all life in ecosystems.”

Hsu has found that primary production in drier regions is more sensitive to precipitation than primary production in wetter areas. Climate models project two types of changes in precipitation: changes in average rainfall and increases in the variability of rainfall over time.

“My results indicate that average levels of primary production are much more sensitive to changes in average rainfall than to changes in rainfall variability,” said Hsu. “However, the variability of rainfall could have a large effect on the variability of primary production.”

“The central goal of global change ecology is to predict how ecosystems are being affected by various types of global change, most of which are caused or exacerbated by humans,” said Hsu. “I hope that my current and future research contributes to this goal.”

USU Home to Four NSF Grad Fellows

In addition to Hsu, three other USU Alumni received the most prestigious of graduate fellowships:

Jan Marie Andersen, Physics

Following graduation from USU with a bachelor’s degree in mathematics and physics, Andersen is now at Boston University, investigating low-mass stars called M-dwarfs that, from Earth, are barely perceptible to the naked eye. Her research affords her opportunities to study data collected from the world’s great observatories, including the Nordic Optical Telescope at Roque de los Muchachos Observatory in Spain’s Canary islands and Las Campanas Observatory in Chile.

Nathan Carruth, Physics

Carruth, who earned a bachelor’s degree in mathematics and physics from USU in 2007 and was completing a master’s degree in physics at time of application, is studying whether it is possible form time to be discrete–if it is necessarily continuous. Tackling these questions is related to a central problem in theoretical physics that has puzzled theorists for decades: how to unify quantum mechanics with general relativity.

Melissa Jackson, Geology

Jackson, who graduated last May as valedictorian of USU’s College of Science, steps back in time to shed light on the origin of rock art in the Colorado Plateau. With faculty mentor Joel Pederson, she conducted research using optically stimulated luminescence (OSL) dating at USU’s lab to refine estimates of the age of alluvial terraces in Horseshoe Canyon in Utah’s Canyonlands National Park.