Native plants evolve to fight off invading species

Far from the lush forests of the American coasts is a dry, dusty place filled with scrubs and grasses. This is the Intermountain West, an area sandwiched between the Rocky Mountains and the Sierra Nevada and Cascade ranges of the West Coast. Its desert-like appearance may give the impression of lethargy, but the plants here are engaged in a pitched battle for survival.

This is the story of the little grass that could — a native species known as ‘big squirreltail’ that has been evolving rapidly enough to fight back against the menace of an invading weed. Cheatgrass, the invasive species, threatens native plants, wildlife, and ranchers and hunters, allowing wildfires to rip though millions of acres of land with ease.

The struggle in the high desert is documented in a study published online on October 29, 2010 by the journal Evolutionary Applications.
In a parched region like the Intermountain West, plants time their growth precisely to seasonal changes to make the most of their limited resources. Cheatgrass (Bromus tectorum), a resource hog, sprouts in early March — before the native grasses have started growing — and saps away nutrients and water the natives can’t survive without.

Cheatgrass then dies early. It drys out in April or May, leaving behind a layer of highly flammable remains. “Imagine a bunch of tissue paper spread across the landscape,” said Mike Pellant, a rangeland ecologist with the federal Bureau of Land Management.

This extra fuel has contributed to an increase in the frequency of wildfires from once every 30 to 40 years to once every 5, Pellant explained. The wildfires, in turn, clear the land of native vegetation and allow the cheatgrass to spread further, depositing as many as 10,000 seeds per square meter.

If cheatgrass is the villain, then the hero of our story is a grass named Elymus multisetus, or big squirreltail. Biologist Elizabeth Leger of the University of Nevada, Reno found that pockets of this native grass still hold on in the face of cheatgrass’ fiery domination. They persist by rapidly evolving in response to the cheatgrass’ presence, investing more energy in their root structure.

Roots are especially important for this species, Leger said, because in the desert, “all the resources that are contested are below ground.”

To gather their data, Leger and her team collected and weighed big squirreltail seeds from invaded and non-invaded areas all over the Intermountain West. Some of the hardiest native plants from invaded areas managed to survive in a hostile environment, and consequently passed along their favorable traits to their seedling offspring.

Leger then grew the samples in a greenhouse, and finished the process by carefully washing the roots of thousands of plants — a process Leger compares to washing hair. She and her research team gradually untangled the fragile tendrils from the soil and each other, and then recorded the size and number of the roots on a special scanner.

Leger found that while the big squirreltail shoots from invaded areas were smaller, they allocated more resources to their roots. The altered roots allow big squirreltail to suck up nutrients and suppress cheatgrass growth by as much as 30 percent, even though cheatgrass has a head start in the resource race with its earlier sprouting date. This reduction in cheatgrass could even allow other native plants to surge back into the Intermountain West.

The idea of native evolution in response to invasive species isn’t new. “Evolution is happening every day,” said Jen Lau, a biologist at Michigan State University. Biologists are “looking at a whole wealth of different species.”

However, Leger’s study is one of the first in which both species have been plants. Most research into evolution due to invasion has looked at species that are in direct contact with each other, such as flowers that adapted to grazing deer by blooming later, or native snakes in Australia that adapted to the invasive (and highly toxic) cane toad by evolving jaws that were too small to eat the poisonous amphibian. Cheatgrass and big squirreltail, in contrast, are only in contact indirectly — cheatgrass isn’t physically doing anything to the big squirreltail, just competing for the same resources.

Leger’s work might do more than expand our evolutionary knowledge base. Government ecologists like Pellant hope to eventually use this work to reintroduce hardier native plants to the region in order to reduce wildfires and bring back the native biodiversity of the Intermountain West.

Previously, ecologists reintroduced grasses like big squirreltail using seedlings from uninvaded areas. These seeds failed to thrive after transplanting, partially because they were not evolutionarily equipped to handle a cheatgrass-dominated environment.

Even under the most optimistic scenario, it could take decades to restore the Intermountain ecosystem to what it looked like in the 1800s, before cheatgrass was introduced in contaminated grain bags from Eurasia. Leger plans to do more studies, seeking out new pockets of resistance in the West in an effort to find populations that are “the best of the best.” Then she must wait for the seeds to grow — a long process, as big squirreltail takes up to two years to reproduce. Eventually she hopes to reintroduce seedlings into cheatgrass-imprisoned areas and help them lead the fight to take back their land.

“All our native species evolved in the great basin without cheatgrass. When it came in, it just upset the apple cart,” said Pellant. “I’d like to get our native plants back.”