How to be a Really Successful Weed? Break the Rules

Most plants face trade-offs in how they grow and survive. For example: Do I grow fast to beat the competition, or grow more slowly and save resources to deal with potential stress later on?

But some invasive plants don’t play by the rules. Instead, they combine characteristics that normally come in an either-or relationship, one at the expense of the other. These unusual combinations may help explain why some species become especially successful weeds.

In a new paper published in Trends in Ecology & Evolution, UC Davis plant ecologists Justin Valliere and Jennifer Funk propose a framework for understanding how some invasive weeds escape the either-or trade-offs that rule native species. Their approach could help us improve efforts to predict and manage plant invasions.

“We often think of invasive plants as succeeding because they're really good at one thing, like growing quickly or dispersing their seeds,” Funk said. “But some invaders appear to combine abilities that usually come at a cost to one another.”

“This idea could have practical value for making restoration efforts more successful,” Valliere added. “It could help us identify native plants with similar combinations of traits as the invaders that could be good candidates for planting in invaded areas.”

Valliere is an assistant professor of Cooperative Extension in the department specializing in ecological restoration and invasive plant management, and a co-founder of UC EcoRestore. Funk is a professor in the department who focuses on ecology in the context of restoration and environmental change.

Three rules and three rule-breakers

In their review of published studies, Valliere and Funk focused on three classic either-or trade-offs: 

• Competition vs. colonization: Do I make big seeds that help my developing seedlings be competitive, or do I make lots of little seeds to increase their chances of dispersing far and landing in a good spot?
• Growth vs. stress tolerance: Do I make tissues that help me get resources and grow, or do I make tough, durable tissues that help me fight stress?
• Growth vs. defense: Do I make tissues that help me get resources and grow, or do I invest limited resources in making chemicals or building structures that protect me from things that want to eat me?

To understand how some non-native plants “escape” the bounds of these and other trade-offs to become “superinvaders,” the scientists compared:

• Non-native plants that become invasive with non-native plants that don’t.
• Native and non-native plants living in the same area.
• The habitat of invasive plants with their original habitat.
• Plant communities in different parts of the invaded area, such as the center of the invasion compared with the expanding edge.

Here are three examples of “superinvaders” that defy classic trade-offs made by most plants:

Star thistle (Centaurea solstitialis): Take the yellow star thistle, for example, which breaks the rule pitting competition against colonization: Plants that are good at dispersing their seeds and colonizing disturbed habitats are often weaker competitors. On the other hand, strong competitors may be less effective at spreading to new locations. But yellow star thistle does both really well.

Originally from Southern Europe, Western Asia and Northern Africa, yellow star thistle grows in disturbed soils and, in its native zones, is typically considered a minor annoyance. But in the United States, it’s a noxious weed on more than 12 million acres in California alone, causing havoc on cattle rangelands and other habitats. It’s a big headache in South America and Australia, too, Valliere and Funk wrote.

Yellow star thistle has seeds built to get picked up and dispersed by the wind – good for colonization. Usually, colonizing plants would have small seeds, but the yellow star thistle has relatively large seeds, allowing seedlings to emerge faster and compete more effectively.

Common stork's-bill (Erodium cicutarium): Plants usually trade vigorous growth for tolerating stress. But not this plant. Brought to the Sonoran Desert in the American Southwest by the Spanish in the 1700s, this robust, lavender-flowered member of the geranium family is transforming the native plant community. 

How? In the desert, where water is scarce and fast growth is risky, native plants usually grow more slowly and use water more efficiently. The common stork's-bill breaks this rule with both fast growth and high water efficiency.

Common mullein (Verbascum thapsus): Usually, plants put a lot of energy into avoiding getting eaten and defending against disease – energy that could be spent on growth and reproduction. It’s the growth-defense trade-off. However, the researchers found that, in places where common mullein has been introduced, it grew faster and was more competitive than mullein plants their native range.

At the same time, the plants in new areas maintained their defenses against herbivores.

“This suggests that some invaders may evolve combinations of rapid growth and strong defense after they’re introduced into a new region,” Funk said.

Solve current problems, predict future threats

All this points to the value of considering trade-off “escape” when planning to restore areas.

"The fact that some of the world's most problematic invaders appear to escape key trade-offs suggests this may be an important mechanism underlying their success,” Valliere noted.

“If we focus on individual traits, such as growth rate or drought tolerance, we may miss what makes some invasive plants so successful,” Funk added. “On the other hand, if we consider combinations of traits, we may be better able to predict which plants will become invasive and identify native plants that can compete more effectively during restoration.”