Researchers Examine How Some Invasive Plants Gain a Foothold

University of California, Riverside genetics Professor Norman Ellstrand led a team of researchers whose findings suggest that harnessing the sexual requirements of some plants can help control the establishment of invasive species.

Using the California wild radish as their model, Ellstrand and graduate student Caroline Ridley at the UCR Department of Botany and Plant Sciences co-authored the research study titled Population size and relatedness affect fitness of self-incompatible invasive plants, published in the Dec. 29 edition of the Proceedings of the National Academy of Sciences. The current article originated from a doctoral dissertation project by former UCR graduate student Diane Elam. Fellow graduate student Karen Goodell also worked on the project. Elam is now with the U.S. Fish and Wildlife Service’s Sacramento office. Karen Goodell now teaches at Ohio State University.

The experiment involved population groups ranging in size from two to 20 plants and was carried out at UCR’s Agricultural Experiment Station. The experiment examined whether a biological phenomenon known as the Allee effect could be used to battle the spread of invasive plants. The Allee effect, named after ecologist W.C. Allee, says that the smaller and sparser a given population, the harder and slower it is for that population to establish itself and expand its range.

Because the California wild radish is what plant biologist call self-incompatible – it needs another plant to reproduce – researchers wanted to know if its fertility would be impacted by population size and the degree to which the mating pairs were genetically related. They found that, as with humans, mates were harder to find in smaller populations and most likely to be compatible if unrelated.

“We call this the 'single’s bar effect,'” Ellstrand said. “Namely, that mating success increases both with the number of possible mates and their sexual receptivity.”

Using populations of full sibling, half-sibling and unrelated plants, the researchers found that both population size and the degree of relatedness had an impact on the plants’ reproductive success, increasing with larger populations as well as when the plants were more genetically diverse. Population size had the greatest effect on numbers of seeds per fruit – the larger the population size, the more seeds per fruit.

Relatedness had an impact primarily on the number of fruits per plant. Unrelated plants had significantly greater numbers of fruit than half- or full-sibling plants.

The Allee effect could therefore be used as a weapon against invasive species if, for example, authorities who regulate industries that traffic in exotic plants allow the import of only a single genotype. This could minimize the likelihood that the plant will spread by seed propagation, according to the article.

The California wild radish, Raphanus sativus, is a hybrid between the cultivated radish and a weed known as the jointed charlock. It is most commonly pollinated by bees and syrphid flies. Introduced more than a century ago, the California wild radish currently ranges throughout California, south into Baja California and north into Oregon.

Other invasive plants that need partners to reproduce and are found in California, include jointed goatgrass, Queen Anne’s lace or wild carrot, marijuana and the wild sunflower, according to Ellstrand.

The California Invasive Plant Council defines invasive plants as those that humans move from one region of the globe to another, and which then crowd out native vegetation and the wildlife that feeds on it. Some invasive plants can even change the processes of ecosystems such as the hydrology, the fire regimes, and the soil chemistry. These invasive plants often have a competitive advantage because they are no longer controlled by their natural predators and can quickly spread out of control. The council estimates that about 3 percent of the plant species growing in the wild are considered invasive, but they inhabit a much greater proportion of the state's landscape.

The following agencies supported the UCR project: The National Research Initiative; the Cooperative State Research, Education, and Extension Service; a grant from the U.S. Department of Agriculture to Ellstrand; and the U.S. Environmental Protection Agency’s Science to Achieve Results fellowship to Ridley.