Resource: Leafy Spurge

It's happened time and time again: A species hitches a ride into a new ecosystem and throws it into disorder. Zebra mussels have infested the Great Lakes, house cats are driving small marsupials to extinction in Australia, and a weed called leafy spurge is devastating native grasses all over western North America.

A cousin of the familiar Christmas poinsettia, leafy spurge (Euphorbia esula) is a perennial weed that spreads extremely fast, both by underground roots that send up new plants and by long-lived seeds that literally explode out of their capsules. In its native ecosystems in Europe and Asia, spurge populations are held in check by various pests and parasites, including insects (mainly flea beetles) that coevolved to feed on the plant.

If spurge populations increase, the flea beetle populations expand to take advantage of the excess food supply. Then, as the supply of spurge decreases, so does the flea beetle population. Yet even though they share a very close ecological relationship, leafy spurge and flea beetles do not exist in isolation. Numerous other organisms are also at work keeping spurge and beetle populations in balance in the larger ecosystem.

Until the early 1800s, leafy spurge did not grow in the United States. When the first seeds arrived, as contaminants in ships' ballast water and in crop seed shipments, leafy spurge found itself in an ecosystem without any flea beetles to keep it in check. The weed, aided by human transport and its own rapid growth and dispersal, soon made its way across the Plains states of the U.S. and into most of western Canada.

Farmers and ranchers began to notice that this new weed was outcompeting both wild and cultivated plants. In no time, leafy spurge could take over entire fields, killing off the resident plants by sucking up water and nutrients faster, and by producing toxins that prevented other plants from growing near them. To make matters worse, cows wouldn't eat spurge because it contains a toxic latex sap that tastes bitter and can kill them if they ingest enough of it. So leafy spurge rapidly became plant enemy #1 in farming and ranching areas.

Until the late 1980s, the only weapons against leafy spurge were physical or chemical: pulling up the weeds, burning them, or dousing them with herbicides. None of these methods was very effective, some injured other plants, and the spurge kept spreading. But in 1989, growers turned to a biological weapon that has proven to be the most effective one yet: imported flea beetles.

Where they have been introduced, flea beetles have reduced spurge coverage by up to 90 percent. From an ecologist's perspective, using one introduced species to control another starts a potentially dangerous cycle. No one knows what may happen if the flea beetles run out of spurge: In a new ecosystem, they may find other native plants that suit their taste. And then, someone will likely suggest bringing in another exotic species to take care of the flea beetles. But hopefully this won't be the case. All anyone can do is wait and see.

What is leafy spurge? How did it get to the US? Why is it considered a "biological invader"?

What gives leafy spurge an ecological advantage over native plant species?

Cattle ranchers and farmers are introducing the flea beetle to areas with leafy spurge. Why are they doing this? How does the beetle affect leafy spurge?

Do you think that the introduction of another non-native species is a good strategy to control a biological invader? Why or why not?