Yellow starthistle (Centaurea solstitialis) is an invasive weed from the Mediterranean region. It was introduced to California accidentally over 120 years ago and was first recorded in MDSP by the pioneering botanist Mary Bowerman in 1944. It is now abundant throughout the Park's grasslands and oak woodlands and infests nearly 15 million acres statewide.

Impacts
Yellow starthistle is toxic to horses, reduces forage for cattle and native herbivores, and displaces California's native plants. Its deep taproot depletes the water available in the soil for other plants, which contributes to its tendency to form dense stands where few other species persist. Star-like spines attached to its flowerheads can be painful for humans, dogs, and wildlife to walk through.

Management
Because of these impacts, we are actively seeking to reduce the abundance of yellow starthistle throughout the Park. Doing so will help restore native ecosystems, maintain rangeland for native herbivores, and protect recreational resources.

Research
Although yellow starthistle is among California's most problematic invasive plants, relatively little is known about certain aspects of its biology. We are conducting research in the Park to learn more about two key questions: Is there an “Achilles’ heel” in its life cycle that we can target to reduce its abundance? Do native herbivores help keep its abundance in check? Yellow starthistle is an annual, which means that it germinates from seed, grows, flowers and dies in a single year. This means that a yellow starthistle infestation must regenerate itself anew from seed each year. Reducing seed production may seem like an obvious way to reduce its abundance, but because each plant can produce over 5,000 seeds, seed production would have to be dramatically reduced before it would affect the plant's abundance.

By tracking the fate of thousands of individual plants as they progress from seedling to flowering and back to seed, we will be able to determine if yellow starthistle is particularly vulnerable during any part of this life cycle. Some invasive species have a Achilles’ heel during the seedling-to-flowering transition: a small reduction in the number of plants that survive to the flowering stage results in a big reduction in the size of the infestation.

If yellow starthistle is similarly vulnerable, management targeted at this stage may enable us to reduce its abundance more effectively. Some of the study plots we are using for this research are protected by wire cages. This allows us to learn more about how native wildlife affect its abundance. Mice and chipmunks eat the seeds after they disperse from the flower and rabbits and deer consume the seedlings before they flower and produce the spines for which the plant is notorious. Our initial results indicate that native herbivores can substantially reduce survival during the potentially vulnerable seedling-to-flowering transition. In this way, native animals may keep the plant from forming even larger infestations and spreading further.

Sarah Swope
Dept. of Ecology & Evolutionary Biology, University of California, Santa Cruz
Exotic and Invasive Weeds Research Unit, USDA, Albany, CA