Biological invasions pose threats to biodiversity, habitat quality, and ecosystem function. According to the Ecological Society of America, invasive species are the second most important threat to native species – behind habitat destruction – contributing to the decline of over 40% of U.S. endangered and threatened species.
Starthistle Invasion and Biodiversity
by Jeffrey S. Dukes
Photo by Harry Brett/UMass Boston
Prickly and aggressive, scientists and land managers consider yellow starthistle (Centaurea solstitialis) the "Jesse James" of California's outlaw plants – seemingly uncontrollable invasives that range the wildlands unabated.
As a member of the genus Centaurea, starthistle is in a group called "knapweeds," collectively considered the most abundant noxious weeds in the western United States.
Native to the northeastern rim of the Mediterranean, yellow starthistle spread to other regions with Mediterranean climate as a contaminant in agricultural seed lots. Today, the annual weed infests grasslands and oak woodlands throughout much of the West, including California. This sprawling infestation causes poorly quantified, but presumably vast economic and ecological effects.
Although yellow starthistle has one genuine economic benefit – it makes good honey – its drawbacks far outweigh its benefits. Starthistle lowers the value of rangeland and the utility of grasslands for recreational use. Chemicals in the plant's tissues cause brain damage in grazing horses. In summer, the sharp spines of starthistle's flowerheads can injure incautious cattle, hikers, and mountain bikers. These properties of the weed have economic impacts that are widely appreciated, but until recently few researchers had investigated starthistle's ecological effects.
As a graduate student at Stanford under Hal Mooney, I began studying this notorious invader. Specifically, I was interested in understanding why yellow starthistle has become so successful, and how it affects other members of California's grassland communities. I also wondered whether the success of starthistle and its effects on a community might vary depending on which and how many species are in that community.
It seemed possible that the success and effects of an invading species such as starthistle would be related to prior patterns of water and nutrient use in the community. I set out to design a thesis project that would test this hypothesis, with the hope that my results would also prove useful to those interested in restoring invaded grassland communities.
To better understand how species and resources in a grassland could affect yellow starthistle growth, I created grassland communities based on a pool of 16 species common to the Jasper Ridge grasslands. This pool consisted of four species from each of four "functional groups," collections of species that have similar lifespans and morphology. I expected that species sharing these characteristics would seek the same resources in similar spaces and at similar times. I chose annual grasses, perennial grasses, spring-flowering annual forbs and summer-flowering annual forbs as functional groups. From each functional group, I chose two species to compete individually with starthistle. In addition, to examine whether several species would compete more effectively than one, I also created several communities of 4, 8, 12, and 16 species. These communities had an equal membership from each of the four functional groups.
To ensure a similar physical environment across the treatments, I planted the communities in deep pots. Each different community type grew alone, as well as in competition with starthistle. Over the course of the growing season, I monitored the availability of water and soil nutrients (such as nitrogen and phosphorus) in these pots. One set of pots was started a year early, allowing the soil to settle, the perennial grasses to become established, and a litter layer to form before the introduction of starthistle. The second set of pots, representing a disturbed community, was planted a year later, and at the time of starthistle introduction resembled the tops of gopher mounds in that the soil was recently disturbed, all plants grew from seed, and there was no litter layer.
As I had suspected, some communities suppressed yellow starthistle growth more than others. Of the eight species grown alone, the species that stunted starthistle's growth the most was a California native known as hayfield tarweed, Hemizonia congesta ssp. luzulifolia. This species, like starthistle, is a summer-flowering annual forb. Established perennial grasses were also effective at suppressing starthistle growth, but starthistle thrived in competition with perennial grass seedlings. Additionally, the experimental plant communities that had one or more species from each of the four functional groups also effectively suppressed starthistle growth.
Naturally, these answers led to further questions. Why were certain species better competitors, and why were the diverse communities also good competitors? Were the communities most successfully invaded by starthistle those that, when grown alone, had an excess of some resource? Fortunately, my experiment was designed to examine resource availability in communities that grew without starthistle.
It turned out that patterns of soil nutrient availability did not correlate with the success of starthistle in a given community type. However, there was a tight positive relationship between soil moisture in uninvaded communities during June and July and starthistle growth in paired invaded communities. This relationship strongly suggests that competition for soil moisture during the late spring and early summer limits the growth of starthistle. This makes sense-as a late-flowering annual, starthistle needs water to continue growing and reproducing. This result also supports one theory about starthistle's great success in California grasslands.
That theory, put forth by researchers in Dr. Kevin Rice's Agronomy and Range Science group at U.C. Davis, suggests that previous immigrants opened the door to the western United States for yellow starthistle. The researchers speculate that starthistle would not have been as successful in grasslands dominated by native perennial grasses, because these grasses dry out the soil to a greater extent than the introduced European annual grasses that replaced them. The researchers point out that the European species that currently dominate the grasslands leave soils wetter in the summer, making it easier for yellow starthistle to survive and reproduce.
My artificial plant communities also showed that starthistle affects grassland species differently in disturbed and undisturbed communities. In disturbed conditions, starthistle had very little effect on annual grasses, and strongly negative effects on perennial grass seedlings. Its effects on forbs were intermediate. In contrast, in undisturbed communities, starthistle had the least effect on the established perennial grasses and on hayfield tarweed, and fairly strong negative effects on most other species. This suggests that established perennial grass communities are likely to persist fairly well after starthistle invasion where there is little soil disturbance, but where disturbance is frequent, annual grasses may be better able to coexist with the invader. My results also hinted, although not conclusively, that in disturbed conditions, more diverse communities might persist better than less diverse communities.
Although starthistle decreases growth of its competitors in many cases, it does not necessarily decrease growth of the grassland as a whole. In disturbed conditions, pots that contained both starthistle and a dominant annual grass, either slender oat or soft brome, were more productive than pots that contained either the grass alone or starthistle alone. This pattern, where the mixture outproduces either of its components, is a marker of a phenomenon known as complementarity. Complementary species are thought to share resources well. In this case, the annual grasses may be using water at different times or in different locations than starthistle.
In designing this project, I hoped that people who combat the spread of yellow starthistle and other invasive problem species would find the results useful. My research has identified species and combinations of species that are likely to compete effectively with yellow starthistle in disturbed and undisturbed conditions. Ideally, restoration strategies will build on and incorporate this knowledge.
The discovery that plant competition limits starthistle's success by affecting water availability may be of use in predicting the response of California grasslands to climate. Recent results from climate models, reported on by the Intergovernmental Panel on Climate Change, suggest that western North America will experience wetter (and warmer) winters in the future. If these changes lead to wetter grassland soils in the late spring and summer, starthistle might benefit. I hope to explore this possibility in future research.
Increased prevalence of starthistle might have effects beyond those now known. Because the plant reduces summer soil moisture in grasslands and oak woodlands, the invasion may be affecting other species that depend on this resource. For instance, oak seedlings depend on summer soil moisture to become established, and may be affected by the starthistle invasion. Few studies have examined whether starthistle threatens native biodiversity, but there is evidence that the invader is putting at least one rare plant species at risk in the Agate Desert Preserve in southwest Oregon.
Outlaws like Jesse James didn't play by the rules, and neither do invasive species. While all species must compete for light, water, and nutrients, invasive species have often escaped the diseases, herbivores, or predators that helped to keep them in check in their native range. In addition, researchers in Dr. Ragan Callaway's Plant Ecology laboratory at the University of Montana are finding evidence that some other knapweeds exude chemicals into the soil that can hinder growth of North American species, while leaving species from the knapweed's native lands unaffected. Although knapweeds may pose a longer-term threat to the West than any human outlaw, there is some hope that, through vigilance and a variety of intensive management techniques, the impacts of these species can be held in check.
The starthistle invasion at Jasper Ridge Biological Preserve provides many opportunities for basic research, as well as for studies that help to address the problems created by knapweed invasions across the western United States. I hope that other researchers share my excitement about this topic, and will take advantage of these opportunities.
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