None identified in literature. Dead skeletons remain standing throughout the summer, often along roadways or other human corridors. These skeletons may serve to carry human-ignited fires into neighboring grasslands or shrubs. This could increase fire frequency in some grassland areas. Mostly found in disturbed areas and along roadsides and fencelines. Not very common in wildlands. No mention of increases in fire frequency is made in the literature. My observations are just that.

Personal observation, Brianna Richardson brichardson@cal-ipc.org.

Dense clumps provide shelter for rabbits. S. marianum invasion increases the food supply (seeds) for rodents and birds. More than 47 species of insect have been found to feed or reproduce on S. marianum in southern California. At least 26 of these insects are economically damaging crop pests. S. marianum can be toxic to sheep and cattle, especially under wet conditions or times of high soil moisture. Though potentially poisonous to grazers (deer) no mention was made of grazers eating S. marianum. It is believed to be unpalatable unless damaged by cutting or herbicide application. No other significant impacts on higher trophic levels were mentioned. Though S. marianum can push out native plant cover (thereby impacting wildlife) this is usually limited in scale [see 1.2].

Plants are larger, have more flowering heads, and a higher number of achenes per head in nutrient-enriched sites like waste places and ruderal zones, than those that grow in un-enriched sites. Plants are less likely to colonize and thrive in sites undisturbed by human activity.
Occurs in pastures, waste lands, irrigation banks.
Seedlings prefer disturbed soil, and don't establish in perennial pasture if the soil is well covered with vegetation during late summer and autumn.
Requires disturbance to spread. Infestations will remain localized unless disturbance becomes more widespread.
Grows on ant hills and rodent burrows. Requires human or animal disturbance to establish and spread.

Increasing but not rapidly. Plants are larger, have more flowering heads, and a higher number of achenes per head in nutrient-enriched sites like waste places and ruderal zones, than those that grow in un-enriched sites. Plants are less likely to colonize and thrive in sites undisturbed by human activity. Requires disturbance to spread. Infestations will remain localized unless disturbance becomes more widespread. Though the plant is not usually targeted by management efforts, its requirement for disturbance and its preference for nutrient rich soils limits its spread.

Hay contamination is a common problem in Victoria, Australia. Seeds disperse long distances with human activities, often found along trail margins in grassland and chaparral. Long distance transport is probably rare though. Human dispersal does spread S. marianum, but the means (as a seed contaminate, by trail users) are less troublesome than if the plant were sold horticulturally or used in revegetation work.

1) DiTomaso, J., E. Healy. unpublished. Weeds of California and Other Western States.
2) Wheatley, W.M. 1971. Thistles--Prickly problem of pasture improvement. The Agricultural Gazette of New South Wales.

Invades pastures, ruderal areas, and ant hills in Australia, Tasmania, Israel. Also found in Alabama, Arizona, Arkansas, Connecticut, Indiana, Lousiana, Maryland, Massachusetts, Michigan, Nevada, New Hampshire, New Mexico, New York, Ohio, Oklahoma, Oregon, Pennsylvania, Tennessee, Texas, Virginia, Washington, West Virginia. S. marianum is an invader in other Mediterranean climates, but largely invades habitat similar to the habitat it is already found in here.

1) Gabay, R.; U. Plitmann; A. Danin. 1994. Factors affecting the dominance of Silybum marianum L. (Asteraceae) in its specific habitats. Flora v. 189: 201-206.
2) Parsons, W.T. 1973. Noxious Weeds of Victoria. Inkata Press, Melbourne.
3) Plants Database http://plants.usda.gov

Widely naturalized in California, particularly in coastal counties, but also inland. First recorded in CA in 1854. Common in coastal and inland valleys. Is found in at least 3 major ecological types and 7 minor ecological types.

1) Klinger, Rob. 1999. Weed Report: Silybum marianum. TNC Wildland Weed Survey.
2) Goeden, R.D. 1971. The phytophagous insect fauna of milk thistle in southern California. Journal of Economic Entomology. V.64, no 5: 1101-1104.
3) Roche, Cindy. 1991. Weeds: Milk thistle. Pacific Northwest Extension Publication #382.
4)San Francisco Estuary Institute: www.sfei.org/wetlands/Reports/Final%20Draft%20Plant%20speci~000.pdf
5) Los Angeles CNPS chapter. Invasive Plant List: www.lacnps.org
6) Catalina Island Conservancy www.catalinaconservancy.org
7) Plants of the Kaweah River Delta Region www.kaweahoaks.com
8) Big Creek Reserve plants http://www.redshift.com/~bigcreek/index.html