I 



17 

 H. POPULATION ECOLOGY 



1. BIOLOGICAL INTERACTIONS 



a. COMPETITION: No studies have been done on 

 competitive interactions, either 

 intraspecific or interspecific. However, the 

 frequent occurrence of C. longistvlum in 

 disturbed sites and in open, unshaded areas 

 suggests that it is not very tolerant of 

 shading. It would likely do poorly on sites 

 with closed canopies. 



b. HERBIVORY: The seed heads of some plants in 

 the Little Belt Mountains have been attacked 

 by a weevil, Rhinocyllus conicus, which was 

 introduced to North America from Europe as a 

 biological control agent for Carduus nutans 

 (Rees 1982, 1987). The weevil was introduced 

 in several locations including Bozeman 

 Montana in the early seventies. Within 

 several years, the weevils had moved out 

 within a 500 miles (800 km) radius of the 

 original establishment location. Weevil 

 infestation rates, and the likely effect on 

 seed production and population viability are 

 unknown, although preliminary studies have 

 been begun. Dr. Charles Turner (USDA, 

 Albany, California, pers. comm.), a 

 specialist in the fauna of members of the 

 Asteraceae (Sunflower Family) , has recently 

 completed a study of the weevil's impact on a 

 rare thistle native to California. Although 

 the data have not been completely analyzed, 

 it was his opinion that the weevil was having 

 little effect on this rare thistle that is 

 restricted to serpentine warmspring sites. 



He also made the observation that new world 

 members of the Cirsium genus were very 

 depauperate in herbivorous fauna in 

 comparison to old world species. It should 

 be noted that the weevil has infested a 

 number of native species in the genus Cirsium 

 (Turner et al. 1987) . 



While the rate of seed predation is not clear 

 in the case of C. lonqistylum , it is a matter 

 of concern because seed predation can have a 

 magnified effect on succeeding life history 

 stages, resulting in greatly reduced seedling 

 establishment and recruitment of new plants 

 (Louda et al. 1990) . Herbivory of other 



