Invasion in a diversity hotspot: exotic cover and native richness in the Californian serpentine flora

• Published in: Ecology (Durham) 2006-03, Vol.87 (3), p.695-703
• Main Authors: Harrison, Susan, Grace, James B., Davies, Kendi F., Safford, Hugh D., Viers, Joshua H.
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biodiversity; Biological and medical sciences; California; Conservation of Natural Resources; Demecology; diversity hotspot; Ecological invasion; Ecological modeling; Ecosystem; endemic plants; Endemic species; Environment; exotic richness; Fundamental and applied biological sciences. Psychology; General aspects; Herbs; indigenous species; Introduced species; invasive species; Models, Biological; native richness; Native species; Overstory; path models; Phylogeny; Plant Development; plant ecology; Plants; Plants - classification; Population Dynamics; refuge habitats; SEM; Serpentine; serpentine flora; Serpentine soils; Soil - analysis; Soils; species diversity; Species Specificity; structural equation modeling; weeds
• ISSN: 0012-9658; 1939-9170
• DOI: 10.1890/05-0778

Exotic species have been observed to be more prevalent ill sites where the richness of native species is highest, possibly reflecting variation among sites ill resources, propagule Supply, heterogeneity, or disturbance. However, Such a pattern leaves unclear whether natives at species-rich sites are Subject to especially severe impacts from exotics as a result. We considered this question using path models in which relationships between exotic cover and native richness were evaluated ill the presence of correlated environmental factors. At 109 sites on serpentine soils across California, USA, exotic cover was positively correlated with total native herbaceous richness and was negatively correlated with the richness of both serpentine-endemic and rare native herbs. However, in path models that accounted for the influences of soil chemistry, disturbance, overstory cover, and regional rainfall and elevation, we found no indication that exotic cover reduced any component of native herb richness. Rather, Our results indicated similarities and differences in the conditions favoring exotic, native, endemic, and rare species. Our results suggest that, in spite of some localized impacts, exotic species are not exerting a detectable overall effect on the community richness of the unique native flora of Californian serpentine.