title: Plant communities in harsh sites are less invaded: a summary of observations and proposed explanations.
abstract: Plant communities in abiotically stressful, or 'harsh', habitats have been reported to be less invaded by non-native species than those in more moderate habitats. Here, we synthesize descriptive and experimental evidence for low levels of invasion in habitats characterized by a variety of environmental stressors: low nitrogen; low phosphorus; saline, sodic or alkaline soils; serpentine soils; low soil moisture; shallow/rocky soils; temporary inundation; high shade; high elevation; and high latitude. We then discuss major categories of hypotheses to explain this pattern: the propagule limitation mechanism suggests invasion of harsh sites is limited by relatively low arrival rates of propagules compared with more moderate habitats, while invasion resistance mechanisms suggest that harsh habitats are inherently less invasible due to stressful abiotic conditions and/or increased effects of biotic resistance from resident organisms. Both propagule limitation and invasion resistance may simultaneously contribute to low invadedness of harsh sites, but the management implications of these mechanisms differ. If propagule limitation is more important, managers should focus on reducing the likelihood of propagule introductions. If invasion resistance mechanisms are in play, managers should focus on restoring or maintaining harsh conditions at a site to reduce invasibility.
authors.affiliation: Department of Plant Sciences, University of California, Davis, CA 95616, USA Present address: Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37917, USA ezefferm@utk.edu.
authors.forename: Emily
authors.initials: E
authors.lastname: Zefferman
authors.affiliation: Department of Plant Sciences, University of California, Davis, CA 95616, USA John Muir Institute for the Environment, University of California, Davis, CA 95616, USA.
authors.forename: Jens T
authors.initials: JT
authors.lastname: Stevens
authors.affiliation: Department of Plant Sciences, University of California, Davis, CA 95616, USA.
authors.forename: Grace K
authors.initials: GK
authors.lastname: Charles
authors.affiliation: Department of Plant Sciences, University of California, Davis, CA 95616, USA.
authors.forename: Mila
authors.initials: M
authors.lastname: Dunbar-Irwin
authors.affiliation: Department of Plant Sciences, University of California, Davis, CA 95616, USA.
authors.forename: Taraneh
authors.initials: T
authors.lastname: Emam
authors.affiliation: Department of Plant Sciences, University of California, Davis, CA 95616, USA.
authors.forename: Stephen
authors.initials: S
authors.lastname: Fick
authors.affiliation: Department of Plant Sciences, University of California, Davis, CA 95616, USA.
authors.forename: Laura V
authors.initials: LV
authors.lastname: Morales
authors.affiliation: Department of Plant Sciences, University of California, Davis, CA 95616, USA.
authors.forename: Kristina M
authors.initials: KM
authors.lastname: Wolf
authors.affiliation: Department of Plant Sciences, University of California, Davis, CA 95616, USA.
authors.forename: Derek J N
authors.initials: DJ
authors.lastname: Young
authors.affiliation: Department of Plant Sciences, University of California, Davis, CA 95616, USA.
authors.forename: Truman P
authors.initials: TP
authors.lastname: Young
keywords: Environmental stress; invasibility; invasive/exotic plants; native plant refuges; propagule pressure; resource availability