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.
Highlights
Discerning the patterns and underlying causes of plant invasions is a central goal of invasion ecology
While we have provided many examples that support the hypothesis that native species are superior competitors in harsh sites, this pattern is not consistent
The descriptive and experimental evidence from a variety of ecosystems summarized in this paper show a pattern of lower plant invadedness in harsh sites
Summary
Discerning the patterns and underlying causes of plant invasions is a central goal of invasion ecology. Despite the general pattern of lower relative performance of invasive species in highly shaded terrestrial environments, a number of studies have shown that certain invasive species outperform natives under all light conditions (Pattison et al 1998; Zheng et al 2009), and that increasing shade does not necessarily reduce invader abundance (Baars and Kelly 1996; Cabin et al 2002). High-elevation regions are characterized by low temperatures, a short growing/productive period (daily or seasonal), high UV exposure, low available soil nutrients, increased water stress and, in some locations, daily freeze-thaw cycles These stressors may help explain the observation that high-elevation areas have relatively few non-native species (Alpert et al 2000; Rejmanek et al 2013). These examples may more appropriately relate to the hypothesis of speciessite mismatch (see above), but in the context of well-suited genotypes rather than species
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