myth of plant species saturation

• Published in: Ecology letters Vol. 11; no. 4; pp. 313 - 322
• Main Authors: Stohlgren, Thomas J, Barnett, David T, Jarnevich, Catherine S, Flather, Curtis, Kartesz, John
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford, UK : Blackwell Publishing Ltd 01-04-2008; Blackwell Publishing Ltd; Blackwell
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biodiversity; Biological and medical sciences; Biotic resistance; Botany; colonizing ability; competitive exclusion; Demecology; Ecology; Flowers & plants; Fundamental and applied biological sciences. Psychology; General aspects; Geography; habitat heterogeneity; indigenous species; introduced plants; invasion; invasive species; Nonnative species; Northwestern United States; plant communities; plant competition; plant diversity; Plants; saturation; Southwestern United States; spatial variation; species richness; temporal variation; Time Factors; Northwestern United States; Southwestern United States; Pacific Northwest; Competition; plant diversity; Competitive exclusion; Diversity; Biotic resistance; Invasion; saturation; Heterogeneity; Plant; Sensitivity resistance; Population dynamics; Habitat; habitat heterogeneity; Species richness
• ISSN: 1461-023X; 1461-0248
• DOI: 10.1111/j.1461-0248.2008.01153.x

Plant species assemblages, communities or regional floras might be termed 'saturated' when additional immigrant species are unsuccessful at establishing due to competitive exclusion or other inter-specific interactions, or when the immigration of species is off-set by extirpation of species. This is clearly not the case for state, regional or national floras in the USA where colonization (i.e. invasion by exotic species) exceeds extirpation by roughly a 24 to 1 margin. We report an alarming temporal trend in plant invasions in the Pacific Northwest over the past 100 years whereby counties highest in native species richness appear increasingly invaded over time. Despite the possibility of some increased awareness and reporting of native and exotic plant species in recent decades, historical records show a significant, consistent long-term increase in exotic species (number and frequency) at county, state and regional scales in the Pacific Northwest. Here, as in other regions of the country, colonization rates by exotic species are high and extirpation rates are negligible. The rates of species accumulation in space in multi-scale vegetation plots may provide some clues to the mechanisms of the invasion process from local to national scales.