Date of Award

Summer 2018

Project Type

Thesis

Program or Major

Natural Resources

Degree Name

Master of Science

First Advisor

Jenica M. Allen

Second Advisor

Thomas D. Lee

Third Advisor

Rebecca J. Rowe

Abstract

Invasive plants pose severe threats to ecosystems worldwide. Geographic invasion risk is often assessed using occurrence-based species distribution modeling, which estimates where species can occur but cannot represent variation in abundance, a crucial component of impact. I assembled an ordinal abundance dataset for 155 terrestrial invasive plant species in the contiguous United States based on reports to the Early Detection & Distribution Mapping System, a georeferenced invasive species data repository. I used maximum entropy models to estimate range boundaries and ordinal regression to estimate abundance within ranges. I predicted the areas where establishment is possible (potential establishment ranges), and where high abundance is likely (potential impact ranges) for each species, and compared where many species may establish (richness hotspots) to areas where many species may achieve high abundance (abundance hotspots). The potential impact range encompasses a small portion of the potential establishment range for many species (median: 9%). High abundance populations have not yet been observed across much of potential impact ranges for each species, indicating substantial risk for expansion. Forty-seven percent of the richness hotspots were overlapped by the abundance hotspots. Eastern Temperate Forests constituted large portions of both hotspots (80% of richness hotspots, 49% of abundance hotspots), but abundance hotspots included large areas not identified by richness hotspots, particularly in western U.S. ecoregions. The delineation of impact ranges within establishment ranges can facilitate the prioritization of management to species and geographic areas associated with the highest impact risks. Using establishment ranges alone could over-estimate geographic invasion risk in areas where many species can establish but few are likely to be abundant, and under-estimate geographic invasion risk in areas where not many species can establish, but high abundance is likely for many of the potential establishers. Given the potential for abundance data to refine estimates of geographic invasion risk, more spatial analyses of invasive plants should include abundance data, and more abundance data should be collected to allow additional comprehensive analyses with greater numbers of species.

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