Invasive species are one of the major drivers of biodiversity loss, and it is estimated that invasive species cost billions in damage per year, globally. Given the economic costs and the potential ecological consequences of invasive species, it is important to understand how introduced species become integrated into natural communities and the consequences of invasion over longer time scales. To better predict and prevent future invasions, we must identify the mechanisms driving a small proportion of introduced species to become invasive. Biotic interactions, such as herbivores and competitors, are among the major drivers of plant community structure and population dynamics. Release from antagonistic biotic interactions during the process of introduction may drive the explosive population growth rates of invasive species when they are transported to new ranges. However, subsequent acquisition of novel biotic interactions in the introduced range could explain why so many of the plants introduced around the world fail to become invasive. The Enemy Release Hypothesis (ERH) is one of the leading hypotheses explaining the success of invasive species and states that species once controlled by antagonistic biotic interactions in their native range will be able to reach high abundances once released from this control.My dissertation research takes an integrative approach to rigorously test the oft-cited Enemy Release Hypothesis. Using field experiments including over 50 plant species, and a meta-analysis of the published literature to test ERH across a wider range of environments and species, I address four main questions: (1) Do invasive, noninvasive exotic, and native species experience different amounts of damage from enemies in the introduced range? (2) Does enemy release result in increased performance for invasive species compared to native and noninvasive exotic species? (3) Is enemy release lost with increased residence time and geographic spread in the introduced range? (4) Does tolerance to enemy damage or competitive ability drive invasiveness?I found no evidence suggesting enemy release is a general mechanism contributing to invasiveness. Invasive species received the most damage from enemies and were equally affected by the presence of antagonistic biotic interactions, compared to native and noninvasive exotic species. Invasive species were no more tolerant to enemy damage than were native or noninvasive exotic species. For both invasive and noninvasive introduced plants, damage and the performance effects of that damage, increased with longer residence times and larger areas of spread in the introduced range. Our results show that invasive and exotic species fail to escape enemies, particularly over longer temporal and larger spatial scales. Key differences between introduced species that become invasive and those that do not may be the formation of successful mutualisms in the introduced range, and release from competition compared to native and noninvasive exotic species in the introduced range.
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