Final Report - Center for Invasive Plant Management

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ProposalAbstractStudies integrating population genetic with demographic data are needed toaddress a fundamental paradox of biological invasions: if genetic bottlenecks are harmfulthen why do so many successful invaders have depauperate gene pools in the invadedrange when compared to their native range (Allendorf & Lundquist 2003)? Newlyinvasive species are particularly well suited to understand these phenomena aspopulations are expected to be highly genetically structured, with some harboring morevariation than others. In spite of overall reduced genetic diversity in the invasive range,some populations will harbor more variation than others. Our population genetic surveysof a newly invasive perennial grass Brachypodium. sylvaticum in Oregon confirm thisexpectation. The more genetically diverse invasive populations are expected to becomemore vigorous and produce more seeds. These populations would have greater intrinsicpopulation growth (λ) and increased propagule pressure contributing more to the invasion(Lockwood et al. 2005). This has not been tested in any invasive species. We propose toinitiate demographic surveys of invasive populations ranging in genetic diversity andspanning a range of regions with different invasion histories to identify the characteristicsof populations that make them a more imminent threat (i.e. higher genetic diversity,growth rate, and fecundity).Problem description and literature review and preliminary results.Demographic studies can help us model the spread of invasive species and informus about which life history stages should be targeted for management (Davis et al. 2006;Parker 2000; Shea et al. 2006). Population genetics studies demonstrate that successfulinvaders may have more (Lavergne & Molofsky), or less genetic diversity than in theirnative range (Meimberg etal. 2006; Tsutsui & Case2001, also see our databelow). However, we donot fully understand howgenetic diversity affectspopulation dynamics ininvasive species. This isprobably because studiesintegrating both approachesare less common in general(Hanski & Saccheri 2006)and more frequently calledfor in conservation of rareFigure. 1. A hypothetical scenario for range expansion in a newly-invasivespecies. Each dot represents an individual population. Darker dots indicate sourcepopulations. The model assumes that a secondary lag phase may be induced byinbreeding depression in newly established populations. Once the genetic load ispurged through population bottlenecks and inbreeding in small populations, the rateof range expansion is expected to increase dramatically. See Deitz and Edwards(Davis et al. 2006; 2006; Meimberg et al. 2006; Parker 2000; Shea et al. 2006) forpossible underlying dynamic ecological processes.or threatened species (e.g.,Oostermeijer et al. 2003)rather than in invasivespecies research. This issurprising since newlyintroduced species arelikely to face the same
challenges as rare species and / or isolated populations.Exotic plant range expansion may be characterized by three major steps:introduction, colonization, and naturalization with each step occurring at a successivelygreater spatial scale (Cousens & Mortimer 1995; Groves 1986; Radosevich et al. 2003).Each step is characterized by specific ecological and genetic phenomena (Fig. 1). Theinitial introduction involves the establishment and survival of a few individuals. At thebeginning of this “lag phase” (Primary Lag Phase, Fig. 1) these individuals becomeestablished, perhaps hybridize (i.e., interbreeding among genotypes from disparateportions of the native range), and produce offspring recruits. As the primary sourcepopulation(s) develops, these recruits are also subject to selection for adaptation to localconditions (Kliber & Eckert 2005). Over time, seeds from the founding source regionwill disperse, near and far, to form new satellite populations in peripheral regions (Fig. 1,Phase 2). The time necessary for a species to reach the next threshold (Fig. 1 Phase 3)may well be determined by the time required to recover from lower genetic diversityand/or inbreeding depression associated with colonizing populations (i.e., purging geneticlmillimeters (mm)Gene Diversity200180160140120100806040200CentralPeripheralPlant Height Leaf Length GrowthVegetative Biomass (g)8.50.0 0.1 0.2 0.3 0.4Gene Diversity (Hs)Figure 2. Size growth and biomass for central (putative source) and peripheral (sink) populations inOregon. Left panel: Bars are least square family means (n=8 to 16 families) nested in 7 central and 4peripheral populations (p< 0.01) indicating significant genetic variation for these traits. Right panel:Vegetative biomass harvested at the end of the second growing season for the same populations.1.41.21.00.80.60.40.20.0-0.2-0.4r 2 = 0.24 nsM14M11M6M3M8M1M16M50 20 40 60Reproductive Culms m -280 100Figure 3. Relationship between genediversity (Hs) and number of reproductiveculms per unit area for eight peripheralpopulations.12.011.511.010.510.09.59.0E4M1S2C6C7oad, Fig. 1, Phase 3) (Ellstrand &Schierenbeck 2000; Groves 2006; Lande& Schemske 1985; Sakai et al. 2001).Ultimately, the invading populations reachsome threshold genetic or demographiclevel and the species appears to spreadexponentially (Fig. 1, Phase 4).Unfortunately, little is known about howthese genetic and demographic processeswork in biological invasions.We have already found that populations inperipheral regions of the invasive range aregenetically depauperate and less vigorous(Fig. 2 left panel) and there appears to berelationship between genetic diversity andC1NC4E1C2C10E9CentralPeripheral
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Final Report - Center for Invasive Plant Management

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