restricted access 20. Exotic Plants and Enemy Resistance
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280 The increasing movement of organisms to new regions by humans is enabling species to breach natural dispersal barriers that normally constrain their geographic distribution. Oddly enough, despite being introduced to areas that may be very different from their home region, some exotics become spectacularly more successful in evolutionarily novel environments than in areas in which they evolved. How some exotics come to dominate these new habitats, despite being often inconspicuous members of their native community, is one of ecology’s central mysteries. Unraveling this mystery involves understanding how introduced organisms faced with novel abiotic or biotic conditions make accommodations to their new environments. In this chapter, we consider one dimension of this accommodation process: how exotic plants respond to the altered assemblage of natural enemies they face within their introduced ranges. Where they are native, plants are attacked by a diverse group of pests, including both specialists and generalists. Together, these enemies can impose diffuse, conflicting, or shifting selection pressures on plants (Hare and Futuyma 1978; Fox 1988; Marquis 1990; Rausher 1992; Pilson 1996; Juenger and Bergelson 1998; but see Maddox and Root 1990). In contrast, exotics face a greatly simplified naturalenemy landscape. Human-mediated transport to new areas instantaneously liberates exotics from their coevolved specialist fauna (Mitchell and Power 2003; Blair and Wolfe 2004; Hinz and Schwarzlaender 2004; Vilà et al. 2005). Indeed, this escape from specialist natural enemies has both been a leading hypothesis for exotic success and a founding principle on which biological control is based (Williams 1954; Maron and Vilà 2001; Keane and Crawley 2002). In addition to escaping from specialists, exotics that lack taxonomic relatives within recipient communities and therefore possess defenses that are chemically unique to the community they invade may also escape from generalists. Host switching by specialists from natives to exotics is also less likely for exotics with no close taxonomic relatives in the recipient community. Although many exotics escape from consumers, some may actually face greater selection pressure by enemies in their introduced than in their native ranges. The most powerful example of this flip side of enemy escape is weed biological control. When a biocontrol insect is introduced, almost instantaneously, plants that have for decades grown free from specialists can suddenly face devastating attack by these herbivores. Since biocontrol insects are often freed from trophic control themselves, they can build to high numbers and potentially impose greater selection on exotic target plants than they might on their native hosts. Moreover, since some exotics represent a new and underutilized food source for generalists, they may actually accumulate generalist pests in their introduced range (Strong 1974; Auerbach and Simberloff 1988; Jobin et al. 1996; Memmott et al. 2000; Graves and Shapiro 2003; Carroll et al. 2005). This may particularly be the case if exotics have reduced genetic diversity and hence reduced genetic diversity of polymorphic defenses against generalists (Colautti et al. 2004). Historically, ecologists have emphasized phenotypic plasticity as a primary mechanism by which exotic plants coped with novel abiotic or biotic circumstances in their introduced range (Baker 1974; Wu and Jain 1978; Rice and Mack 1991; Williams et al. 1995). High levels of adaptive plasticity (which might be present in particular species prior to their introduction) have often been often posited as a predictor of invasiveness (Baker 1974; Rejmanek and Richardson 1996; Mal and Lovett-Doust 2005) and important in allowing exotics to cope with a range of heterogeneous environments . Genetic impoverishment from founder effects (Baker 1974; Morgan and Marshall 1978; Barrett and Richardson 1986), the perceived long time span over which it took TW E NTY Exotic Plants and Enemy Resistance JOHN L. MARON AND MONTSERRAT VILÀ evolution to operate, and the relatively short invasion history of many exotics have appeared to make evolution an unlikely mechanism that could account for rapid phenotypic adaptation to new conditions. In terms of response to natural enemies, it was assumed that nonnatives that faced reduced selection by enemies should lower levels of defense in a plastic manner, enabling reallocation of resources to growth (Bazzaz et al. 1987). Recently, however, these assumptions have met with reconsideration. Rapid Evolution of Exotics in Response to Enemy Pressure Recently, evolution has been considered more seriously within the ecological context of invasions (Huey et al. 2000; Bone and Fares 2001; Lee 2002; Rice and Emory 2003; Stockwell et al. 2003). A growing number of studies show that organisms faced with novel abiotic or biotic conditions can rapidly evolve adaptations to these new conditions (Reznick et...