Resistance and susceptibility of plant communities to invasion:revisiting Rabotnov's ideas about community homestasis

One of the strangest but most interesting phenomena in ecology is the tremendous increase in abundance that some species experience when introduced to new ecosystems by humans (see Louda et al. 2003). Although most species that establish in new ecosystems appear to be well behaved and act more or less like the native species, some become overwhelming dominants-far more abundant than they are in their native regions. Equally impressive is the collapse of many native communities when new species are introduced (see D'Antonio and Vitousek, 1992; Mack et al. 2000; for reviews on community-level effects of plant invasions). Diversity can be transformed to virtual monocultures (Braithwaite et al. 1989; Malecki et al. 1993; Meyer and Florence, 1996; Bruce et al. 1997). The extreme nature of invasive success and native collapse suggests that unusual and very powerful ecological processes and mechanisms are at work. The goal of this chapter is to scrutinize the mechanisms that drive the impressive "transmogrification" (Watterson, 1988) of some exotic invaders from minor components at home to competitive monsters away from home and how this transmogrification devastates native plant communities. The list of plant species that transmogrify from good citizens to outlaws is remarkable. For example, Heracleum mantegazzianum, appropriately called giant hogweed in North America, is a relatively rare endemic restricted to a small and remote part of the Caucasus Mountains in the Republic of Georgia (Kolakovsky, 1961; Z. Kikvidze, pers. com.). Introduced as a "garden curiosity" because of its 50 cm inflorescences and 6 m height, it is now a major problem in continental Europe and Great Britain and a federally listed noxious weed in North America where it forms dense monocultures. One of the most invasive plants of wild lands in California, Centaurea solstitialis (Howald et al. 1999; DiToamso and Gerlach, 2000; Rejmánek, and Reichard 2001), only occurs in discrete, highly disturbed patches in its native Turkey (O. Eren and J. Hierro, pers. obs.). In invaded regions, C. solstitialis appears to have substantial competitive abilities against the locals and occurs at much higher densities than at home (Dukes, 2001; Hierro et al. unpublished data). Cytisus scoparius is a minor weed where it is native in western and central Europe, but when introduced to New Zealand, Australia, and the United States it became a major invader of pastures and forest and a target of biocontrol programs (Paynter et al. 1998; Memmot et al. 2000). Similarly, Echium plantagineum is an occasional weed in its native range in the Mediterranean region; however, after its introduction to Australia as a garden plant in the 1800s, it has spread to infest and dominate large areas of agricultural land, becoming one of the Australia's worst weedy species (Grigulis et al. 2001). A rare South American legume, Mimosa pigra, sharing an uncannily similar etymology with giant hogweed, has spread throughout many other tropical areas of the world after human introduction. Mimosa pigra has been particularly successful in tropical Australia where it forms dense monospecific stands with an understory composed only of conspecific seedlings (Miller and Lonsdale, 1987; Braithwaite et al. 1989). The primary hypotheses for the remarkable success of many exotics as community invaders relative to their success in their native communities are: 1) they have escaped the consumers that control their population growth-the "natural enemies hypothesis" (Darwin, 1859; Williams, 1954; Elton 1958), 2) certain invaders can take advantage of 'empty niches' that no local natives utilize (Mack et al. 2000; Dukes, 2001; Sakai et al. 2001), 3) because of a longer history with humans, invaders can take advantage of disturbances more efficiently than natives or even engineer disturbances to their own advantage (Simberloff and Von Holle, 1999; D'Antonio et al. 1999; Mack et al. 2000); and 4) invaders have undergone rapid natural selection in response to new environments which makes them unusually aggressive (Blossey and Nötzold 1995; Mack et al. 2000; Gaskin and Schaal, 2002). However, as described below many highly successful invaders do not always behave in ways that support these hypotheses. The puzzling behavior of many successful exotic invaders provides an unparalleled opportunity to examine fundamental ecological theory including the relative importance of top-down versus bottom-up control of communities, the nature of niches, the importance of local adaptation, and the individualistic versus holistic nature of plant communities. In this chapter we focus on the latter issue and argue that the peculiar behavior of many exotic species may challenge our conceptual paradigm for plant communities.