Novel weapons: invasive success and the evolution of increased competitive ability

www.frontiersinecology.org © The Ecological Society of America I the 13th century, eastern and central Europe experienced one of the most surprising invasions in human history. The Mongols, once restricted to a small portion of central Asia, swept into Russia, Hungary, Poland, and Germany and consistently defeated much larger European armies. It was only the death of the Mongol leader, Ogedai Khan, in 1242, and the ensuing internal politics, that prevented Europe from becoming a collection of vassal states. The reasons for the success of the Mongol invasion were complex. Although it was in large part due to brilliant strategy and discipline, these strengths were derived from the possession of a novel weapon, the recurve bow carried by most Mongolian soldiers (Figure 1). The recurve bow fired arrows faster, farther, and with greater force than anything the Europeans had ever experienced. Such novel weapons have played a role in many human invasions, but novel biological weapons have only recently been considered as possible drivers of invasions by non-human exotic species. Here, we explore the possibility that novel weapons, biochemical in nature, play a part in the exotic plant invasions that are currently sweeping the world. One of the mysteries in ecology is how exotic plants occurring at low densities in their native ranges attain extremely high densities in their introduced ranges (Figure 2). Although only a small fraction of introduced species have enjoyed such dramatic changes in fortune, these have had extensive economic impacts and suggest the existence of very powerful, yet poorly understood, ecological processes. A number of explanations for this invasion success have been proposed (Mack et al. 2000), but consumer-based hypotheses predominate – the “natural enemies hypothesis” (Williams 1954; Elton 1958; Crawley 1987, 1997; Mack et al. 2000; Maron and Vilà 2001; Levine et al. 2002) and a recent expansion of the natural enemies hypothesis, “the evolution of increased competitive ability” (EICA; Blossey and Nötzold 1995; Müller-Schärer et al. 2004). The natural enemies hypothesis attributes exotic plant success to the fact that upon introduction many exotics are liberated from their specialist herbivores and pathogens. Exotics are thought to gain a substantial advantage because their populations are no longer directly suppressed by specialist consumers and pathogens, and because they obtain a competitive advanCONCEPTS AND QUESTIONS