One prominent explanation for the success of some invasive plant species is the 'enemy release hypothesis', which predicts that plants are less controlled by their natural enemies in areas where a plant species has been introduced
(e.g. Keane and Crawley, 2002). Plant species which are strongly regulated by their enemies in the native range should immediately benefit from reduced attack in the invaded range. In contrast, well-defended plants gain a competitive advantage only after evolutionary adaptation (Colautti et al., 2004). In the absence of enemies, selection pressure should favour re-allocation of resources from defence towards growth or reproduction, which leads consequently to an 'evolution of increased competitive ability' (EICA hypothesis - Blossey and Notzold, 1995). Evidence is accumulating that invasive plants can undergo a rapid evolution in their new environment (Bossdorf et al., 2005). Muller-Scharer et al. (2004) argued that plant invaders are not released from all enemies, but only from specialists. Defence traits might be associated with indirect costs in the presence of specialists. For instance, plant toxins can deter generalists, while adapted enemies use these chemicals as recognition cues or as feeding and oviposition stimulants in the native range. In the invaded range, where high levels of toxins are not associated with the attraction of specialists, plant toxins might increase in concentration, contrary to the prediction of the EICA hypothesis. In return, the plant is able to invest fewer resources in defence traits against specialists which might have higher allocation costs such as digestibility-reducing compounds or mechanical protection. This line of argumentation was supported by Joshi and Vrieling (2005). In common garden experiments, invasive populations of Senecio jacobaea have on average, higher alkaloid concentrations and higher resistance against general-ists than native populations. However, at the same time they were less protected against specialist herbivores.
Hattendorf (2005) investigated the toxicity of plant extracts and the performance of plant hairs on leaf petioles in native and invasive populations of H. mantegazzianum. Trichomes are assumed to have particularly high allocation costs, because their formation depends on growth processes (Gutschick, 1999). In addition, trichomes can limit the efficiency of predators of herbivorous insects (Gassmann and Hare, 2005). In bioassays, plant extracts from invasive plant populations showed a higher toxicity against brine shrimps, Artemia salina (L.), as compared to plants from the native range. In contrast, plants in the invaded range had fewer and shorter trichomes. It seems that H. mantegazzianum exhibits the pattern predicted by Muller-Scharer et al. (2004). Allocation of resources at higher furanocoumarin levels might be over-compensated by lower resource investment in the more costly defence by tri-chomes. Ode et al. (2004) found similar patterns for furanocoumarins in Pastinaca sativa. Several furanocoumarins were found in lower concentrations in native populations in Europe as compared to populations in the USA, where the plant is invasive. Unfortunately, their study included only two European populations and therefore other explanations such as geographical distinctions might cause the differences.
Two studies investigated the variation in furanocoumarins between plant populations of H. mantegazzianum. Satsyperova and Komissarenko (1978) analysed fruits of four Caucasian populations. The samples covered a large area of the native range and originated from the northern and the southern slopes of the mountains (North Ossetia; Republic of Adygea; Russian
Federation: Krasnodar Krai; Abkhazia). The composition of furanocoumarins was almost identical in all cases (Table 13.1). In contrast, Molho et al. (1971) found significant differences in the number of compounds present in fruits from three H. mantegazzianum populations in Central Europe. The number of isolated furanocoumarins ranged from four to nine compounds (Table 13.1). This pattern of divergence in the invaded region as compared to native distribution range is quite surprising and could indicate different adaptation processes in the new environment. However, Molho et al. (1971) stated that the complex taxonomy of giant hogweeds (see Jahodova et al., Chapter 1, this volume) could have led to misidentifications because similar or closely related species not belonging to H. mantegazzianum may have been identified as such.
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