Conclusions Master of all Traits

Plant ecological determinants of giant hogweed's invasiveness are simple and easy to determine. The species does not seem to possess any special characteristic/mechanism; extremely high fecundity, rapid growth, capability of self-pollination, extended germination period by means of short-term persistent seed bank, high germination, negligible impact of natural enemies - all these characteristics can be found in other plant invaders (Pysek and Richardson, 2007).

A recent review of species traits associated with invasiveness identified, among others, fecundity, rapid growth and associated physiological measures, height, resistance to herbivory, early germination and flowering and persistent seed bank as important (Pysek and Richardson, 2007). This volume illustrates that H. mantegazzianum has many such attributes (Fig. 19.5) and some of them compensate for the lack of some others typical of invasive species. For example, vigorous vegetative spatial growth is consistently recognized as an attribute of invasiveness (Callaway and Josselyn, 1992; Vila and D'Antonio, 1998; Larson, 2000; Morris et al., 2002). However, the main advantage provided by this ability is efficient and rapid space pre-emption, which in

Fig. 19.5. Traits identified as contributing to the successful invasion by H. mantegazzianum, plotted on the scheme of the life cycle of a monocarpic plant: I - soil seed bank; II - stage of seedling recruitment; III - growth phase; IV - the terminal phase of seed production (stages according to Harper, 1977). Drawing: J. Pergl.

Fig. 19.5. Traits identified as contributing to the successful invasion by H. mantegazzianum, plotted on the scheme of the life cycle of a monocarpic plant: I - soil seed bank; II - stage of seedling recruitment; III - growth phase; IV - the terminal phase of seed production (stages according to Harper, 1977). Drawing: J. Pergl.

H. mantegazzianum is achieved by rapid growth of large leaves above the surrounding vegetation and the production of large numbers of vigorously growing offspring in the vicinity of fruiting plants.

Invasions by some plant species are facilitated by special mechanisms resulting from their specific biochemical features, e.g. substances resulting in a high flammability that changes the fire regime in invaded regions (D'Antonio and Vitousek, 1992). H. mantegazzianum has two different defence mechanisms: glandular trichomes and a phototoxic sap containing furanocoumarins that protect the plants against vertebrates, invertebrates, fungi, bacteria and viruses (Hattendorf et al., Chapter 13, this volume). However, the role both mechanisms may play in facilitating the invasion by H. mantegazzianum in Europe is unclear. The resource-costly defence by glandular trichomes is less pronounced in the invaded area, thus releasing the plant from some defence costs. This would enable the invading plant populations to invest more in growth. The second defence mechanism by less costly qualitative toxic compounds did not change as suggested by the hypothesis of Blossey and Notzold (1995). H. mantegazzianum showed higher levels in the invaded range of Europe even though no damaging specialized herbivores occur there. The plant appears to invest more in biochemical defence than necessary and the reason for this is not clear. However, we know that the levels and types of fura-nocoumarins in this species are variable, and this may reflect the original stock from which the introductions were made.

Therefore, it is a combination of superior traits associated with a single species and acting at different stages of the life cycle (Fig. 19.5) that provides

H. mantegazzianum with remarkable invasion potential and makes it a 'master-of-all-traits' of plant invasions. This has practical implications, as the species does not seem to have a weak link in its life cycle, on which the control measures could be most efficiently targeted. Appropriately conducted long-term mechanical and chemical control (Nielsen et al., Chapter 14 and Pysek et al., Chapter 7, this volume) associated with suitable landscape management and revegetation schemes (Ravn et al., Chapter 17, this volume) can be used, with reasonable success, to reduce the extent of invasion in heavily affected areas and prevent the species from further spread. It is unlikely, however, that the species can be completely eradicated by classical means of control. So the answer to the question raised in the title of this chapter is - yes, we can fight giant hogweed with some success but, for now ... giant hogweed lives.

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