Molecular systematics and ecology of invasive Kangaroo Paws in South Africa: Management implications for a horticulturally important genus

Most legislation pertaining to non-native organisms is implicitly focussed at the individual species level. However, in some cases interspecific hybrids can be more invasive than any of the parent species. This is problematic for policy makers, and for horticulturists developing or trading in new ornamental cultivars. We explore these issues in the context of the need to manage naturalized populations of Kangaroo Paws (Anigozanthos species) in South Africa. Self-sustaining, dense populations of naturalized Kangaroo Paws occur at several localities and are highly attractive to local nectar-feeding birds. The populations show high levels of seed set with or without bird pollination. Given the known propensity of Kangaroo Paws to hybridise in their native range in Australia, and confusion about the species identity of naturalized populations in South Africa, it was essential to resolve some key taxonomic issues in the group. We constructed the first molecular phylogeny for all species of the Kangaroo Paw group (genera Anigozanthos and Macropidia; family Haemodoraceae). As previously determined by taxonomists working on herbarium specimens, naturalized populations were identified as A. flavidus. In addition, we also identified a second species, A. rufus. Relative genome size estimates for Anigozanthos species indicated that small inter-specific differences in genome sizes are positively correlated to hybrid fitness. Anigozanthos flavidus and A. rufus have relatively 'compatible' genomes and may produce fertile hybrids under field conditions. However, for species whose genome size differ more than ~30%, there is little inter-specific compatibility and consequently a very low risk of producing fertile hybrids. In conclusion, we recommend that trade in Kangaroo Paws in South Africa should be temporarily restricted and that particular cultivars should first be subjected to a careful risk assessment. © 2010 Springer Science+Business Media B.V.