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TOWARDS RESPONSIBLE HORTICULTURE: PREDICTING WHICH PLANTS ARE LIKELY TO BECOME NATURALIZED OUTSIDE THEIR NATIVE RANGE

Moraea flaccida, a member of the Iridaceae family. (Photo: Mark van Kleunen)

To prevent new biological invasions, it is important to find predictors of naturalization that can be assessed in the native range prior to a species’ introduction elsewhere. Mark van Kleunen and Steve Johnson of the DST-NRF Centre for Invasion Biology tested the importance of horticultural usage and species characteristics for naturalization status of the 1036 species of Iridaceae (iris family) native to southern Africa.

Of the 67 species which have become naturalized elsewhere, no less than 62 are used as garden plants, indicating that horticultural trade is the main source of naturalized Iridaceae. Species introduced elsewhere for horticultural usage are not a random subset of species but have a larger distributional range, a lower maximum altitude and more subtaxa, and grow taller. Moreover among the horticultural species, naturalization differs between genera, and is more likely for species with lower maximum altitude, species with higher numbers of subtaxa and for taller species.

In a subsequent field study on 20 species and a garden experiment on 60 species, they also showed that naturalized species of Iridaceae have a higher capacity of seed production in the absence of pollinators, and have faster and more profuse seedling emergence than related non-naturalized species. This shows that naturalization of South African Iridaceae in other regions of the world is associated with species characteristics that can be assessed in the native range.