Far from the garden path : an identikit picture of woody ornamental plants invading South-eastern Australian bushland

Abstract

This thesis establishes a predictive model to identify woody plants with a high invasive potential in South-eastern Australia. The model evolved through a comparison of the hundred or so introduced species that have already become invasive with the several thousand introduced species which have not. Several hundred naturalised, but not yet invasive, species were also compared against the non-spreaders. In the comparisons consideration is given to three major factors: - pressure of introduction; - environmental suitability; and - inherent plant attributes. Pressure of introduction is gauged through the use of nursery catalogues for the cities of Adelaide, Melbourne and Sydney and by planting and sales records for Canberra, where one government body has been responsible fol' the majority of planting. The pitfalls, benefits and histories provided by these documents are discussed. There are strong correlations between the inferred amount a particular species has been planted and the probability that this species is a bushland invader, for all four cities studied. Strong correlations were also found between period of first recorded introduction and invasiveness. In relation to geographical origin, invasive species were found to originate from areas of similar climate to that of point of introduction, but to have a wide distribution over different climate, vegetation and soil types. The thesis also compares the biological environment of source and introduced habitats. However, no firm conclusions relating biotic environment to invasiveness are made. The key inherent identifying attributes of invasive woody species in Southeastern Australia are that they tend to: - reproduce vegetatively; -belong to the Fabaceae, Rosaceae and Mimosaceae and not the Myrtaceae; and - are either summergreen broadleaved trees, Mediterranean evergreen shrubs or broad evergreen vines, and not mesic summergreen shrubs. Other important attributes include: - a large chromosome number; - a long seed longevity; - a middle of the range seed weight; - early lifetime maturity; • being dioecious; - being ant or bird dispersed; - belonging to the Salicaceae and Oleaceae families and not the Cupressaceae; - and being a bush stem succulent. Many more genotypic and phenotypic attributes, to those listed above, are examined in the thesis, but are not found to be related to invasiveness. Another important identifying feature of invasive species in South-eastern Australia, is whether a species or a close relative of similar biology (i.e of the same genus, lifeform and dispersal and reproductive mechanism) is invasive in similar climates elsewhere. Relationships between and amongst all the environmental and inherent plant identifying attributes of invasive species are examined. The Invasive Species Model combines and weights only those features that are required for maximum precision in identification. The model requires answers to a question concerning each of the key attributes and accrues a score, depending on the answers received. Total scores range from -300 to +400. Scores were calculated for 791 woody species commonly planted in Southeastern Australia. None of the 286 species (36% of all common introductions) with a sub-zero score was invasive. There are only two invasive species amongst the 425 species with a invasive score below fifty. Sixty-four of the 109 species with a score over 250 are invasive. Of the ninety-one invasive species eight-one (89%) have an invasive score of above 150. The model is able to provide reasonably accurate and consistent statements of invasive probability, but not categorical distinction. The precision is greatest when taken over the wider South-eastern Australia area than at a more specific focus. The model is also best at distinguishing groups of uninvasive rather than invasive species, a focus which has previously been ignored in studies of invasive plant species. The application of the model, both within South-eastern Australia and globally, is tested and discussed. The basic conclusion being that the model, or a derivation, can provide a means by which the potential of a woody plant to become invasive can be gauged .