Invasive plants are of critical concern on a world-wide scale due to their adverse impacts on native biodiversity, agriculture and economics. To effectively address the threat of invasive plant species, an understanding of the factors that control the process of invasion is essential. One of the key requirements for the naturalization of introduced plants is an ability to reproduce outside their native range. However, research to date has not provided conclusive evidence regarding the reproductive characteristics of invasive plants. Recognising the lack of empirical data, this thesis aimed to determine the reproductive biology of three milkweed species that have become highly successful invasive plants in Australia, namely, Asclepias curassavica, Gomphocarpus fruticosus and G. physocarpus. These species are of particular interest due to their characteristically highly evolved floral morphology.
Baker's Rule hypothesises that plant species capable of uniparental reproduction are more likely to become successful colonists than species that are self-incompatible or dioecious. Although this hypothesis is relevant in the context of biological plant invasions, few empirical studies have tested the applicability of Baker's Rule to invasive plant species. Hand-pollination experiments were undertaken as part of this study, and provided support for Baker's Rule as all three study species were found to be self-compatible, although dependent on pollinators for reproductive output. Furthermore, progeny performance trials indicated that selfed progeny are sufficiently viable such that they may contribute to the demographic trajectory of populations. These results are noteworthy, given that genetic self-incompatibility is almost ubiquitous among milkweeds.
Following on from the finding that the study species are dependent on pollinators for reproductive output, the pollination systems and floral rewards of the three study species were investigated. It is logical to predict that introduced plant species with specialised pollination systems are less likely to become invasive due to difficulty in establishment of pollinator relationships in their invaded ecosystem, whereas introduced plant species that can be served by numerous pollinator species have a greater chance of reproduction. Observations of flower visitors in natural populations of the three study species revealed that their pollination systems are essentially specialised at the taxonomic level of order, but generalised and thus highly flexible at the species level. Specifically, pollinators of the two Gomphocarpus species predominantly included various Hymenoptera species, while pollinators of A. curassavica were primarily Lepidoptera. Attraction of diverse assemblages of insects by the study species is likely attributed to the accessibility of large volumes of remarkably concentrated nectar.
The ability to reproduce in an introduced range is particularly important during the initial stages of invasion when populations are small. According to the Allee effect, plants in small populations should experience reduced reproductive success relative to plants in larger populations. However, field surveys of seed production in natural populations of varying sizes did not detect Allee effects in any of the study species. Instead, plants in small populations exhibited remarkably higher levels of reproductive output compared to those in large populations. Such a trend has rarely been documented in plant ecology studies. Supplemental hand-pollination experiments found pollen limitation in large but not in small populations of the study species, therefore suggesting that this trend is a consequence of competition among plants for pollinators in large populations.
Following on from the finding that the study species are self-compatible, and that even small populations are capable of high levels of reproductive output, AFLP markers were used to determine realised patterns of mating in various-sized populations of the three milkweed species. Most study populations were found to produce approximately 60 percent of their progeny through outcrossing, and no relationships between population size and outcrossing rates were detected. This suggests that production of seed through pollinator-mediated selfing is common in both founder and established populations of the study species. For comparison, the mating system of a single G. physocarpus population in the South African native range was investigated. Interestingly, all progeny from the native range were outcrossed, thereby providing conclusive evidence for the breakdown of self-incompatibility as part of the invasion process.
In conclusion, the data presented in this thesis add to the growing evidence that reproductive characteristics are significant with regards to the process of biological invasion. The results of this study have implications for the management of invasive plant populations, in particular, the importance of early intervention and eradicating small populations in the establishment phase. Furthermore, the results of this study are discussed in terms of incorporating reproductive traits into screening protocols to predict invasiveness. Specifically, traits that appear to be consistently associated with invasiveness include self-compatibility, generalised pollination systems and highly rewarding flowers. The ability to accurately predict species invasiveness is one of the long-term goals of invasion biology and will not be accomplished by a single study, but is likely to benefit greatly from further case studies examining modes of reproduction in invasive species.