Australia’s national flower is an invasive species – even in Australia

Australia’s national flower, Acacia pycnantha (the golden wattle), is native to New South Wales, Victoria and South Australia. And very pretty it is too. But this species was introduced and has become invasive in Western Australia and is probably naturalizing in some areas of New South Wales and South Australia from cultivated plantings in revegetation projects and along roadsides. A. pycnantha is also an invasive species in the Eastern and Western Cape Provinces of South Africa, in Portugal, and possibly in California.

Understanding botanical introduction and invasion histories has important practical implications. The selection of effective host-specific biocontrol agents for invasive plants can depend on identifying which subspecific entities of the plant were introduced. Following the success of other biological control agents against Australian acacias in South Africa, a gall-forming wasp and a seed-feeding weevil have been used to try to control the plant.

A recent paper published in Annals of Botany aims to place invasive populations of Acacia pycnantha in the context of historical biogeographical patterns in the native range of the species in south-eastern Australia. The authors use plastid and nuclear DNA markers to reconstruct phylogenetic relationships among invasive and native populations, and to compare genetic diversities in these invasive and native populations. They show that the invasive genotype found in South Africa is similar to the invasive genotypes in Portugal and Western Australia and thus introduction of the same variant of gall-forming wasp successfully used for biological control in South Africa is recommended.

Elucidating the native sources of an invasive tree species, Acacia pycnantha, reveals unexpected native range diversity and structure. (2013) Annals of Botany 111 (5): 895-904. doi: 10.1093/aob/mct057
Understanding the introduction history of invasive plant species is important for their management and identifying effective host-specific biological control agents. However, uncertain taxonomy, intra- and interspecific hybridization, and cryptic speciation may obscure introduction histories, making it difficult to identify native regions to explore for host-specific agents. The overall aim of this study was to identify the native source populations of Acacia pycnantha, a tree native to south-eastern Australia and invasive in South Africa, Western Australia and Portugal. Using a phylogeographical approach also allowed an exploration of the historical processes that have shaped the genetic structure of A. pycnantha in its native range. Nuclear (nDNA) and plastid DNA sequence data were used in network and tree-building analyses to reconstruct phylogeographical relationships between native and invasive A. pycnantha populations. In addition, mismatch distributions, relative rates and Bayesian analyses were used to infer recent demographic processes and timing of events in Australia that led to population structure and diversification. The plastid network indicated that Australian populations of A. pycnantha are geographically structured into two informally recognized lineages, the wetland and dryland forms, whereas the nuclear phylogeny showed little geographical structure between these two forms. Moreover, the dryland form of A. pycnantha showed close genetic similarity to the wetland form based on nDNA sequence data. Hybrid zones may explain these findings, supported here by incongruent phylogenetic placement of some of these taxa between nuclear and plastid genealogies.
It is hypothesized that habitat fragmentation due to cycles of aridity inter-dispersed with periods of abundant rainfall during the Pleistocene (approx. 100 kya) probably gave rise to native dryland and wetland forms of A. pycnantha. Although the different lineages were confined to different ecological regions, we also found evidence for intraspecific hybridization in Victoria. The invasive populations in Portugal and South Africa represent wetland forms, whereas some South African populations resemble the Victorian dryland form. The success of the biological control programme for A. pycnantha in South Africa may therefore be attributed to the fact that the gall-forming wasp Trichilogaster signiventris was sourced from South Australian populations, which closely match most of the invasive populations in South Africa.