Hydrocharis dubia - a 'native' frogbit not native after all

In the serene waters of eastern Asia, a delicate aquatic plant, Hydrocharis dubia, blooms with its white 3-petaled flowers. However, its presence in Australia has sparked debates among weed control authorities regarding its origin. While initially accepted as indigenous, recent scrutiny challenges this notion.
The Hydrocharis genus is comprised of five species: the recently reclassified H. laevigata and H. spongia, H. morsus-ranae, H. chevalieri and H. dubia. Hydrocharis morsus-ranae grows in Europe, H. laevigata is native to South America, H. chevalieri calls central Africa home, and H. dubia finds its niche in southeastern Asia, from India to Japan and beyond. Interestingly, it's been labeled a "noxious weed" in Japan, hinting at its invasive potential.
Historical records of H. dubia in Australia date back to Bentham's observation in 1873, albeit under a different name. Specimens collected from Queensland and New South Wales further support its presence. However, its habitat preferences raise eyebrows; thriving in disturbed sites rather than remote, untouched areas. Nowhere does it seem to have a natural range; instead its rapid spread over the last four decades and disjointed range defy distribution norms.
A test of origin status has revealed the following observations made on Hydrocharis dubia in Australia:
- Hydrocharis dubia prefers disturbed or modified habitats, indicating potential human introduction.
- Like the introduced H. laevigata, it exhibits unrestricted growth and nowhere does it grow in balance with surrounding biota.
- There is an absence of pests or predators, suggesting recent arrival in evolutionary terms.
- Over the last 40 years, its geographical range has significantly expanded.
- It has a disjointed distribution beyond natural explanations like climate or geography, indicating human involvement.
- Initial Australian locations are geographically distant from known occurrences in Asia.
- The first Australian record dates within the period for potential human introduction.
These traits all together mean H. dubia fails to meet the standards of indigenous species. Instead, it aligns more closely with traits typical of introduced species, possibly hitchhiking its way into Australia through human activities. This evidence has led to this species being de-listed as a native species in Queensland, and classified as naturalised instead.
Like Hydrocharis laevigata, Hydrocharis dubia is an attractive and easy to grow floating plant. While there's certainly no issue with appreciating and cultivating exotic species, understanding their background and true characteristics is essential. Determining whether a plant is truly native is vital in preventing unintended introductions into the wild, particularly if your aim is to maintain native species exclusively.