From ‘Immigrants’ to ‘Invaders’? Old World Ludwigia species in the Asian-Pacific Region

Nimal R. Chandrasena 1, Iswar C. Barua 2 and Hirohiko Morita

Keywords:

Primrose Willow, Ludwigia species, Ludwigia peruviana, Ludwigia longifolia, Ludwigia adscendens, Ludwigia peploides, Ludwigia hyssopifolia, Ludwigia octovalvis, Ludwigia decurrens

Abstract:

Several Ludwigia L. species (Onagraceae) have become significant aquatic weeds, creating problems in the Asian-Pacific region, extending from part of the Old World (India and Sri Lanka, eastwards to the Malayan peninsula, Indonesia, the Philippines, Southern China, Japan) and Oceania (Australia, Papua New Guinea and the Pacific Islands). Many are important rice weeds, and a few others are aggressive colonisers of permanently or frequently wet habitat and are already well-established, presenting local problems due to their abundance and dominance of such occupied habitat. The wide distribution of several Ludwigia species is directly linked to human introductions, via botanic gardens and aquarium industry and large-scale habitat disturbances caused by human activities. As evidenced from recent introductions of New World species to Western Europe by the nursery industry, several of the newly introduced species have the potential to become serious pests in aquatic habitat. Both the established species in the Old World and the ‘new immigrants’ require attention, so that the risks of them becoming ‘invaders’ can be mitigated. In this article, we highlight some ecological and management-oriented studies from Australia, and observations from India, Malaysia and Japan, where several Ludwigia species are well established. While not all of them present problems, a few are significant weeds of agriculture and aquatic habitat. We focus our attention on factors that influence the successful colonisation of habitats by Ludwigia species. We also discuss why they are so successful and the limitations of implementing integrated weed management to locally contain those species. Clearly, in the case of relatively recently introduced Ludwigia species that have the potential to spread more widely, containment and/or local eradication strategies should include early detection, early intervention to control individuals reaching maturity and prevention of spread via stormwater runoff, wind and other dispersal mechanisms. Successful colonisers, such as Ludwigia spp., demonstrate tenaciousness, resistance to control and the capacity to expand their bio-geographical range with little or no help from humans. Weed Science has the tools, i.e. integrated and holistic, ecology-based weed management approaches, including biological control, that could reduce any future risks these species may pose. Learning from the studies presented, we encourage more research directed at understanding the ecology and biology of the most significant ‘weedy’ Ludwigia species, so that they can be better managed.