Distribution of the invasive alien aquatic weed species Pontederia crassipes Mart., Pistia stratiotes L., Salvinia molesta D.S.Mitch. and Azolla filiculoides Lam. in Madagascar

Résumé

Madagascar is a biodiversity hotspot with extensive freshwater ecosystems.It is also one of the world's least developed countries and relies on its wetland habitats for essential services such as rice production and fisheries.The floating aquatic weeds Pontederia crassipes Mart.(Pontederiaceae) (water hyacinth), Salvinia molesta D.S. Mitch.(Salviniaceae) (giant salvinia), Pistia stratiotes L. (Araceae) (water lettuce) and Azolla filiculoides Lam.(Salviniaceae) (red water fern) have been recorded from Madagascar, and some at least are known to have significant negative impacts.However, their identities and distribution have not been well documented.Here, we confirm identities and map distributions of the four species using field survey data, herbarium records and literature records.All species were widespread, each occurring in at least 13-16 of the 23 Administrative Regions (ARs) and in four of the five bioclimatic regions of Madagascar.In only four ARs were none of the four species recorded.Pistia stratiotes has the earliest collection records (1847) but there is some dispute as to whether it is an alien species or not.Pontederia crassipes was first collected in 1931, while S. molesta was first collected in 1995 and often confused with the native congener S. hastata.Finally, A. filiculoides had not previously been collected, and was often confused with the native congener A. pinnata.All four species occupied mainly lentic systems, including both rural and urban settings, and natural and agricultural (rice-growing) systems.Salvinia molesta is the most widespread species and causes the most problems, although both P. crassipes and A. filiculoides are also frequently problematic.Pistia stratiotes is the least aggressive of the species.An understanding of the distribution and frequency of occurrence of these weeds is essential in developing management programmes for them.

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