Hydrilla verticillata (Hydrocharitaceae), a recent invader threatening Brazil's freshwater environments: a review of the extent of the problem

Hydrilla verticillata (Hydrocharitaceae) is a highly prolific, rooted submerged macrophyte native to Asia that has invaded aquatic systems worldwide, causing many ecological and human-related problems. Hydrilla recently invaded the Parana River basin in Brazil, making other ecologically and socially important Brazilian watersheds more susceptible to invasion by this plant. Here, I summarize the relevant information about Hydrilla, focusing on its biotic attributes, abiotic tolerance and effects on ecosystems. The aim of this review is to provide background information to assist with planning for the potential impacts of this species in the Neotropical region (particularly in Brazil) and the development of research, monitoring and management strategies. A wide ecological amplitude, resistance organs, and high growth rates and dispersion ability provide Hydrilla with great potential to invade and infest a variety of habitats, often resulting in important physico-chemical and biotic effects on the environment. Hydrilla is similar in its morphological and ecological aspects to Egeria najas and Egeria densa (South American Hydrocharitaceae), but this non-native species is a superior competitor and can be expected to exert significant pressure in habitats colonized by these native Hydrocharitaceae. Socially important rivers (such as the So Francisco River) have a high risk of being infested with Hydrilla, especially in stretches affected by dams, which could prejudice important human activities like hydropower generation. Important wetlands for biodiversity conservation (such as the Pantanal) may also be invaded, but they seem to be more resistant to infestations as long as their natural hydrologies are preserved. Before investing substantial effort to control Hydrilla, managers should weigh the potential costs and benefits of available techniques and consider the potential benefits of Hydrilla in providing ecosystem services.