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The largest area of intact vegetation in the Mascarene's archipelago is found on Reunion Island, but the remaining natural areas are under threats. Biological invasions are one of the main threats to biodiversity loss on this island and globally worldwide. This study aimed to quantify invasion degree by alien plants species in Reunion Island. This work was possible thanks to a large partnership between researchers and managers. All existing spatial data on invasion pattern were combined and completed by expert knowledge to develop the first 250 x 250 m map of invasion degree at the island scale. To fill the gaps where no field survey data or expert knowledge was available, we used a Random Forest model using nine climatic, landscape and anthropogenic variables. This model also provides a preliminary assessment of drivers of invasion at Reunion Island. Results showed that 85% of the extant native vegetation was invaded in different proportions; 38% are slightly invaded, 26% moderately invaded and 22% very heavily invaded. Despite the high levels of invasion in some places, more than 50% of the extant vegetation is not invaded or slightly invaded. Most of the invaded areas are located in the lowland and in the leeward coast although alien plants invade all types of vegetation from the coast to the top of the island. These results highlight a clear increase in the distribution of alien species over time. This study constitutes a key first step for about the ongoing prioritisation of management interventions on Reunion Island.

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-In Réunion Island, expanding human populations, urbanization and agriculture during the last 50 years have all contributed to a steady increase in the level of nitrates in drinking water. Various nitrate point sources are responsible for the nitrate contamination around the island including chemical fertilizers, animal effluent applied to pasture and crops, and urban waste such as sewage and domestic waste water. In terms of agricultural fertilizers, pig effluent is the most widely used, but the cumulative effects of slurry applications on soil water and groundwater are unknown. Our objectives were (1) to characterize and follow in situ the fate of nitrogen through the subsurface after application of pig effluent onto a cultivated soil using stable nitrate isotopes, δ 15 N and δ 18 O, and (2) to compare the isotopic signatures of Réunion Island's principal aquifers with results from the experimental site to infer potential contamination sources. The study was conducted on an experimental field site planted with maize in the western part of Réunion Island during the rainy season. A control site with no fertilizer application to the maize was compared with the investigation site which had pig effluent applied once a year. The site which had pig effluent applied over one year had an average maximum surface soil water 15 N-NO − 3 value of +9.0% at 0.45 m depth. This signature was significantly more enriched in 15 N than the corresponding subsurface soil water 15 N-NO − 3 value of +3.8% at 10 m depth. The control site average maximum surface soil water 15 N-NO − 3 value of +3.6% at 0.45 m is similar to the subsurface pig effluent application plot. This indicates that nitrates derived from pig effluent have not reached 10 m depth in the subsurface, even though over the last 18 months this site was subjected to two effluent applications, each around 200 kg N ha −1 , and more than 1900 mm of rain, more than half of which drains directly into the root zone. This slow migration shows that mobilization of nitrates through cultivated soil can take many tens of years before infiltrating and contaminating the saturated zone situated at several tens, and in places, hundreds of meters depth. On an island-wide scale, an isotopic assessment of nitrates from the experimental site's soil water and other drinking water wells highlights a nitrogenous contamination derived primarily from urban and/or agriculture via effluent application.nitrates / nitrogen and oxygen isotopes / pig effluent / groundwater contamination / tropical island

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