Standardising field-based assessment of invasion degree: A case study in two habitats of Reunion Island
Biological invasions represent one of the major threats to biodiversity conservation, especially in island ecosystems. Field-based assessments of the invasion degree are required for managing and monitoring invaded habitats but there is no unifying measure available in the literature. Here, we developed a standardised method for quantifying invasion degree based on four metrics: alien and native species cover, alien species richness, the occurrence frequency of alien species and invasion level, and demonstrated its use in two contrasted habitats of Reunion Island. This rapid survey quantified relative abundance and diversity of alien plant species among three vegetation strata (herbaceous, shrub and tree) in a systematic sampling. In lowland tropical forest, it took 40 person days to survey 150 ha (344 plots), and 52 person days in subalpine vegetation to survey 125 ha (304 plots). Our results showed that the herbaceous stratum was the most invaded in terms of invasion level; more than 45% in lowland tropical forest and almost 55% in subalpine vegetation. Based on cluster analysis and kriging, we mapped three different levels of invasion highlighting potential invasion fronts. We found that it was important to differentiate invasion degree per vegetation strata as mean alien cover was significantly different between strata at each site. This method is suitable for the majority of protected areas where fine-scale assessment is required to characterise the spatial pattern of plant invasions, monitor invasive species and identify invasion foci to control or populations of emerging species to eradicate.