Summary: Understanding the consequences of realistic species loss on the functioning and persistence of vulnerable ecosystems is key to devising conservation strategies when environmental changes are immediate threats. Yet, few studies have provided direct evidence for conservation prioritization and decision-making. We incorporated the quantification of functional diversity based on the Rao index of diversity (FDrao) into this applied context and examined the consequences of realistic species loss on functional diversity in moorland plant communities widely interspersed within a subalpine zone in northern Japan. The realistic order of species loss was derived from the nested subset pattern in the moorland communities, which was corroborated by selective species tolerance and selective extinction. We analysed the relationships between the FDrao half-life, as an index of each moorland's vulnerability to species loss, and a range of environmental variables describing the moorlands. We then mapped this index across the entire landscape. At most sites, ordered species loss caused a relatively small decline in FDrao until a certain number of species was lost and an accelerating decline thereafter, suggesting relatively low initial vulnerability to species loss. At the other sites, however, ordered species loss caused an approximately proportional decrease in FDrao, suggesting relatively high vulnerability to species loss. The model indicated that sites with higher elevation, higher carrying capacities, or increasing isolation have a shorter FDrao half-life. The mapping of this index allowed us to identify the geographical distribution of sites of high conservation priority. Synthesis and applications. We performed fine-scale assessments of the vulnerability of moorland plant communities to species loss, which is likely to occur under future environmental conditions, by simulating the consequences of realistic species loss for functional diversity. The methods used here can provide urgently needed information to support the prioritization and decision-making involved in conserving ecosystems in the face of global biodiversity loss.
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