RLI values relate to the proportion of species expected to remain extant in the near future without conservation action.
RLI values ranges from 0 to 1. An RLI value of 1.0 equates to all species being categorised as Least Concern, and hence that none are expected to go extinct in the near future. An RLI value of 0.0 indicates that all species have gone Extinct. Therefore, the lower the value the closer the set of species is toward extinction.
Put another way, a downwards trend in the graph line (i.e. decreasing RLI values) means that the rate of species extinctions is increasing i.e. that the rate of biodiversity loss is continuing. A horizontal graph line (i.e. unchanging RLI values) means that the rate of species extinctions remains unchanged. An upward trend in the graph line (i.e. increasing RLI values) means that there is a decrease in rate of species extinctions (i.e. an improvement in the status of species).
Red List Index for the world’s mammals, birds, amphibians and corals. Source: http://www.iucnredlist.org
More information on interpreting an RLI can be found in: The IUCN Red List Index – Guidance for National and Regional Use