Development of a new continental-scale index for freshwater assessment based on dragonfly assemblages

摘要

African freshwater ecosystems are increasingly being impacted by humans, requiring an effective tool to assess these impacts for future conservation action. Such a tool, the Dragonfly Biotic Index (DBI), was earlier developed to assess the quality of South Africa's freshwater ecosystems and is based on combining the scores of three subindices (geographical distribution, threat status, and habitat sensitivity) for each South African dragonfly species. The sum of the DBI scores for all the species recorded at assessed sites indicates the relative quality of these sites. The International Union for the Conservation of Nature/Species Survival Commission (IUCN/SSC) has assessed the threat status of certain aquatic taxa in Africa, including dragonflies. These assessments, coupled with the latest information on the geographical distribution of each species, makes it possible here to geographically expand the South African DBI into a continental-scale assessment index (the African Dragonfly Biotic Index (ADBI)) by adapting the South African DBI sub-indices. We develop this continental index here. However, there are challenges when undertaking an assessment at the continental scale compared to a national scale. In particular, the habitat sensitivity sub-index of the South African DBI is a relative, quantitative measure based on numbers of individual dragonflies recorded from natural versus human-modified or artificial freshwater systems. While the data for the two sub-indices, species' geographical distribution and Red List threat statuses, are available across the continent, this is not the case for the habitat sensitivity sub-index at this large spatial scale. This meant that an alternative sub-index measure was required. We overcame this challenge by exploring an alternative sub-index, i.e. the 'species vulnerability sub-index', based on knowledge of the vulnerabilities of the species to certain types of landscape transformation. Then, the species vulnerability sub-index scores were calculated and combined with the geographical distribution and Red List threat status sub-index scores to develop ADBI scores for a core of 604 dragonfly species with adequate data across the African continent. These ADBI scores provide a workable framework and baseline for determining freshwater quality, both lotic and lentic, relative to human disturbance at a continental spatial scale. The ADBI enables the monitoring of quality changes, for better or worse, over the continent in years to come. Overall, the ADBI also has the potential to help identify threats to, and sensitivities of, African freshwater ecosystems, leading to conservation action.