The IUCN Red List of Threatened Species is the most comprehensive database pertaining to the conservation of biodiversity. This database includes species assessments against a set of criteria to determine to what degree a species is threatened with extinction. These assessments are generally widely accepted and highly regarded. They are also often used to make conservation decisions, including the allocation of funding. Therefore, it is important that IUCN Red List assessments are both reliaible (i.e., of high quality) and comprehensive (i.e., cover a large proportion of species in a particular area or taxonmoic group). For regions or taxonomic groups of high biodiversity but low coverage in the IUCN Red List, rapid assessments that utilise the knowledge of experts are sometimes undertaken (Fig. 1).
Therefore, a trade-off between the quality and quantity of assessments potentially arises when large numbers of IUCN Red List assessments are required. Highly reliable assessments are based on detailed knowledge of the species by the person(s) undertaking the assessment. Unless an expert has undertaken recent, detailed work on the species to be assessed, existing knowledge should be supplemented by targeted field surveys. Field surveys are important because they provide current information and can allow harnessing a wealth of local knowledge. However, they entail considerable costs with regard to time and money – resources that could be used to assess more species based on existing, but unrecorded, knowledge among experts. Such assessments are often undertaken in workshops involving several experts (Fig. 1).
A good example of this conundrum is the Global Tree Assessment, which had the goal to assess the IUCN Red List status for all tree species in the world by 2020. While this goal was clearly overly ambitious, the project succeeded in assessing about half of all tree species. Most of the newly added assessments were based solely on expert knowledge. I was involved in two such workshops. Each assembled the best botanical experts in the Pacific, who enthusiastically and committedly worked towards the best possible assessments (Fig. 1). Assessments were generally based on recollections of particular species from past field visits with a broad focus, such as biodiversity surveys. Therefore, while these expert-based asessements clearly improved the quantity of assessments and collected vital new information, one could question their reliability.
In 2019, I had the opportunity to test the reliability of one of our expert assessments, when I visited a subpopulation of the Critically Endangered Pterocymbium oceanicum (Fig. 2) with students from the University of South Australia, Abaca Villagers and NatureFiji-MareqetiViti staff. Three years earlier our team of experts conservatively estimated this subpopulation near the village of Abaca as consisting of about 100 individuals during a Global Tree Assessment workshop. This retrospective assessment was possible because P. oceanicum is a very large tree that protrudes above the canopy (Fig. 2) and we had passed the subpopulation several times when ascending Mount Batilamu.
Our detailed assessment of the stand near Abaca village suggested that the expert assessment had been good, counting 132 individuals. However, we also found out that there were other stands in close proximity (within 5km), increasing the total size of this subpopulation four-fold to 433 individuals. In addition, we were able to collect invaluable ecological information on the species and identified the invasive African Tulip Tree, Spathodea campanulata, as a key threat, as both species are fast-growing pioneer species.
Therefore, in this case, the Global Tree Assessment IUCN Red List assessments helped to obtain a more comprehensive conservation status of Fijian trees, captured vital information about tree conservation among experts, produced a good estimate of the size of a particular stand, and helped to raise awareness that one of the biggest trees in Oceania is threatened with extinction. However, the assessment had important shortcomings. It severely underestimated the total population size of Pterocymbium oceanicum and did not identify an important threat. Therefore, while our findings for the Abaca subpopulation highlight the relevance and importance of expert knowledge for conservation and assessments of conservation status, they also show that field assessments for threatened species are vital for obtaining high-quality assessments.
While it is dangerous to extrapolate from a single case, it appears likely that many expert-based, rapid assessments for other tree species have similar shortcomings. Therefore, providing an indication about the likely quality of the assessments in the IUCN Red List would be a useful addition. This could be achieved by indicating which assessments are based on recollections by experts and which are based on detailed field surveys, and by introducing a measure of expert confidence in the assessment.
Further Reading: Keppel G, S Peters, J Taoi, N Raituku & N Thomas-Moko (2021) The threat by the invasive African tulip tree, Spathodea campanulata P. Beauv., for the critically endangered Fijian tree, Pterocymbium oceanicum AC Sm.; revisiting an assessment based on expert knowledge after extensive field surveys. Pacific Conservation Biology, early view. (you can request the article from the author of this blog, if you do not have access to the journal)