Trends in botanical exploration in Nigeria forecast over 1000 yet undescribed vascular plant species.

Background and aims: Taxonomists are primary actors of biodiversity assessment. At the same time, there is awareness by the taxonomic community at large that the field is going through a crisis, sometimes referred to as the 'taxonomic impediment'. Coupled with the ongoing biodiversity crisis, or 6th mass extinction, this biodiversity impedance puts at risk the target set in the Convention on Biological Diversity's (CBD) Global Biodiversity Framework vision 2050, which calls for urgent action to '… put biodiversity on a path to recovery by 2030 for the benefit of planet and people'. This risk is particularly pronounced in tropical African countries where taxonomic studies are done on an ad hoc basis. In this study, our aim is to investigate the historical trends in botanical exploration of vascular plants in Nigeria and forecast the near-term (50-year) description of presently unknown species, which we use to discuss scenarios of taxonomic effort that may be necessary for a comprehensive biodiversity assessment in the country.

Methods: The study is based on a dataset from the World Checklist of Vascular Plants, containing all vascular plant species reported to occur in Nigeria. We fit nested Bayesian time series regressions to estimate the long-term trend in the rate of description of vascular plant species in Nigeria. From these models, we use an ensemble forecast to estimate the number of species descriptions by the year 2070, and then evaluate the description rates per taxonomist required to meet this estimate under different totals of active taxonomists.

Key results: We find a striking difference in species description between Nigerian botanists and their foreign counterparts, with the former contributing relatively small numbers. Additionally, only a fraction of the authors involved in describing Nigeria's vascular plants are of indigenous origin. Our study reveals that the number of new species described annually exhibits a long-term increasing trend, with an average of 19.5 species described per year. However, after taking into account year-to-year variability and the number of taxonomists active in a given year, the long-term trend in species descriptions credibly declines over time. While the number of authors involved in describing species has generally increased over time, it has remained stable since the 1950s. Predictions for the number of new species descriptions by 2070 vary by model, with an ensemble prediction estimating 1140 species descriptions, but ranging from 1004 to 2239 between individual models.

Conclusions: The study estimates that current levels of taxonomic activity should lead to a 20 % increase in known species of vascular plants in Nigeria over the next 50 years, which is still probably an underestimate of the true, unknown species richness. Urgent action is needed to address the taxonomic impediment so that local taxonomic studies in tropical African countries can achieve the CBD's Global Biodiversity Framework vision 2050. Here, we outline some key pathways to achieving this goal.