Locating sufficient plant distribution data for accurate estimation of geographic range: The relative value of herbaria and other sources

In a megadiverse country such as South Africa, plant locality data are routinely sourced from the South African National Herbarium (PRE). Evidence suggests that large areas of the country remain poorly collected and that locality records are not always adequately represented in PRE. Our aim was to assess whether distribution information obtained exclusively from PRE adequately represented the known range of selected species. We also assessed the relative value of regional herbaria and supplementary sources of locality data. Locality information was sourced from PRE, 17 regional herbaria, sight records and literature for a subset of 121 ethnomedicinal plant species that are currently regarded to be threatened with extinction or of conservation concern according to the IUCN Red List criteria. Geographic range (km2) was calculated using distribution information (Quarter-Degree Squares, QDS) obtained from PRE and non-PRE sources. The species' ranges were examined to compare the differences in range size and the overall proportion of QDS records represented in PRE and non-PRE sources. Supplementary data obtained from regional herbaria and other sources increased the number of known QDS records by ± 45% per species across the various IUCN Red List threat categories, and the ranges increased by ± 28% per species. As the threat status of a species increased, proportionally more QDS were likely to come from supplementary sources. Rarer species tended to be found only in herbaria within their province of occupancy. 'Return for effort' analyses indicated that QDS records should be sourced from PRE plus one other herbarium located within each province in which a species of interest occurs. QDS coverage within species' geographic ranges was under-represented using only data obtained from PRE, reducing the accuracy of species occurrences and distributions relying solely on information sourced from that repository. We demonstrate that this can impact on the accuracy of conservation planning resources such as Red Lists. Our results highlight the relative importance of regional herbaria.