The threat by the invasive African tulip tree,

Abstract

Expert knowledge is widely used for assessing the conservation status of little-known species. Pterocymbium oceanicum is a large, emergent tree endemic to Fiji. Experts classified the species as ‘Critically Endangered’ (IUCN Red List) in 2016 and estimated the population size at <250 mature individuals with the largest subpopulation consisting of a single stand of 100 individuals. We surveyed this stand and counted 133 individuals, which validated the expert estimate. However, we discovered an additional three stands nearby, increasing the total subpopulation size to 433 individuals. Therefore, the expert estimate reliably and conservatively estimated population size but missed important information. Field surveys suggest that P. oceanicum has a narrow ecological niche as a long-lived early pioneer at mid-elevations (400–800 m) in seasonally dry forests, one of Fiji’s most degraded ecosystems. The introduced African Tulip Tree, Spathodea campanulata, is invading the sampled subpopulation and shares the characteristics of being a large, fast-growing and early successional tree. Therefore, the narrow niche of P. oceanicum may now be threatened by S. campanulata. Our study highlights that expert knowledge can provide valuable information about threatened species but also has limitations. IUCN Red List assessments based solely on expert assessments should therefore be cautiously interpreted. The invasion by S. campanulata poses a serious threat to P. oceanicum and needs to be carefully monitored, with the impacts of S. campanulata on the establishment and growth of P. oceanicum requiring urgent investigation.