Ecological niche information supports taxonomic delimitation of Irvingia gabonensis and I. wombolu (Irvingiaceae)

Abstract

African bush mango trees (Irvingiaceae) are priority food trees in West and Central Africa. There are bitter- and sweet-fruited species, which are difficult to distinguish based on morphological characters. This has led to a debate about their correct taxonomic status. Furthermore, it is unclear whether they are native to the Dahomey Gap, the dry and hot area, separating the two West African forest blocks. This study evaluates the ecological differences between bitter- and sweet-fruited species in tropical Africa, and the nature (wild vs. cultivated) of the occurrences in the Dahomey Gap, in order to discuss the current taxonomical opinions. Irvingia gabonensis and I. wombolu occurrence data were combined with climate and soil data in MaxEnt to produce environmental niche models. Environmental niche identity tests were carried out in ENM-Tools. Wild sweet-fruited trees were predicted in the Guinean–Congolian phytogeographical region, while the predicted occurrence of bitter-fruited trees extended to the Guineo–Congolia/Sudania and Lake Victoria transition zones. The related niche difference is significant, supporting the taxonomical opinion that bitter- and sweet-fruited species are two different taxa. We also conclude that bitter-fruited trees occur naturally in the Volta forests (Dahomey Gap). Moreover, our results support that I. gabonensis is not native to the Dahomey Gap. In historical times, they were probably introduced from Nigeria.