It grows in lower montane myrtisilva (myrtle forests) from 400 to 700 meters elevation, where Myrceugenia schulzei is the dominant tree.[1][2] According to historical records Fagara externa was once abundant, but is now scarce. In 2006 it was estimated that there are 500 to 1,000 individuals on the island, with the largest concentration in the El Blindado sector. The species' area of occupancy (AOO) is estimated to be <10 km2. In 2023 the IUCN Red List assessed the species as critically endangered.[1]
^Josef Greimler, Patricio López-Sepulveda, Karl Reiter, Carlos Baeza, Patricio Peñailillo, Eduardo Ruiz, Patricio Novoa, Alejandro Gatica, and Tod Stuessy "Vegetation of Alejandro Selkirk Island (Isla Masafuera), Juan Fernández Archipelago, Chile," Pacific Science 67(2), 267-282, (1 April 2013). https://doi.org/10.2984/67.2.9
^Zanthoxylum externum (Skottsb.) Stuessy in GBIF Secretariat (2023). GBIF Backbone Taxonomy. Checklist dataset https://doi.org/10.15468/39omei accessed via GBIF.org on 2025-02-02.
^Penneckamp D. 2019. Suplemento a la flora vascular silvestre del Archipiélago Juan Fernández (Primera Edición). Capítulo adicional. 724–750. [1] Accessed 1 February 2025.
^Niklas Reichelt, Jun Wen, Claudia Pätzold, Marc S Appelhans, Target enrichment improves phylogenetic resolution in the genus Zanthoxylum (Rutaceae) and indicates both incomplete lineage sorting and hybridization events, Annals of Botany, Volume 128, Issue 4, 17 September 2021, Pages 497–510, https://doi.org/10.1093/aob/mcab092
