Genetics in Threatened Plant Management: The Diversity, Morphology and Habitat of northern North Island ewekuri (Streblus banksii)

Abstract

One of the greatest unresolved concerns in plant conservation is the genetic management of fragmented populations and threatened species. Although increasingly popular as a tool for ecological management, little is known of how to best utilise population genetics within conservation. Recently, a need to balance ecosourcing practices and maximise genetic diversity within conservation management has arisen. Streblus banksii (known as ewekuri or the large-leaved milk tree) is a poorly-studied endemic tree species considered ‘At Risk’ in Aotearoa New Zealand, and reestablishing populations in appropriate habitats could improve the species’ conservation status. Ewekuri was common across much of northern Aotearoa prior to human arrival, and has declined due to deforestation and herbivory by rat (Rattus) species. Ewekuri also may be threatened by genetic swamping due to suggested hybridization between it and a close relative, tūrepo (Streblus heterophyllus). The aims of this study were to determine the genetic, morphological and demographic variation of ewekuri, putative hybrids and tūrepo across the wider Tāmaki Makaurau region then use the resulting genetic structure and associated community composition to advise conservation efforts and suggest avenues of further research. I found that ewekuri are readily hybridising with tūrepo, forming a polymorphic and introgressive hybrid swarm spread across the study range. Those populations recognized as ‘pure’ ewekuri appear to have low levels of genetic variation. I identified key leaf characteristics of sampled ewekuri, putative hybrids and tūrepo, and found that ‘pure’ ewekuri are largely restricted to offshore islands generally in coastal broadleaf forest. I suggest that a variety of conservation methods are utilised in situ and ex situ to reduce predicted risks to ewekuri populations in the face of climate change, deforestation and pest incursions. I also suggest further study of risks to ewekuri from their hybrids, taxonomic recircumscription of the ewekuri type suite and assignment of hybrids to a nothotaxon, as well as further ecological, genetic and morphological study of ewekuri across their entire range. My findings are applicable to other woody species as a case study in conservation genetics, both in Aotearoa and globally.