Streblus banksii is a shrub or small tree endemic to Aotearoa / New Zealand where it occurs on Te Ika a Maui / North Island and offshore islands. The total population size is suspected to be 8,000 mature individuals in situ. Of these, most are secure on predator free islands where there is ample recruitment, however, there is ongoing loss in sites where browsing animals are not managed. The area of occupancy is estimated to be less than 2,000 km² but the species occurs in more than ten locations. It is assessed as Near Threatened.

This species is endemic to Aotearoa / New Zealand where it occurs on Te Ika a Maui / North Island (and offshore islands) – northern portion from Te Aupouri south of the Bay of Plenty thence disjunct to Tairawhiti / East Cape, and the Wairarapa, Horowhenua and Wellington region as well as on the Te Wai Pounamu / South Island (Marlborough Sounds, Golden Bay Region). Distribution map is generalised.

No nation-wide accurate counts of individuals have been made. Population estimates are also clouded by the recent (2023) discovery that many plants referred to Streblus banksii are part of an introgressive hybrid swarm, S. banksii × heterophyllus (McKenzie 2023; but note S. hterophyllus is a synonym of Paratrophis microphylla, POWO 2023). S. banksii – as circumscribed here comprises those plants with entire rather than lobed or sinuate-lobed leaves. Such plants are now mostly found in eastern Northland on islands with some remnant stands on coastal headlands and in coastal forest; on some islands in the Hauraki Gulf; on islands and headlands of the Coromandel Peninsula south to Tuhua / Mayor Island; in coastal forest remnants along the eastern portion of Tairawhiti / East Cape; and in alluvial forest, coastal headlands and islands from the Wairarapa east to the Horowhenua, Wellington Region (Upper and Lower Hutt, Porirua), portions of the Marlborough Sounds (headlands and islands) and Golden Bay. Collectively there are probably <8,000 mature individuals (de Lange et al. 2018 gave a range of 5,000–20,000), with strongholds confined to those sites free of browsing animals, including rats (Rattus exulans, R. norvegicus and R. rattus) – which predate seed and seedlings.

Outside of sites where all browsing animals have been excluded, S. banksii remains as often isolated mature trees, which, over time will become senescent and die. The longevity of the species is unknown, though based on historic images and accounts of significant trees, Streblus banksii may live in excess of 200 years. At a national level a decline rate for the species of 5% over the next three generations seems likely, considering that most of the decline in this species happened historically through habitat loss. Outside those areas where threats cannot be controlled, the species is recovering such that as a whole the total population of this species can be considered to be increasing.

This is a dioecious shrub or tree up to 12 m tall of coastal and lowland forests, preferring deep, fertile soils. The largest trees are often found on alluvial terraces, valley heads or on colluvial aprons. On offshore islands Streblus prefers forested habitats, such as valley floors but it is also found in drier habitats and on skeletal soils, and it may at times be found on steep cliff faces, rock ledges, or as stunted shrubs on cobble/boulder beaches (de Lange 2023). Streblus is rarely a canopy dominant, exceptions to this are locations such as Simmonds Islands (Motu Puruhi and Terakautuhaka) and Middle Island where it forms 80% of the forest cover. More usually trees grow as a minor component of coastal and lowland forests dominated by Dysoxylum spectabile, Beilschmiedia tawa, and Metrosideros excelsa. In these habitats, it can be easily overlooked. The inflorescences of Streblus banksii are often attacked, distorted, and rendered dysfunctional by the endemic eriophyid mite, Eriophyes paratrophis. Severe infestations of the mite help make Streblus trees more conspicuous in the dense forested habitat these trees often grow in.

Streblus banksii undoubtedly experienced a range contraction following human settlement of Aotearoa / New Zealand. The fertile alluvial lowland forests where the largest trees are known were rapidly converted in farmland. There are now very few functional remnants containing this species left. As the species is dioecious there is the added issue that some forest remnants now contain single or highly isolated trees, which are thus unable to reproduce. Streblus is also highly palatable, seedlings, saplings and adult trees are sought out by livestock, feral goats (Capra hircus) deer (Cervidae), feral pigs (Sus scrofa), possums (Trichosurus vulpecula) and rats (three species). Rats consume seeds, so on those offshore islands with rats present, Streblus is only found as adults that predate rodent arrival. During the summer of 2022 / 2023 the exceptionally high rainfall added a new threat; mature trees in Tairawhiti / East Cape coastal forests overlying soils prone to waterlogging, collapsed, and died – evidently Streblus cannot tolerate long periods of waterlogging. Finally, hybridism between Streblus banksii and S. heterophyllus (=Paratrophis microphylla POWO 2023), that had been inferred by the numerous morphologically intermediate specimens found between these two species, has been confirmed using DNA analysis (McKenzie 2023). It is now evident that wherever the range of the parent trees overlaps, hybridism is common, and it is the hybrids rather than S. banksii which are more commonly seen on the mainland portion of the range of S. banksii. An additional threat to these island refugia is fire (human induced or otherwise) especially as some of the islands occupied by this species are remote.

At a regional level Streblus banksii has been listed as a ‘Relict’ (Simpkins et al. 2022). Nationally the species was assessed as ‘At Risk / Relict’ by de Lange et al. (2018), qualified ‘PD’ [Partial Decline], ‘Sp’ [Sparse]. The qualification of ‘PD’ reflecting the gradual loss of specimens from sites where the threats cannot be managed. These losses are of course balanced by the increasing number of predator free sites with this species.

This species is occasionally seen in cultivation and sometimes sold by specialist native plant nurseries. Streblus banksii has been used for street and park plantings and in some locations, such as Tamaki Makaurau / Auckland City this has led to plants appearing in the bird dispersed ‘seed rain’ of residential gardens.

The majority of the remaining Streblus banksii populations are now found on offshore islands historically free or recently made free from the majority of the browsing animals responsible for its extirpation from the main islands of Aotearoa / New Zealand. On those islands now free of browsing animals provided there are male and female Streblus, recovery of this species is rapid. Therefore, to secure this species requires excellent biosecurity to detect pest incursions and undertake a rapid response to these. An additional threat to these island refugia is fire (human induced or otherwise) especially as some of the islands occupied by this species are remote, so a fire would be difficult to detect. Fortunately, biosecurity and plans for the management of fires and pest incursions have been prepared for the majority of the islands groups supporting Streblus, by the New Zealand Department of Conservation in partnership with iwi, not so much for this species but rather because the islands it inhabits support a range threatened biota sensitive to the same threats. 

Streblus banksii has been assessed as a ‘Relict’ species by de Lange et al. (2018) in recognition that most of the decline in the species happened historically, and that with the exception of some parts of this species range, the remaining populations are either stable or increasing in size. The threats facing this species are well known, they can be managed for, and provided they are there is no reason to furnish this species with a higher threat assessment.