Polygraphus nobuchii is an endemic species of the Republic of Korea, specifically in the summit area of Halla Mountain on Jeju Island. Its area of occupancy (AOO) and extent of occurrence (EOO) are 8 km², meeting the threshold for the Critically Endangered (CR) category under criterion B. Regarding sub-criteria (a) and (b), its sole habitat is deteriorating due to the decreasing availability of its host plant, driven by climate change. A single location has been identified for this species. As a result, it qualifies as Critically Endangered.

Polygraphus nobuchii is a nut weevil species endemic to the Republic of Korea (National Institute of Biological Resources 2015). The distribution of this species is limited to Halla Mountain, Jeju Island, and no additional habitats have been found outside of its type locality, despite the several research on Halla Mountain’s invertebrate diversity and the insect species that inhabit its host.

Due to the limited knowledge of these species’ ecology and the small number of samples, a habitat map covering the entire distribution on Jeju of its only known host plants, Abies koreana (Korean Fir) and A. holophylla (Needle Fir) was used for analyses (National Institute of Biological Resources 2015, Park 2018). Recent work has found that the Korean Fir’s distribution has become mostly restricted to areas with elevations above 1,510 meters (Kim 2022). The genus Abies is mainly classified into alpine/subalpine conifer forest communities, and is affected by temperature rise caused by climate change driving alterations in precipitation, moisture stress (Ahn et al. 2019, Koo and Kim 2020), and competing species (Song 2011, Kim et al. 2015). According to Kim et al. (2024), in 1918, the area of the A. koreana forest was analysed to be 1,168.4 hectares. Through continuous decline, the forest area had shrunk by 562.4 hectares, leaving 606 hectares of forest in 2021 (Kim et al. 2024). The same trends were predicted in the future (Kim et al. 2015, Koo et al. 2016, Koo and Kim, 2020, Choo et al. 2023). Koo et al.'s (2016) model predicted that rising temperatures, along with fluctuating weather patterns would decline the distribution of A. koreana and, consequently, cause the risk of extinction due to changes in their current habitat (Koo et al. 2016).

Following the decline of its host trees, this species is suspected to be decreasing throughout time. Currently, the area of occupancy for this species is estimated to be 8 km² based on a 2x2 km grid overlay on the range map. Its extent of occurrence (EOO) is estimated to be 6.6 km² based on total area within a minimum convex polygon around all known and inferred occurrences, but because this is less than the AOO value, the EOO takes the AOO value.

This species' population is suspected to be declining due to the decline of its host plants.

Polygraphus nobuchii has been found only from the genus Abies in Halla mountain. It is well known that relatives who live in coniferous forests are host plant specific because these habitats provide critical features such as stable temperatures, high humidity, and shaded environments, which are crucial for their growth (Park 2016). Furthermore, it is well known that this genus uses the interior of fruit as food, so its structural suitability is important (Kim et al. 2014). The regional characteristics of coniferous forests specifically support their development by offering optimal temperature and humidity conditions, along with protective shade (Park 2016). Additionally, these species rely on fungi and microorganisms hosted inside host trees, and it cannot utilize the symbiotic microorganisms associated with other tree species (Mehlhorn 2016). This strict ecological dependency underscores the species’ vulnerability to changes in its coniferous forest habitats, proving the significance of known host species and their impact on P. nobuchii.

The decrease of host plants is a dominant threat. As this species is suspected to depend on genus Abies as host plants, this species' population is likely to be influenced by host plant population dynamics. Climate change significantly impacts the survival of host plants, driving the continuing decline of the host species’ distribution. Specifically, the distribution of conifers has reduced by 34% between 1988 and 2002 (Kim and Lee 2013), and 15.2% during 2006 to 2015 (Koo and Kim 2020). Therefore, this species is facing inevitable continuing decline, and has one location defined by this threat.

There is no trade in this taxon.

Conservative actions need to address the decline of this species’ host plants, Abies koreana and Abies holophylla, due to climate change. Therefore, actions should be taken to conserve A. koreana and A. holophylla. To find proper conservation methods, more research should be conducted on the population size, distribution, and life cycle of this species. Meanwhile, the adults of this species were all found on Abies species, but further investigation into the larvae is needed to confirm whether these trees are definitively used as hosts.