Cryptic invasion by non-native reed canary grass, Phalaris arundinacea, in Japan endangers native conspecific populations through displacement and hybridization

Invasions by non-native conspecific populations can result in genetic disturbance and even local extinction of native populations. In cases of intraspecific cryptic invasion, where distinction between native and non-native individuals by morphology is difficult, it is challenging to delineate the extent of invasion and assess its ecological impact. Phalaris arundinacea is a cosmopolitan grass species native to Eurasia and North America. Although it is also native to Japan, commercial strains from the USA have been introduced and grown for nearly 150 years. The cryptic spread of these non-native genotypes has raised concerns regarding their impact on native populations. Here, we investigated the distributions of native, non-native, and admixed P. arundinacea in eastern and northern Japan. We collected over 270 individuals in the Hokkaido, Tohoku, Kanto, and Koshinetsu regions and analyzed genome-wide single nucleotide polymorphisms and chloroplast sequences. Nearly all individuals north of 39.7°N were genetically identified as purely non-native, whereas admixed individuals were widespread south of this latitude. These findings suggest that the effect of non-native P. arundinacea on native individuals varies by region: native individuals have been largely displaced in the Hokkaido and northern Tohoku regions and genetically introgressed in the rest of the Tohoku region. Furthermore, in the Kanto region, pure native genotypes were detected only in a restricted area. Native populations in both eastern and northern Japan are at substantial risk of local extinction.