Urban Parks of Northeast China Harbor Surprising Earthworm Genetic Diversity, First Large Survey Reveals

Beneath the manicured lawns and shaded pathways of city parks across Northeast China, an unassuming but ecologically powerful workforce is quietly shaping the soil. Earthworms, often called ecosystem engineers for their ability to transform soil structure, nutrient cycling, and microbial communities, have now received their most comprehensive census in the region’s urban green spaces. A new study published in Subterranean Biology surveyed 42 urban parks spanning Liaoning, Jilin, Heilongjiang, and eastern Inner Mongolia, collecting and identifying 604 earthworm individuals. The results reveal a surprisingly rich community of 12 species across seven genera and three families, and, more strikingly, expose deeply divergent genetic stories hidden within the four most dominant species.

The research team focused on urban parks because these habitats serve as replicated, human-designed green islands surrounded by asphalt and concrete. Soils in such parks are repeatedly disturbed by trampling, irrigation, pruning, and turf management, pressures that re-engineer physical and chemical profiles far beyond those of adjacent natural soils. Yet parks also buffer urban heat islands, fine-tune microclimates, and sustain a substantial share of city biodiversity. In the frost-dominated cities of Northeast China, where long, severe winters act as a primary filter on earthworm survival, the central question was whether urban parks function mainly as ecological traps for cold-sensitive species or as climate-buffered refugia that conserve and even disperse genetic diversity.

To answer this, the researchers sampled three microhabitats preferred by earthworms within each park: shrub patches, ditch margins, and forest-floor litter hotspots. In each microhabitat, a one-square-meter quadrat was placed randomly, surface litter was removed, and soil was excavated in 10-centimeter increments down to 30 centimeters. All earthworms encountered were hand-sorted, counted, and fixed in 95 percent ethanol for concurrent morphological and molecular analysis. Morphological identification followed established taxonomic frameworks, examining external features such as body segmentation, coloration, clitellum position, and genital pore locations, alongside internal structures like the gizzard, spermathecae, and prostates observed through dissection.

The molecular work centered on the mitochondrial cytochrome c oxidase subunit I gene, widely used as a DNA barcode. DNA was extracted from tail tissue, amplified with universal primers, and sequenced. Sequences were translated into amino acid sequences to screen for nuclear mitochondrial pseudogene interference, and no abnormal internal stop codons were detected. Genetic diversity indices, including haplotype number, polymorphic sites, haplotype diversity, and nucleotide diversity, were calculated for each dominant species, and median-joining haplotype networks were constructed to visualize evolutionary relationships among geographic populations.

Species composition results showed a balanced representation across three families: Lumbricidae, Megascolecidae, and Moniligastridae each contributed four species. Geographically, Megascolecidae exhibited a restricted distribution, occurring only in Liaoning, Jilin, and eastern Inner Mongolia, with no records from Heilongjiang Province. In contrast, Lumbricidae and Moniligastridae were widely distributed across all three provinces and eastern Inner Mongolia. Dominance analysis identified four dominant species: Metaphire tschiliensis, Drawida japonica sensu lato, Drawida ghilarovi sensu lato, and Aporrectodea caliginosa. The remaining eight species fell below the dominance threshold and were classified as non-dominant.

Genetic analysis of the dominant species revealed striking differences in their evolutionary histories. Drawida ghilarovi sensu lato exhibited the highest genetic diversity of the four, with 34 haplotypes, 214 polymorphic sites, haplotype diversity of 0.953, and nucleotide diversity of 0.123. Its haplotypes formed six independent clades in the phylogenetic tree, and the haplotype network displayed a highly complex reticulate structure with no shared haplotypes among geographic populations, indicating significant population differentiation. This pattern is characteristic of historically continuous and stable populations, suggesting that D. ghilarovi sensu lato in northeastern China may represent a species complex containing cryptic species or significantly differentiated geographic subspecies.

Drawida japonica sensu lato also showed high haplotype and nucleotide diversity, with 27 haplotypes and a star-like radiation pattern in the haplotype network centered on a primary node. This topology suggests the population may have undergone a recent rapid expansion event, possibly associated with the rapid establishment of park green spaces and habitat expansion during urbanization. Notably, shared haplotypes between the Shenyang and Changchun populations strongly suggest long-distance gene flow mediated by human activities. Although earthworms possess limited natural dispersal capacity, they can achieve long-distance passive dispersal through the transport of plant root systems and soil, and the establishment and maintenance of urban green space systems involve frequent transfers of soil and nursery stock.

In contrast, Aporrectodea caliginosa and Metaphire tschiliensis exhibited relatively low genetic diversity. Aporrectodea caliginosa, an exotic invasive species, displayed a simple genetic structure likely arising from a founder effect, and its successful invasion into China and survival in relatively cold environments are likely associated with its parthenogenetic reproductive strategy. Parthenogenesis in earthworms facilitates rapid population establishment in new habitats and enhances adaptation to diverse environmental conditions. Metaphire tschiliensis, a thermophilic and widely distributed taxon, contained 213 sequences with only 69 variable sites. However, multiple shared haplotypes across geographically distinct populations were detected in both species, which may reflect historical gene flow or shared ancestral origins. The northernmost distribution point of M. tschiliensis was recorded at Nanhu Park in Changchun, Jilin Province, likely approaching the natural northern limit of its geographical range.

The phylogeographic patterns uncovered in this study carry broader implications. As global warming reduces the frequency and intensity of extreme low-temperature events in northern winters, the suitable habitat of thermophilic species like M. tschiliensis has the potential to expand northward. Urban green spaces may play a key role in this process, as the urban heat island effect can lead to higher winter soil temperatures compared to surrounding natural areas, providing localized suitable microhabitats for species at their distributional edges. Meanwhile, urban green space management may create favorable conditions for human-mediated dispersal of earthworms, which can enhance gene flow, promote population admixture, and potentially lead to genetic homogenization.

This study represents the first systematic investigation of earthworm communities in urban green spaces across Northeast China, offering an important foundation for understanding the biogeographic evolution of earthworm communities in the region’s urban parks. The distinct genetic patterns observed across the four dominant species reveal that urban green spaces, as habitats under substantial anthropogenic disturbance, have differentially shaped earthworm lineages. Future research building on these differentiation patterns, incorporating multi-gene markers, microsatellite data, and long-term monitoring, would help elucidate the mechanisms by which urban environmental factors drive succession in earthworm communities and shape their genetic patterns, contributing to biodiversity conservation and ecosystem service enhancement in urban green space ecosystems.

Subject of Research: Earthworm species composition and genetic diversity in urban green spaces of Northeast China

Article Title: Survey of earthworm diversity and genetic diversity analysis of dominant earthworms in urban green spaces in Northeast China

Article References: Zhang, S., Xie, C., Ma, S., Wu, D., Zhang, Y., & Zhao, H. (2026). Survey of earthworm diversity and genetic diversity analysis of dominant earthworms in urban green spaces in Northeast China. Subterranean Biology, 56, 161-178. https://doi.org/10.3897/subtbiol.56.188497

Image Credits: AI Generated

DOI: 10.3897/subtbiol.56.188497

Keywords: earthworms, urban parks, Northeast China, COI barcode, genetic diversity, phylogeography, Drawida ghilarovi, Drawida japonica, Metaphire tschiliensis, Aporrectodea caliginosa, urban green spaces, cryptic species