Introduction

The Himalayan landscape has undergone extensive deforestation, primarily driven by agricultural expansion, urbanization, and large-scale development projects. The remaining forests have been significantly degraded due to indiscriminate logging for timber, fuelwood collection, overgrazing, and frequent forest fires. These disturbances, along with habitat loss, overexploitation of species, the spread of invasive alien species, and climate change, are major drivers of biodiversity decline in the region. According to several studies, if the current rate of deforestation continues, approximately 25% of the Himalaya’s endemic species, including 366 indigenous vascular plants, could face extinction by the end of the century1,2,3.

The population of Taxus contorta Griffith, a gymnosperm native to the moist temperate forests of the Himalayas, has declined sharply. This decline is largely attributed to intensive commercial exploitation for extraction of anti-cancer compounds, along with ongoing habitat degradation and poor natural regeneration. Consequently, the species is listed as endangered on the IUCN Red List of Threatened Species4,5,6.

The Indian National Mission on Himalayan Studies (NMHS) has initiated a dedicated programme to investigate the factors contributing to the poor regeneration of T. contorta, with the aim of preventing its extinction and restoring its population in natural habitats. The initial phase of the programme focuses on analyzing abiotic and biotic factors affecting its growth, assessing suitable habitats for reintroduction, and identifying biotic indicators to monitor soil and ecosystem health. In alignment with the Mission’s goals, the present study evaluates soil biodiversity in a moist temperate forest in Himachal Pradesh, Northwest Himalayas, where T. contorta shows significant regeneration.

Earthworms constitute a major component of animal biomass in many terrestrial ecosystems7. As ecosystem engineers, they actively modify soil structure and influence the distribution of resources8,9,10. Earthworms also affect key soil properties such as pH, organic matter content, nitrogen concentration, nutrient availability, and overall fertility11,12. Globally, approximately 5406 earthworm species have been described, with more than 87 recognised as cosmopolitan or “peregrine” species, which commonly thrive in disturbed habitats13. Some of these peregrine species can become invasive, posing significant threats to native biodiversity and stability of various forest ecosystem stability14,15.

Earthworm diversity in Himachal Pradesh, a northwest Himalayan state and the focus of present study, is notably high, with 47 recorded species16. This diversity is primarily composed of exotic and native cosmopolitan earthworms, which together constitute 91.5% of the region’s earthworm diversity. These species have successfully colonized a range of land-use systems, including agricultural fields, orchards, grasslands and forests17,18,19,20.

Aligned with NMHS goals, the present study assesses soil biodiversity in a moist temperate forest in Himachal Pradesh, which supports a significant regenerating population of T. contorta. Additionally, it explores the spread of exotic earthworms within this ecosystem. Extensive sampling revealed a substantial population of O. tyrtaeum (Savigny), an exotic lumbricid earthworm of European origin. Exotic earthworms can significantly alter soil chemistry and contribute to decline of native biodiversity14,21. Specifically, O. tyrtaeum has been reported to influence soil nutrient dynamics, drastically reduce microbial and microarthropod communities, and significantly decrease litter thickness, organic layers, C/N ratio, and soil pH22,23.

Studies indicate that lumbricid earthworms have predominantly colonized the Northwest Himalayas at elevations above 1000 m, where they now outcompete native earthworm species24. The Himalayan moist temperate forests in Himachal Pradesh, located at elevations of 1500 to 3300 m, cover 40.24% of the state’s geographical area25. These forests rank among the most complex and productive ecosystems in the world, supporting rich biodiversity and providing essential services26. However, they experience higher biotic pressure and anthropogenic disturbances compared to other forest types26. They are highly prone to fragmentation and degradation, making them particularly vulnerable to climate change and human activities27.

Understanding the changes in the abundance of exotic earthworms and their impact on soil structure is crucial for evaluating their influence on various microhabitats. Despite their ecological significance, few studies have compared the effects of different earthworm species on the region’s moist temperate forests17,28. To address this gap, we conducted field experiments in the northern, northeastern, and southwestern aspects of a moist temperate forest to assess earthworm abundance and distribution. The objective of this study was to analyze the spatial distribution of O. tyrtaeum populations in temperate T. contorta stands and examine the associated edaphic and environmental factors.

Materials and methods

Study sites overview

The study was conducted in a moist temperate forest with naturally regenerating stands of T. contorta at Himri (31.1421° N, 77.4795° E; altitude 2200–2400 m), located in the Sunni orest division of Shimla District, Himachal Pradesh, India (Fig. 1). The study site was selected based on the availability and feasibility of Taxus contorta forests, using occurrence data provided by the Forest Department, Shimla, Himachal Pradesh. The Himri forest area, covering 556.03 hectares, was selected based on data from the Himachal Pradesh State Forest Department indicating a regenerating T. contorta population. Estimated to be around 300 years old, the forest is subject to occasional rotation felling, hay harvesting, and deadwood collection. The area receives an average annual rainfall of 2000 mm, with temperatures ranging from a minimum of 7 °C to a maximum of 19 °C. The soil is classified as brown hill type with a sandy loam texture.

Fig. 1
Fig. 1The alternative text for this image may have been generated using AI.
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Map of study area, showing sampling sites.

Earthworm sampling

Based on preliminary field surveys, three study sites were selected where T. contorta formed part of the tree canopy. These sites were located on the north, northeast, and southwest-facing slopes, each characterized by distinct vegetation compositions (Table 1) and varying degrees of canopy openness. They were delineated using a compass, and their locations were recorded with GPS coordinates to ensure accurate sampling and consistency across surveys. The occurrence points of T. contorta and other selected plant species were recorded using a Garmin eTrex 10 GPS receiver with an accuracy of ± 5 m.

Table 1 Aspect-wise phytological attributes (%) in moist temperate forest at Himri, District Shimla, Himachal Pradesh.
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Earthworm sampling was conducted over two consecutive years (2020–2022) during the spring (March 2021 & 2022) and autumn (November 2020 & 2021) seasons, when soil moisture and earthworm activity were most favourable. A total of 57 plots, each measuring 5 × 5 m2, were randomly selected for sampling across the three different aspects: 24 in the north, 24 in the northeast, and 9 in the southwest. The smaller number of plots in the southwest was due to the limited extent of Taxus canopy. Plot locations were determined using an online random number generator.

In each plot, three soil monoliths (25 cm × 25 cm × 30 cm) were extracted for earthworm collection following the Tropical Soil Biology and Fertility Program protocol for soil macroinvertebrate sampling29. Earthworms were placed in a flat-bottomed tray with a small amount of water and narcotized by gradually adding 70% ethyl alcohol. Once narcotized, they were transferred to another tray and fixed in 5% formalin for 4–6 h. Subsequently, they were preserved in vials containing 5% formalin, which were labelled with the location name and collection date.

Environmental and edaphic parameters

On-site data collection involved measuring various environmental factors in the field. A Spherical Crown Densiometer was used to assess tree canopy cover, while a steel scale was used to measure litter thickness. The ground temperature was recorded hourly using an iButton Temp Logger placed 5 cm below the soil surface. However, rainfall, air temperature, and humidity data were obtained from ERA5 through Elistar Geomatics, Delhi.

A total of 228 composite soil samples and an equal number of litter samples, each weighing 150 g, were collected from the same earthworm sampling sites in each plot over two seasons per year for laboratory analysis. The physicochemical properties of the soil and litter were analyzed using specific techniques: soil pH was measured by a pH meter30; electric conductivity with a digital electrical conductivity meter31; organic carbon was determined using Walkley and Black’s rapid titration method32; available nitrogen by the Kjeldahl method33; available potassium using the Flame photometer method34; and available phosphorus was measured using Bray’s method for acidic soils35.

Additionally, 50 g Soil and litter samples were collected and weighed in the field for moisture analysis. Soil moisture was determined using the gravimetric method36, while litter moisture was determined using the Litter Moisture Content Monitoring (LMC) method37.

Species identification

In the laboratory, earthworms were identified to the species level using a Magnus MSZ-Bi microscope to examine taxonomic characteristics with identification keys from Gates and Blakemore38,39,40. Biomass was determined by weighing preserved specimens after drying them on filter paper, without gut content removal, and expressed in grams per square meter (g m⁻2).

Statistical analysis

The soil data were analyzed and are presented as means and standard error ( ±). Differences in soil characteristics across sampling sites and earthworm diversity parameters were assessed using one-way ANOVA, followed by Tukey’s post hoc test at a 5% significance level. Principal Component Analysis (PCA) was conducted to evaluate the influence of soil variables on earthworm distribution. Components with eigenvalues greater than 1 were extracted, and the percentage of variance explained by each component was reported. Statistical analyses were performed using PAST (Version 4.13) and SPSS (Version 16).

Results

Edaphic and environmental parameters

The northeast aspect exhibited significantly higher average soil moisture content (30.5%) and litter moisture content (34.4%) compared to the north and southwest sampling sites. Similarly, the highest concentrations of soil organic carbon (4.9%) and available nitrogen (475.9 kg/ha) were recorded in the northeast, whereas the southwest showed the lowest levels of organic carbon (4.1%) and nitrogen (463.6 kg/ha). Additionally, the northeast had the highest canopy coverage (93.8%), followed by the southwest, while the north had the lowest (90.9%). In contrast, litter thickness was greatest in the southwest (1.9 cm), followed by the northeast (1.7 cm), with the north exhibiting similar values.

Soil pH ranged from 5.8 to 6.0, with the highest values in the north and southwest and the lowest in the northeast. Similarly, available phosphorus was highest in the north (36.5 kg/ha), followed by the southwest (35.4 kg/ha), and lowest in the northeast (31.2 kg/ha).The southwest recorded the highest average electrical conductivity (101.1 ppm) and soil temperature (8.8 °C), followed by the north, while the northeast had the lowest values (77.4 ppm and 8.3 °C). Available potassium was also highest in the southwest (423.1 kg/ha), followed by the north (413.7 kg/ha), and lowest in the northeast (404.3 kg/ha) (Table 2).

Table 2 Mean values of edaphic and environmental parameters across three aspects in a mixed moist temperate forest stand of Taxus contorta in Northwest Himalaya.
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Species diversity and abundance

A total of 1101 earthworms (961 adults and 140 juvenile) were collected during the study, all identified as Octolasion tyrtaeum (Savigny, 1826)41, which belongs to the Family Lumbricidae and the Order Crassiclitellata. As shown in Table 3, the highest mean density of O. tyrtaeum was recorded in the northeast aspect (30.17ind m−2), followed closely by the north aspect (30.00 ind m−2). The lowest density was observed in the southwest aspect (15.86 ind m−2).

Table 3 Density (ind m⁻2) and biomass (g m⁻2) of Octolasion tyrtaeum across three aspects (north, northwest, and southwest) in Taxus contorta stands within a mixed moist temperate forest in the northwestern Himalaya.
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Juvenile worm density was highest in the north aspect (6.00 ind m−2), followed by the northeast (1.72 ind m−2), and lowest in the southwest (0.11 ind m−2). The adult worm population was most abundant in the north (23.87 ind m−2), followed by the northeast (23.39 ind m−2), with the lowest density recorded in the southwest (16.67 ind m−2).

The highest earthworm biomass was recorded in the northeast aspect (8.45 g/m−2), followed by the north (6.28 g/m−2). The lowest biomass was observed in the southwest aspect (5.88 g/m−2). Among juvenile worms, the highest biomass was found in the north aspect (0.36 g/m−2), followed by the southwest (0.19 g/m−2), with the lowest in the northeast (0.12 g/m−2). In contrast, adult worm biomass was highest in the northeast (8.33 g/m−2), followed by the north (5.92 g/m−2), while the lowest was recorded in the southwest (5.69 g/m−2) (Fig. 2, Table 3).

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Fig. 2The alternative text for this image may have been generated using AI.
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Density (ind m⁻2) and biomass (g m⁻2) of Octolasion tyrtaeum across three aspects (north, northwest, and southwest) in Taxus contorta stands within a mixed moist temperate forest in the northwestern Himalaya. JD, Juvenile density; AD, Adult density; TD, Total density (adult + juvenile); JB, Juvenile biomass; AB, Adult biomass; TB, Total biomass (adult + juvenile). A Tukey test was performed at the 5% significance level. Mean values followed by different letters within each row indicate statistically significant differences at p < 0.05.

Principal component analysis (PCA)

Principal Component Analysis (PCA) was performed to investigate the influence of edaphic factors on the density and biomass of O. tyrtaeum across different slope aspects (North, Northeast, and Southwest). The analysis included 11 physicochemical soil parameters measured at 57 sampling points to identify key variables affecting earthworm distribution. Components with eigenvalues greater than 1 were retained for interpretation.

The scree plot (Fig. 3) indicates the extraction of six principal components from the dataset. The PCA biplot (Fig. 4) illustrates the distribution of sampling sites based on earthworm abundance and associated soil properties. PC1 accounted for 13.77% of the total variance, primarily driven by positive loadings of soil and litter moisture, which also corresponded with increased earthworm abundance. PC2 explained 13.74% of the variance, influenced by positive loadings of pH and electrical conductivity (EC). PC3 contributed 10.36% of the variance, associated with soil organic carbon and litter thickness. PC4 (10.28% variance) showed negative loading for temperature and positive loadings for potassium and canopy cover. PC5 and PC6 explained 7.8% and 7.2% of the variance, respectively, driven by loadings of nitrogen and phosphorus (Table 4).

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Fig. 3The alternative text for this image may have been generated using AI.
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Scree plot of principal components based on the given data. Principal components with eigenvalues greater than 1 were retained for analysis.

Fig. 4
Fig. 4The alternative text for this image may have been generated using AI.
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Principal component biplots showing relationships between earthworm density and biomass and various soil and climatic parameters across three aspects (north, northeast, and southwest) in Taxus contorta stands within a mixed moist temperate forest in the northwestern Himalaya.SM, Soil moisture; EC, Electrical conductivity; ST, Soil temperature; SOC, Soil organic carbon; AN, Available nitrogen; AP, Available phosphorus; AK, Available potassium; CA, Canopy; LT, Litter thickness; LM, Litter moisture.

Table 4 Principal components and eigenvalues with total and cumulative variance of for the soil and earthworm.
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Pearson correlation coefficients

Pearson correlation analysis revealed significant positive relationships between earthworm abundance and variables such as soil moisture, organic carbon, canopy cover, leaf litter, and litter thickness. Conversely, significant negative correlations were observed with pH, EC, and temperature (Table 5).

Table 5 Pearson correlation coefficients showing the relationships between soil parameters and earthworm abundance.
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Discussion

Contrary to our initial expectations, this study found that the Himri moist temperate forest has low earthworm diversity, represented exclusively by Octolasion tyrtaeum, an exotic European species of the Family Lumbricidae. The forest environment appeared to provide favourable conditions for the survival and reproduction of this species. O. tyrtaeum belongs to the functional group of endogeic earthworms, which are generally less sensitive to environmental disturbances than epigeic and anecic species20,42.

Earthworm abundance and edaphic parameters

The population dynamics of earthworms are influenced by a range of edaphic and environmental factors, including soil structure, temperature, pH levels, total nitrogen, phosphorus, potassium, electrical conductivity, organic matter content, and the depth of leaf litter22,43,44. Our findings, based on Pearson correlation and Principal Component Analysis (PCA), corroborate these relationships. An analysis of the population dynamics of O. tyrtaeum in the Himri forest across three different geographical aspects revealed that the northern and northeastern aspects exhibited higher average density and biomass than the southwestern aspect. This variation may be attributed to higher levels of soil moisture, litter moisture, soil organic matter, available nitrogen and other related factors.

Principal Component Analysis (PCA) was applied to the 11 different physico-chemical and environmental variables, resulting in six principal components (PCs), which collectively explained 63.26% of the total variance (Table 4). PC1 accounted for 13.78%, primarily driven by soil and litter moisture. Earthworms rely largely on soil moisture for cutaneous respiration and overall survival45,46,47, often completing most of their life cycle in moist environments48. Increased soil moisture generally supports larger earthworm population49.

PC2 explained 13.74%variance, a value comparable to that of PC1, and was primarily influenced by soil pH and electrical conductivity (EC), indicating earthworms’ preference for soils with specific salt concentrations50.The soil in Himri forest is acidic, with an average pH ranging from 5.8 to 6.0. The highest pH values were recorded on the north and southwest aspects, while the lowest were found on the northeast aspect. These results are consistent with earlier studies by Minhas et al.51 and Rawat et al.52, who reported pH ranges of 5.0–5.8 in Himachal and 5.2–6.1 in the Garhwal Himalayas, respectively. The slight variations in pH observed among aspects may be attributed to their proximity and shared soil and vegetation characteristics, suggesting limited influence on earthworm density and biomass.

The relationship between pH and earthworm activity remains complex, as earthworms can tolerate pH levels beyond their preferred range and may also alter soil pH53. Although soil pH did not directly impact the earthworm population42, it indirectly affects various chemical processes within earthworms, which are crucial for determining nutrient availability in the soil42,54. Higher electrical conductivity (EC) in the southwest aspect may be associated with the lower earthworm density and biomass. This observation aligns with the findings of previous research, which highlighted the influence of EC on earthworm abundance53. EC plays a crucial role in earthworm metabolism and is directly related to soil salinity, which refers to soluble salts in the soil42,55. Moreover, since soil pH can influence salt solubility, it also indirectly affects EC and its impact on earthworm populations55.

PC3 explained 10.37% of total variance and was primarily influenced by soil organic carbon and litter thickness. Soil organic carbon is a key factor affecting distribution and abundance of earthworms56,57, serving as a primary energy source58. Several studies have reported a positive correlation between soil organic carbon content and earthworm density and biomass59,60,61.

Litter in forest ecosystems provides a crucial food source for endogeic earthworms, supporting their populations through its high organic matter content7,62. Continuous litter fall has been shown to significantly increase earthworm biomass63. Interestingly, the reduced litter thickness observed on the northern and northeastern aspects of Himri Forest may be linked to higher earthworm population and biomass. This aligns with previous findings suggesting that O. tyrtaeum plays an important role in reducing forest floor litter by incorporating it into the topsoil22. Mesocosm experiments with O. tyrtaeum by Cesarz et al.54 further revealed that a combination of litter and soil characteristics was more effective in promoting earthworm biomass than litter availability alone. It is important to emphasize that these findings represent observed associations, and further experimental work is required to establish direct causal relationships between earthworm activity and changes in litter dynamics.

PC4 accounted for 10.28% of the total variance and was associated with canopy cover, soil temperature, and available potassium. Earthworm populations tend to be higher under tree canopies compared to open areas64. Canopy cover indirectly influences earthworm abundance by enhancing soil moisture levels, which are critical for their survival. The elevated soil moisture observed on the northeastern aspect was likely due to greater canopy density, which reduces sunlight penetration and subsequently lowers soil temperature and evaporation rates42,65,66,67. Dense canopy also contributes to increased litter input and slower decomposition due to cooler temperatures, ultimately resulting in higher organic carbon concentrations68,69. Additionally, available potassium is an important edaphic factor influencing earthworm abundance57. Earthworms significantly enhance the availability of potassium in the soil by their activities and the production of nutrient-dense castings70.

PC5 explained 7.82% of the total variance and was primarily associated with available nitrogen. Earthworm biomass has been positively linked to increased nitrogen availability71. The rise in soil nitrogen can also be attributed to enhanced mineralization of organic matter in earthworm casts20,70. However, Eisenhauer et al.22 reported a decrease in soil nitrogen in areas inhabited by O. tyrtaeum in a North American forest, suggesting that the relationship between earthworms and nitrogen dynamics may be species- or context-dependent.

PC6 explained 7.27% of the total variance and was driven by available phosphorus, highlighting its role as an important edaphic factor influencing earthworm distribution.

Overall, the PCA results demonstrate that earthworm abundance in the Himri mixed moist temperate forest is closely correlated with a range of physico-chemical and environmental variables.

Potential dispersal of Octolasion tyrtaeum and implication for management of Himalayan temperate forests

It is notable that Indian cosmopolitan earthworms were absent in the forest under study, despite their successful colonization of various land-use systems in the northwestern Himalayas17,19,20,28. This suggests that the Himri temperate forest may have been unsuitable for their establishment and survival or that they were out competed by the polyhumic endogeic O. tyrtaeum. According to Szlavecz et al.72, polyhumic endogeic earthworms exhibit high plasticity, allowing them to avoid intense competition with species from other functional categories.

Furthermore, the present study did not find any species belonging to the native West Himalayan genera Perionyx, Argilophilus and Eutyphoeus, as documented by Paliwal and Julka73. These native earthworms may have retreated to more remote and less disturbed areas of the forest, making them more difficult to collect. Alternatively, they may be unable to compete with the exotic O. tyrtaeum, or the Himalayan moist temperate forests in this region may have developed without native earthworms, possibly due to their extinction during past glaciations. Notably, previous glaciation events have been linked to the loss of native earthworms in North American forests14,74,75,76.

The occurrence of O. tyrtaeum as the only species may reflect limitations in our sampling effort, potentially leading to an under representation of earthworm species diversity in this forest. Currently, there are no existing records on earthworm fauna in Himri Forest area for comparison. However, three exotic megascolecid species, Amynthas corticis, A. morrisi and Metaphire houlleti have been reported from Sunni77, a low altitude region (about 670 m) within the same Bhajji Forest Range that includes Himri.

European lumbricid earthworms are among the most successful invasive species globally39,74,75. They were unintentionally introduced to the Northwest Himalaya by humans through various pathways and have since successfully colonized disturbed and degraded terrestrial ecosystems39,78. This introduction likely began with the arrival of European colonial rulers around 300 years ago, who cleared forested areas to create additional land for agriculture, development, urbanization, and revenue generation79.

Invasive lumbricid earthworm species possess a remarkable ability to thrive in extremely cold climates, which has enabled them to survive and establish in the Northwest Himalaya—an area climatic conditions similar to their native range. Their natural resilience to disturbance further facilitates their spread into new habitats75,80. Moreover, their parthenogenetic reproductive strategy allows a single worm to establish a new population, providing a significant advantage in dispersal and colonization81.

Exotic lumbricids are recognised for their detrimental effects on terrestrial ecosystem functioning. They have drawn significant attention due to their negative impacts in invaded regions, including disruptions to nutrient cycling, reduced plant growth and diversity, decreased tree seedling density, diminished fine root biomass, alterations to soil seed banks and microbial biomass, and adverse effects on other soil organisms22,45,82,83.

Additionally, many native plant and animal species in deciduous forests have evolved to thrive in the substantial accumulation of organic matter.The reduction of this organic layer caused by earthworm activity has negatively affected both plant growth and the behavior of soil organisms22,84.

In the forest under investigation, invasive O. tyrtaeum appears to be well-established and is likely to remain a key driver of soil development and ecosystem functioning in the foreseeable future. This assessment is supported by numerous studies on the impact of invasive earthworms in North American forests, which have shown that these worms significantly alter soil microbial community composition, redistribute soil organic matter, modify litter residue chemistry, and contribute to the depletion of soil carbon and nutrients85,86,87,88,89.

Furthermore, invasive lumbricid species have been found to strongly influence the survival of native seedlings and reduce plant diversity, while simultaneously promoting the proliferation of non-native plant species15,90,91.

Major limiting factors for earthworm invasion in temperate forests are tolerance to soil acidity and frost76. The successful colonization of O. tyrtaeum in the Himri moist temperate forest can be attributed to its ability to with stand low soil pH, alter its habitat by increasing pH, and tolerate frost76.

O. tyrtaeum is classified as a polyhumic endogeic species, which not only feeds on soil organic matter but also consumes a significant amount of leaf litter88,92. These polyhumic endogeic species exhibit high plasticity, a trait that helps them avoid direct competition with other endogeic earthworms72.

Under laboratory conditions, O. tyrtaeum cocoons exhibit a synchronized hatching pattern, with juveniles emerging within a short period93. This coordinated hatching increases the chances of juvenile survival and minimizes the risk of predation. It may also facilitate the establishment of dense populations capable of rapid dispersal and outcome coexisting detritivores. Studies have shown that invasive lumbricid earthworms negatively impact other detritivores like millipedes94, reduce microarthropod densities and diversity22, and promote the passive dispersal of microorganisms, thereby affecting microbial biodiversity95,96.

The invasion of the earthworm O. tyrtaeum in Taxus contorta stands within a mixed moist temperate forest may lead to a series of ecological consequences that threaten the health and regeneration of Taxus species. As an endogeic earthworm, O. tyrtaeum actively consumes the litter layer22, which significantly reduces surface organic matter and soil moisture—conditions critical for the germination of Taxus seeds that require damp and shaded environments97. Moreover, Taxus seeds often remain dormant for extended periods and depend on the protective cover of dense herbaceous undergrowth98. Selective earthworm consumption of seeds could reduce herbaceous undergrowth99, thereby increasing Taxus seed exposure and susceptibility to predation. In northern North American deciduous forests, O. tyrtaeum has been shown to alter plant community composition by promoting the growth of certain native seedlings while substantially suppressing the establishment of legume species100. In a mixed hardwood forest, Octolasion sp. biomass has been negatively associated with the abundance of maple seedlings101.

Additionally, O. tyrtaeum has been associated with a shift in soil microbial communities from fungal to soil bacterial dominance82, which can disrupt the mycorrhizal associations essential for optimal root function and growth in Taxus102. Its intensive burrowing activity may also bury seeds in deeper soil layers, potentially placing them beyond the optimal depth for successful germination100. These combined effects suggest that O. tyrtaeum may negatively impact the ecological balance and regeneration potential of the endangered T. contorta. This inference is based on studies involving other earthworm species and ecosystems. Therefore, further empirical research is needed to evaluate the specific effects of O. tyrtaeum in the Himri moist temperate forest, particularly its potential to disrupt Taxus root-mycorrhizal associations and alter seed germination dynamics through deep burial.

The moist temperate forests of the northwest Himalayas are becoming increasingly vulnerable to climate change and anthropogenic disturbances, such as deforestation and land-use changes25,91. These pressures, together with the spread of the invasive earthworm O. tyrtaeum, pose a growing threat to native biodiversity and ecosystem stability, giving rise to significant ecological, conservation, and management challenges. Invasion-driven shifts in plant community composition are also likely to disrupt aboveground food web dynamics15,22,103. To mitigate these emerging threats, regular monitoring of soil biodiversity in both affected and unaffected areas is essential to enable timely and informed management interventions.