The research team, led by Rojen Singh Thounaojam of Manipur University, spent the entirety of 2018 sampling earthworms and soils at two contrasting forest sites in the Imphal region. Site I was a reserved mixed forest in Imphal West district, while Site II was a community-protected mixed forest in Imphal East. Both sit within a subtropical monsoon climate that delivers an average annual rainfall of 1325.7 millimeters, with a wet season from June to October, a hot summer from March to May, and a cool, dry winter from November to February. The soils at both sites were sandy loam, and the vegetation included familiar subtropical trees such as Lithocarpus dealbatus, Gmelina arborea, Pinus kesiya, and Schima wallichii.

The sampling protocol was deliberately rigorous. Each month, the researchers demarcated six random plots of one square meter at each site, and within each plot they excavated four soil monoliths measuring 25 by 25 by 30 centimeters. Over the year, this yielded 72 quadrats per site, with earthworms extracted by digging and hand-sorting following the standard Tropical Soil Biology and Fertility method. Specimens were preserved in formalin and identified under stereo zoom microscopes using established taxonomic keys. Alongside the biological sampling, the team recorded soil temperature in situ each month, measured pH in soil-water suspensions, and determined moisture content gravimetrically.

What emerged from this year-long effort was a community profile that surprised the researchers. Site I harbored seven earthworm species belonging to six genera and four families, while Site II supported five species in four families. The Megascolecidae family dominated the species lists at both sites. Shannon diversity indices of 1.29 at Site I and 1.17 at Site II indicated modestly higher diversity at the reserved forest. But the headline result concerned the exotic component of the community: at Site I, exotic species made up more than 74 percent of the earthworm community, with P. corethrurus alone accounting for 62.37 percent. At Site II, the single exotic species present, P. corethrurus, constituted 38.65 percent of the community, second only to the native Drawida nepalensis at 45.89 percent.

The ecology of P. corethrurus helps explain its success. First described in 1857 by the German naturalist Fritz Müller from Santa Catarina State in Brazil, and believed to have originated in the Guiana Shield region of South America, the species is now one of the most widely distributed tropical earthworms on Earth. According to the Drilobase database, as of July 2025 it had been recorded in tropical, subtropical, and Mediterranean zones across dozens of countries on four continents. In India, it was first reported from Kerala in 1910, likely introduced through human activity associated with cassava or rubber plantations. It has since spread to nine of India’s fifteen agro-climatic zones and to eight states in Northeast India alone.

Several biological traits make this worm a formidable invader. It tolerates an exceptionally broad range of soil conditions and climates, reproduces continuously with high fecundity and hatching success, develops quickly, and can even reproduce by parthenogenesis, meaning a single individual can found a population without a mate. In the Manipur forests, the species showed remarkable spatial and temporal versatility: it was found throughout the year, in every season, and at soil depths of up to 30 centimeters, whereas most other species were confined to the upper 10 centimeters of soil and many appeared only during the rainy season. Notably, the researchers recorded P. corethrurus active at soil temperatures as low as 15 degrees Celsius, below the previously reported minimum of 17 degrees for other parts of Northeast India.

The statistical analysis reinforced the importance of climate in structuring these communities. Soil moisture and soil temperature showed significant positive correlations with every recorded species at both sites, with correlation coefficients generally ranging from about 0.69 to 0.86, while soil pH was negatively correlated with earthworm abundance. Earthworm populations peaked during the rainy season, when elevated moisture and moderate temperatures create ideal conditions for activity and reproduction. Earthworms breathe through their skin, so moisture is a critical constraint on their distribution, and the optimal soil temperature for the species recorded in this study fell between 20.21 and 24.26 degrees Celsius, within the broader 20 to 30 degree range reported for tropical and subtropical earthworms.

Why does the dominance of an exotic worm in an undisturbed forest matter? Earthworms are ecosystem engineers, and established populations of P. corethrurus have been shown elsewhere to alter soil physical structure, modify biogeochemical processes, and reshape plant and microbial communities. By accelerating leaf litter decomposition and mixing soil in the upper layers, invasive earthworms can strip away the litter layer, increase soil bulk density, and reduce soil carbon content, carbon-to-nitrogen ratios, and cation exchange capacity, ultimately degrading forest soil fertility. The density of P. corethrurus is often inversely correlated with the density of other earthworm species, and previous studies have documented the displacement of native earthworms following the establishment of exotic populations. The Manipur findings suggest a real risk that native species at these sites could decline over time.

Yet the story is not one of inevitable defeat for the natives. The native Drawida nepalensis maintained high population densities even in communities dominated by exotics, and it was the most abundant species at Site II, where natives predominated. A parallel pattern has been reported in pineapple plantations in West Tripura, where the native Drawida assamensis outcompeted P. corethrurus, apparently thanks to superior survival and competitive ability. Endogeic earthworms, which live within the soil rather than on its surface, are also known to be the most resistant ecological group to disturbance. These observations hint that some native lineages may possess the competitive arsenal to coexist with, or even resist, the invader, although the researchers caution that long-term coexistence may ultimately depend on how much habitat modification the exotic species causes.

How the worm reached these protected forests remains a question of pathways rather than capability. Earthworms disperse on their own less than 10 to 15 meters per year, so human-mediated transport is almost certainly responsible. Cocoons and worms can hitchhike in soil on vehicle tires, in potted plants, in streams and surface water, or on birds and mammals. In Manipur specifically, the authors point to a worrying and largely unregulated trend: the booming use of exotic earthworms, particularly Eisenia andrei and Eisenia fetida, for vermicomposting, with little ecological oversight or public awareness of the risks of escape and establishment. The team calls for immediate policy formulation, monitoring protocols, and public education to protect native soil biodiversity. As the invasion of P. corethrurus proceeds even in undisturbed forests, the study stands as a warning that no ecosystem, however pristine, is automatically safe from a determined, parthenogenetic, climate-tolerant invader moving quietly beneath our feet.

Subject of Research: Invasion of protected subtropical forests in Northeast India by the exotic earthworm Pontoscolex corethrurus

Article Title: Assessment of Pontoscolex corethrurus, an exotic earthworm, in contrasting protected subtropical forest habitats in North-East India

Article References: Thounaojam, R. S., Singh, K. B., Singh, T. B., Keisham, S., & Singh, T. B. (2025). Assessment of Pontoscolex corethrurus, an exotic earthworm, in contrasting protected subtropical forest habitats in North-East India. Discover Ecology, 1(1), Article 10. https://doi.org/10.1007/s44396-025-00010-z

Image Credits: AI Generated

DOI: 10.1007/s44396-025-00010-z

Keywords: Pontoscolex corethrurus, invasive species, earthworms, soil biodiversity, Manipur, Northeast India, Indo-Burma biodiversity hotspot, subtropical forest, soil ecology, ecosystem engineers, vermicomposting, species invasion

The research team, led by Phishet Sungtong and Daoroong Sungthong, compared conventional composting with vermicomposting, a mesophilic process in which the African nightcrawler earthworm Eudrilus eugeniae works alongside microbes to decompose organic matter. Each reactor contained a carefully balanced feedstock of cow manure and discarded cabbage at a ratio of 89:11 on a dry-weight basis, with some reactors receiving 7 grams of cut PLA/PBAT bag pieces, equivalent to 1 percent of the initial dry mass. The bags, collected from a campus café, were cut into 5 by 5 centimeter squares following the specimen preparation procedure of ISO 20200, the international standard for laboratory-scale disintegration testing. Twenty-eight independent reactors were prepared, with destructive sampling at days 15, 30, 60, 90, and 120.

The temperature story turned out to be central. Conventional composting initially peaked at 27 to 28 degrees Celsius before stabilizing near ambient levels, while vermicomposting held a strictly mesophilic profile of 24 to 26 degrees throughout, a range chosen to keep the earthworms alive and reproducing. Neither system reached the thermophilic phase of 58 degrees Celsius that defines industrial composting, and that matters enormously for PLA. The glass transition temperature of PLA sits around 56 to 63 degrees Celsius; below it, the polymer remains glassy, with stiff chains that resist water penetration and hydrolytic chain scission. PBAT, by contrast, with a glass transition near minus 35 degrees, stays rubbery at any composting temperature, yet its aromatic terephthalate segments remain stubbornly resistant to microbial enzymes regardless.

By day 120, the recovered plastic fragments told a nuanced tale. Vermicomposting achieved a mean disintegration of 59.91 percent, compared with 55.96 percent for conventional composting, a numerically higher result of nearly four percentage points, though the ranges overlapped and the difference was not statistically distinguishable. Notably, the vermicomposting range extended to 72.97 percent, higher than any single composting reactor achieved. The researchers attribute this edge to the combined action of earthworms and microbes: worms ingest and transport plastic fragments into the drilosphere, the biologically rich zone around their burrows, and their gut passage and castings expose the polymer to dense microbial communities in oxygen-limited microsites.

Spectroscopic evidence confirmed that the mass loss was not merely physical fragmentation. Attenuated total reflectance Fourier transform infrared spectroscopy revealed a pronounced decline in the absorbance of the ester carbonyl band near 1712 wavenumbers, dropping from 0.3304 at day zero to 0.1892 in composted and 0.3083 in vermicomposted samples, alongside a sharp decrease in the carbon-oxygen-carbon stretching band. These changes signal cleavage of the ester bonds that link the polymer chains, generating terminal hydroxyl and carboxylic acid groups that microbes can then assimilate through the tricarboxylic acid cycle into carbon dioxide, water, and biomass. Scanning electron microscopy showed the once bumpy, sponge-like surface progressively cracking, pitting, and peeling into layers, with rod-shaped cell-like structures colonizing the fracture grooves.

An intriguing metabolic twist emerged when the researchers compared the two systems chemically. The vermicomposting reactors received fresh frozen cabbage every 30 days to feed the worms, providing microbes with an abundant, easily digestible carbon source. Under carbon catabolite repression, microorganisms preferentially consume such favored substrates and suppress the synthesis of the extracellular depolymerases needed to attack recalcitrant plastic. The composting reactors, starved of new food after day 30, may have been driven by metabolic stress to upregulate polymer-degrading enzymes, using PLA/PBAT as an alternative carbon source. This may explain why the vermicomposted plastic showed a less pronounced reduction in ester absorbance despite its slightly greater mass loss.

Beyond the plastic itself, the study asked whether the resulting compost and vermicompost were safe and effective fertilizers. Both systems matured convincingly: the carbon-to-nitrogen ratio fell from 27.1 to between 16 and 20, meeting Thai agricultural standards, while cation exchange capacity nearly doubled, reflecting the formation of humic substances. Vermicompost proved the richer fertilizer, accumulating roughly twice the calcium of conventional compost, reaching 12.7 percent, thanks to the calciferous glands of earthworms excreting calcium carbonate in their castings, along with elevated potassium and sodium. Germination tests with mung bean seeds showed that phytotoxicity dropped rapidly, with the germination index exceeding 100 percent by day 15 and reaching 136 percent in vermicompost at day 120, compared with 101 percent for compost.

Heavy metal analysis added a cautionary note. Cadmium and lead remained below detection limits in all systems, and chromium traced back to the cabbage feedstock rather than the plastic. However, nickel appeared exclusively in the bioplastic-amended reactors, peaking at 37.78 milligrams per kilogram in vermicompost, likely derived from nickel oxide additives used in PLA manufacturing to improve tensile strength and confer antimicrobial properties. The researchers warn that lactic acid released during PLA degradation could locally acidify the matrix and mobilize nickel into plant-available forms, a dynamic that warrants environmental monitoring even though all measured values fell below Thai regulatory limits.

Bacterial community sequencing of the day-120 samples revealed sharply different microbial worlds. Compost samples hosted greater diversity, with Shannon indices above 9 and over a thousand observed amplicon sequence variants, dominated by Pseudomonadota, Bacillota, and Chloroflexota, with genera such as Lysinibacillus and Litorilinea prominent. Vermicompost samples were less diverse but enriched in Bacillota and Bacteroidota, including anaerobe-associated taxa such as Clostridium and Romboutsia, plausibly reflecting oxygen-limited microsites in earthworm guts and moist organic particles. Both Lysinibacillus and Clostridium are recognized plastisphere colonizers capable of secreting esterases and lipases that cleave polyester bonds, making them candidates for further investigation, though the authors are careful to note that their presence at day 120 does not prove direct involvement in plastic degradation.

The study’s practical message is clear: compostable does not mean gone. Under mesophilic conditions, PLA/PBAT bags lost more than half their mass in 120 days but remained incompletely disintegrated, and the bioplastic-amended compost actually failed the Thai agricultural standard precisely because macroscopic plastic fragments persisted. The authors suggest extending treatment duration, maintaining continuous thermophilic conditions, introducing specific polymer-degrading microorganisms, or employing advanced in-vessel reactors with real-time temperature and moisture monitoring to push degradation to completion. They also acknowledge the limits of their design, including single reactors for most sampling times and endpoint-only microbial analysis, which constrain statistical inference. Until such improvements arrive, the study offers consumers and waste managers a grounded benchmark: earthworms give compostable plastics a modest but measurable boost, yet the thermophilic heat of industrial composting remains the gold standard for making these materials truly vanish.

Subject of Research: Disintegration of PLA/PBAT bioplastic blends during composting and vermicomposting and the resulting fertilizer quality and bacterial communities

Article Title: Lab-scale comparison of PLA/PBAT disintegration in composting and vermicomposting: fertilizer quality, phytotoxicity, and day-120 bacterial community profiles

Article References: Sungtong, P., & Sungthong, D. (2026). Lab-scale comparison of PLA/PBAT disintegration in composting and vermicomposting: fertilizer quality, phytotoxicity, and day-120 bacterial community profiles. Environmental Challenges, 25, Article 101664. https://doi.org/10.1016/j.envc.2026.101664

Image Credits: AI Generated

DOI: 10.1016/j.envc.2026.101664

Keywords: PLA/PBAT, biodegradable plastics, composting, vermicomposting, earthworms, Eudrilus eugeniae, disintegration, FTIR, scanning electron microscopy, bacterial community, compost quality, heavy metals