The study examined Armadillidium nasatum , A. vulgare , Haplophthalmus danicus , Porcellio dilatatus , P. laevis , P. scaber and Porcellionides pruinosus , seven terrestrial isopods treated as exotic in Japan and associated originally with Europe or the Mediterranean region. The underlying Japanese database contained 17,412 specimens compiled from 159 publications, museum records and the first author’s personal collection. After duplicate locations were excluded, the seven species were represented by 1,516 Japanese sites, while the European and North American occurrence datasets supplied 4,135 and 1,953 locations, respectively.

Five species were first reported in Japan in 1927; A. nasatum and P. dilatatus were first reported in 1943. Researchers used Maxent version 3.4.1 to construct environmental niche models from 19 bioclimatic variables in WorldClim version 2.0, mapped at five-arc-minute resolution. The variables included measures of temperature and precipitation.

To reduce multicollinearity, highly correlated predictors were removed when the absolute value of their Pearson correlation coefficient was at least 0.75. Models used linear and quadratic features to describe relationships between occurrence probability and environmental conditions while limiting overfitting. Because the records came from non-random sources, the researchers generated a kernel-density estimate of sampling bias and selected 10,000 background points weighted by that surface, creating a target-group background intended to account for uneven collection effort.

Two modelling approaches were compared for each species. Regional models used occurrence records from Japan alone, whereas global models combined Japanese records with those from Europe and North America. Training area under the curve, or AUC, ranged from 0.681 to 0.910 for the global models and from 0.712 to 0.857 for the regional models.

AUC values above 0.9 indicate high discriminatory performance, values from 0.7 to 0.9 represent reasonable or moderate performance, and values from 0.5 to 0.7 indicate poor performance. Global models performed better for A. nasatum , A. vulgare and P. dilatatus . The researchers used the global approach to map potential Japanese distributions because records confined to an invaded country may understate the full environmental range occupied by a species in its native and other invaded regions.

Temperature-related variables supplied most of the strongest climatic signals in the global models. Annual mean temperature contributed more than 25 percent for three species: A. nasatum , H. danicus and P. scaber . Mean diurnal temperature range exceeded a 25 percent contribution for five species: A. vulgare , H. danicus , P. laevis , P. scaber and P. pruinosus .

Temperature seasonality was also a major predictor for P. dilatatus , contributing 37.6 percent. Among precipitation variables, seasonality contributed more than 25 percent for A. nasatum , P. dilatatus and P. laevis , while precipitation in the wettest month was especially important for A. nasatum , exceeding 65 percent contribution and permutation importance. The response curves revealed shared and species-specific temperature patterns.

Annual mean temperature responses differed among the remaining species: A. nasatum , H. danicus and P. scaber were associated with temperatures above 15°C, while P. pruinosus showed a hump-shaped response across a more moderate range of approximately 15–25°C. The study’s authors noted that the annual mean temperatures assigned probabilities above 0.5 covered roughly 10–25°C across most of Japan except mountainous areas, suggesting that annual mean temperature alone was not the principal national-scale constraint for these species. Global potential distributions generally included western Honshu, Shikoku and Kyushu, although their boundaries varied among species.

Suitable areas for A. vulgare , P. scaber and P. pruinosus extended farther inland than those for A. nasatum and H. danicus . The projected range for P. scaber also included coastal Hokkaido, while that of P. laevis was concentrated more heavily in southwestern Japan. For P. dilatatus , the global and regional models both identified most of Kyushu and Shikoku, but the regional model additionally projected suitable coastal areas of Honshu and southern Hokkaido.

The broader global projections for the other six species suggest that their Japanese occurrence records had not yet reached all areas classified as climatically suitable, not that those areas will inevitably be occupied. Invasion stages were inferred by plotting regional against global model probabilities and dividing the results at a probability of 0.5. Areas classified as highly suitable by both models represented stabilising populations.

High global probability combined with low regional probability indicated colonising populations, whereas low probability in both models identified sink populations. A high regional probability paired with low global probability was interpreted as a locally adapted population. A. vulgare , H. danicus , P. laevis , P. scaber and P. pruinosus contained mixtures of stabilising and colonising areas, with several sink areas also identified.

A. nasatum was composed mainly of stabilising areas and one sink, while P. dilatatus produced the pattern assigned to local adaptation. These labels describe relationships between model predictions rather than observed demographic outcomes. For example, a colonising classification means that an area is climatically suitable in the global model but poorly represented by the species’ Japanese regional occurrence pattern; it does not establish a population trajectory or guarantee establishment.

Likewise, a sink classification is consistent with the presence of a species in an area predicted to be unsuitable, including cooler inland and northern locations, but it does not demonstrate that a population is declining. The four categories did not form sharply separated geographic regions. The authors also warned that occurrence records could not cover every part of the species’ ranges and that the projected maps might change as a larger, more complete database became available.

A. vulgare had the widest recorded Japanese range, extending from 43°10′ north to 24°45′ north, and occurred at 770 sites. P. scaber was recorded at 393 sites and was the only species commonly reported in Hokkaido. The Japanese records for A. nasatum , P. dilatatus and P. laevis numbered 20, seven and 32 sites, respectively, while P. pruinosus occurred at 174 sites and reached Iriomotejima Island at 24°16′ north, the southernmost location in the dataset.

The authors considered the short time since the first Japanese reports of A. nasatum , together with its large projected suitable area and introduction to suitable areas, one possible reason for its limited number of records. They also suggested that the ecological niche and areas of introduction were more relevant than elapsed time to the narrow Japanese range of P. dilatatus . The results show why occurrence records from an invaded region alone may underestimate the potential range of these seven exotic isopods.

Combining Japanese, European and North American data produced wider projections for six species and exposed areas where further establishment may be possible. The analysis did not establish ecological harm, human exposure or the consequences of future expansion, and its macro-scale climate models did not include every local influence on isopods. Within those limits, the study identifies temperature and diurnal temperature variation as major climatic correlates and suggests that six of the seven modelled species retained geographic scope for potential expansion beyond their documented Japanese ranges.

Subject of Research: Ecological niche modelling of seven exotic terrestrial isopod distributions and invasion stages in Japan

Article Title: Invasion stages and potential distributions of seven exotic terrestrial isopods in Japan

Article References: Karasawa, S., & Nakata, K. (2018). Invasion stages and potential distributions of seven exotic terrestrial isopods in Japan. BioRisk, 13, 53-76. https://doi.org/10.3897/biorisk.13.23514

Image Credits: AI Generated

DOI: 10.3897/biorisk.13.23514

Keywords: terrestrial isopods, Japan, ecological niche modelling, species distribution models, invasive species, climatic niche, temperature variables, Maxent, invasion stages, soil biodiversity, Invasion, stages