Research Article |
Corresponding author: Shigenori Karasawa ( dojyoudoubutu@gmail.com ) Academic editor: Josef Settele
© 2018 Shigenori Karasawa, Kensuke Nakata.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
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
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Evaluating potential distribution areas and limiting factors for the distribution of exotic species in invasive regions are essential to identify risks and protect the native ecosystem. However, less research has been conducted on the underground ecosystem than for above-ground. Factors, limiting the distributions of exotic terrestrial isopods, have been identified and their invasive stages and potential distribution areas in Japan evaluated. A database of distribution data has been developed for 17,412 terrestrial isopod specimens in Japan and two ecological niche models constructed using 19 bioclimatic variables; the regional model was calculated using data from Japan (invasive region) only, whereas a combination of data from Japan and North America (invasive regions) and Europe (native region) was used to construct the global model. The global model predicted that annual mean temperature and mean diurnal-temperature range were the important limiting factors for most exotic isopods. It was found that Armadillidium nasatum Budde-Lund, 1833, A. vulgare Latreille, 1804, Haplophthalmus danicus Budde-Lund, 1880, Porcellio laevis Latreille, 1804, P. scaber Latreille, 1804 and Porcellionides pruinosus (Brandt, 1833) were composed of stabilising and colonising populations, which enabled prediction of the future spread of distribution areas for these species in Japan. Porcellio dilatatus Brandt, 1833 was introduced in unstable environments and thus was found in fewer locations.
Ecological niche model, Maxent, Oniscidea , precipitation, temperature
Populations of exotic species are rapidly increasing worldwide with recent globalisation (
Soil arthropods have extremely high species richness and serve important ecological functions, such as decomposition, carbon and nutrient cycling, soil structure and maintenance and biological population regulation (
About 140 terrestrial isopod species have been reported in Japan (
The aims of this study were to 1) generate a potential distribution map in Japan for exotic isopod species, 2) identify important climatic factors associated with their distributions and 3) evaluate stages of invasion and whether the populations have reached equilibrium.
A distribution database of terrestrial isopods was developed in Japan that includes distribution data of 17,412 specimens based on 159 publications, a database of specimens deposited in Japanese museums (S-Net;
Bioclimatic variables (19) were used from the WorldClim version 2.0 dataset at five arc-minute resolution for these analyses (
The environmental niche models (ENMs) were constructed using Maxent version 3.4.1 (
The global model was used to predict the potential distribution of exotic species in Japan for the following reason: if exotic species have not been present for a sufficient length of time to spread to all suitable areas, their distributions are limited to areas smaller than their potential distribution areas. Thus, estimation using the present data in invasive areas may potentially underestimate their distribution areas (
The invasion stages of the exotic species in Japan were evaluated using an approach that was theoretically developed by
Seven exotic species were reported from 1,516 locations in Japan and 4,135 and 1,953 location data were found for Europe and North America, respectively (Table
Potential distribution maps, invasive stage and occurrence locations for A. nasatum. Potential distribution maps predicted by the regional model (A) and the global model (B). Invasive stage of A. nasatum populations based on global and regional model predictions (C) and the locations on the map (D); red: stabilising population, yellow: sink population. Four large islands and three regions were described in A and B, respectively.
Potential distribution maps, invasive stage and occurrence locations for A. vulgare. Potential distribution maps predicted by the regional model (A) and the global model (B). Invasive stage of A. vulgare populations based on global and regional model predictions (C) and the locations on the map (D); red: stabilising population, blue: colonising population, yellow: sink population.
Potential distribution maps, invasive stage and occurrence locations for H. danicus. Potential distribution maps predicted by the regional model (A) and the global model (B). Invasive stage of H. danicus populations based on global and regional model predictions (C) and the locations on the map (D); red: stabilising population, blue: colonising population, green: adapted population, yellow: sink population.
Potential distribution maps, invasive stage and occurrence locations for P. dilatatus. Potential distribution maps predicted by the regional model (A) and the global model (B). Invasive stage of P. dilatatus populations based on global and regional model predictions (C) and the locations on the map (D); blue: colonising population, green: adapted population.
Potential distribution maps, invasive stage and occurrence locations for P. laevis. Potential distribution maps predicted by the regional model (A) and the global model (B). Invasive stage of P. laevis populations based on global and regional model predictions (C) and the locations on the map (D); red: stabilising population, blue: colonising population, yellow: sink population.
Potential distribution maps, invasive stage and occurrence locations for P. scaber. Potential distribution maps predicted by the regional model (A) and the global model (B). Invasive stage of P. scaber populations based on global and regional model predictions (C) and the locations on the map (D); red: stabilising population, blue: colonising population, yellow: sink population.
Potential distribution maps, invasive stage and occurrence locations for P. pruinosus. Potential distribution maps predicted by the regional model (A) and the global model (B). Invasive stage of P. pruinosus populations based on global and regional model predictions (C) and the locations on the map (D); red: stabilising population, blue: colonising population, yellow: sink population.
Exotic isopod species and their first references in Japan and numbers of sites analysed.
Species | First report in Japan | No. of site analysed | ||
---|---|---|---|---|
Japan | Europe | North America | ||
Armadillidium nasatum Budde-Lund, 1833 |
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20 | 147 | 72 |
Armadillidium vulgare Latreille, 1804 |
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770 | 1176 | 1276 |
Haplophthalmus danicus Budde-Lund, 1880 |
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120 | 223 | 28 |
Porcellio dilatatus Brandt, 1833 |
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7 | 44 | 38 |
Porcellio laevis Latreille, 1804 |
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32 | 29 | 162 |
Porcellio scaber Latreille, 1804 |
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393 | 2413 | 236 |
Porcellionides pruinosus (Brandt, 1833) |
|
174 | 103 | 141 |
Training AUC value for each species varied between the global model (0.681–0.910) and the regional model (0.712–0.857); the values for three (A. nasatum, A. vulgare and P. dilatatus) of the seven species were higher in the global models than in the regional models (Table
Species | Training AUC | Annual mean temperature | Mean diurnal rangea | Temperature seasonality (standard deviation *100) | Mean temperature of wettest quarter | ||||
---|---|---|---|---|---|---|---|---|---|
Percent contribution | Permutation importance | Percent contribution | Permutation importance | Percent contribution | Permutation importance | Percent contribution | Permutation importance | ||
Armadillidium nasatum | 0.803 | 33.0 | 23.6 | 22.3 | 24.6 | 5.7 | 0.3 | 2.4 | 5.7 |
Armadillidium vulgare | 0.768 | 23.7 | 25.4 | 52.2 | 57.5 | 3.0 | 0.6 | 0.1 | 0.8 |
Haplophthalmus danicus | 0.776 | 43.6 | 48.2 | 19.1 | 21.1 | 2.4 | 3.1 | 0.0 | 0.0 |
Porcellio dilatatus | 0.910 | 1.3 | 5.3 | 26.4 | 52.1 | 37.6 | 18.6 | 3.5 | 0.0 |
Porcellio laevis | 0.823 | 10.7 | 19.0 | 27.7 | 37.4 | 23.7 | 1.6 | 0.8 | 0.0 |
Porcellio scaber | 0.681 | 29.8 | 23.3 | 28.8 | 33.5 | 1.5 | 8.3 | 0.4 | 0.0 |
Porcellionides pruinosus | 0.802 | 33.5 | 38.9 | 42.2 | 54.9 | 4.3 | 0.0 | 5.6 | 1.9 |
Species | Training AUC | Annual precipitation | Precipitation of driest month | Precipitation seasonality (coefficient of variation) | Precipitation of warmest quarter | ||||
Percent contribution | Permutation importance | Percent contribution | Permutation importance | Percent contribution | Permutation importance | Percent contribution | Permutation importance | ||
Armadillidium nasatum | 0.803 | 2.4 | 17.0 | 7.9 | 4.6 | 25.7 | 23.2 | 0.5 | 1.0 |
Armadillidium vulgare | 0.768 | 0.0 | 0.1 | 3.4 | 1.5 | 14.2 | 12.4 | 3.3 | 1.6 |
Haplophthalmus danicus | 0.776 | 0.2 | 0.7 | 20.1 | 0.1 | 11.8 | 20.0 | 2.7 | 6.8 |
Porcellio dilatatus | 0.910 | 1.0 | 0.3 | 0.9 | 0.0 | 25.3 | 19.8 | 4.0 | 3.8 |
Porcellio laevis | 0.823 | 1.7 | 2.7 | 1.5 | 3.6 | 24.6 | 28.0 | 9.3 | 7.6 |
Porcellio scaber | 0.681 | 14.9 | 24.7 | 18.2 | 0.7 | 6.2 | 7.6 | 0.2 | 1.8 |
Porcellionides pruinosus | 0.802 | 0.4 | 2.1 | 6.4 | 0.3 | 0.5 | 2.0 | 7.1 | 0.0 |
In the global model, relationships between high-contribution (> 25 %) variables based on the percent contribution values and the probabilities of seven species are shown in Figs
Relationships between high-contribution variables (more than 25 % contribution) and the occurrence probabilities. The contribution variables are based on the percent contribution values in the global model. First row: A. nasatum; second row: A. vulgare; third row: H. danicus; fourth row: P. dilatatus. Left: the highest contribution variable; right: the second highest contribution variable.
Relationships between high-contribution variables (more than 25 % contribution) and the occurrence probabilities. The contribution variables are based on the percent contribution values in the global model. First row: P. laevis; second row: P. scaber; third row: P. pruinisus. Left: the highest contribution variable; right: the second highest contribution variable.
The potential distributions of the seven species in Japan based on the regional and global models are shown in Figs
Plotting the probabilities of the regional model against those of the global model showed three patterns (Figs
To estimate the factors limiting distribution areas at a macro scale, ecological niche models (ENMs) and species distribution models (SDMs) have been useful (
The global model indicated that temperature-related variables are more important than precipitation-related variables in limiting the distributions of terrestrial isopods. The distributions of isopods at a macro scale are limited by natural factors, especially temperature and moisture (Harding and Sutton 1985, Hopkin 1991). For example, a cartographic analysis of terrestrial isopods in the former USSR indicated that the length of the period with temperatures above 10 °C plays an important role in limiting isopod distributions (
For six of the seven exotic isopod species, the global model predicted wider potential distribution areas in Japan than did the regional model. These results implied that these species have not yet filled all suitable environments in Japan and that there is a risk that their distribution areas will spread further in the future. The results also indicated that combined data from native and introduced regions were useful for estimating the potential distributions of these exotic species in the invaded region (
Armadillidium nasatum and P. dilatatus were reported from relatively few locations in Japan (20 and 7 locations, respectively). Examination of the literature revealed that A. nasatum and P. dilatatus were first reported 16 years after the earliest reports of the other species. It is speculated that shortage of elapsed time since the introduction of A. nasatum was one of reasons for its distribution in fewer locations in Japan, because the species’ suitable area covered a large area of Japan and because this species had been introduced to suitable areas; this species was composed of stabilising populations. Additionally new locations were recently found (
This was a first study that applied ecological niche modelling to identify factors limiting the distributions of exotic terrestrial isopods and evaluate their invasive stages and potential distribution areas in Japan. The model constructions indicated that EMSs, constructed based on occurrence data in introduced areas alone, were insufficient to evaluated potential distribution maps for exotic isopods, because these species have never occupied many of their suitable environments. The global model indicated that the annual mean temperature and mean diurnal temperature range were the most important limiting factors for most exotic isopods. From evaluating invasive stages, A. nasatum, A. vulgare, H. danicus, P. laevis, P. scaber and P. pruinosus were composed of stabilising and colonising populations; these results showed that these species could spread their distribution areas in Japan in the future. In contrast, P. dilatatus was introduced in unstable environments and was therefore found in fewer locations.
This work was supported by a Grant-in-Aid for Young Scientists (B) Grant Number 26830145.
Row distribution data used to construct ENMs
Data type: occurence
How to use a searching system for distributions of terrestrial isopods in Japan
Data type: multimedia
Pearson’s correlation coefficients (r) between climatic variables for the global model (Japan, Europe and North America)
Data type: statistical data
Pearson’s correlation coefficients (r) between climatic variables for the regional model
Data type: statistical data
AUC and variable contributions of the regional model
Data type: statistical data