Introduction

Studies of spatial differentiation of various taxa are among the most important frontiers of modern biogeography. For some well-studied groups, mainly, vertebrates and plants, such trends are already discovered (Loiselle et al. 2003; Guisan and Thuiller 2005; Grenouillet et al. 2011), but for soil-dwelling invertebrates they are only at the stage of species inventory. However, there are certain groups of invertebrates for which analysis of spatial differentiation is already possible due to the large number of records from different geographical localities. Woodlice are among such groups.

There is no faunistic list of terrestrial isopods for the territory of the former USSR until now, as well as of the territory of Russia. However, there are extensive regional lists (Borutzky 1948, 1953; Zalesskaya and Rybalov 1982; Khisametdinova 2007; Gongalsky and Kuznetsova 2011), and numerous records scattered in the literature devoted to soil macrofauna. At the same time, there are only a few ecological studies about factors affecting woodlice distribution over regions of the former USSR (Gongalsky et al. 2005; Khisametdinova 2009).

The aim of the study is to determine the factors affecting woodlice distribution over the plain area of the former Soviet Union. To achieve this, an inventory of species distribution across the study area was made. The task was to create a database indicating locations with woodlice presence/absence overlaid with several environmental variables values distribution.

Material and methods Database

The first step was to compile a list of species for the study area. We made a database of isopod presence or absence in the locations across the whole territory of the former USSR (both plains and mountains). For each record the database includes information about date, data source, geographical coordinates, location, isopod species list or information about woodlice absence in the soil fauna list, biotope, and natural zone.

Three types of information sources of terrestrial isopod locations were used: i) available literature on soil fauna surveys; ii) collections of the Zoological Museum of Moscow State University (Moscow, Russia) and the Zoological Institute of the Russian Academy of Sciences (St.-Petersburg, Russia); and iii) authors’ personal collections. Here we provide a list of woodlice from the territory of the former USSR since some species and localities were not included in the list of Schmalfuss (2003), although it covered the majority of species. To work with regional databases, a specific list would be useful. Since such a list for this area did not exist, the proposed compilation would be a start to be completed in the future. We used the taxonomic system proposed by Schmalfuss (2003) for species naming. Isopod absence was recorded only in extensively surveyed locations.

For cartographic analysis, 259 locations were chosen, 44 of which with woodlice absence. Due to the difficulty of tracing ecological trends in the mountains, only plain territories were involved into the analysis. Some species were excluded from the analysis: i) synanthropic species and ii) species inhabiting azonal locations, such as sea coasts, caves and anthills.

Then database records with isopod presence or absence locations were laid on the geographic maps to perform cartographic analysis.

Cartographic analysis

The map of woodlice distribution was visually compared with the maps of environmental factors (mean annual temperature; the period with temperature above 10°C; mean precipitation; permafrost distribution; soil pH and soil type; vegetation type; natural zones) found in the Agricultural Atlas of the USSR (Tulupnikov 1960) and the Geographical Atlas of the USSR (Kolosova 1980). The data were verified using the WorldClim database (Hijmans et al. 2005).

The database is maintained in MS Excel. Cartographic analysis is done in MapInfo 8.5.

Results and discussion Limits of isopod distribution

Woodlice have not been recorded northwards the isocline of 120 days a year with temperature >10°C (Fig. 1). The northern border of woodlice distribution matches the distribution of this parameter. Other parameters did not coincide with isopod distribution as well as with this isocline (data not shown).

Species diversity

In total, 190 species were recorded from the territory of the former USSR (Appendix 1). Among them, 64 were recorded from the territory of Russia. Northernmost natural zone with woodlice records is southern taiga. No woodlice records were in tundra, northern and middle taiga. The species diversity increases southwards, but decreases in the deserts. However, this may be due to the low number of locations extensively studied to reveal local faunas.

Distribution of isopods is known to be limited by natural factors, such as temperature and moisture (Harding and Sutton 1985, Hopkin 1991). In our study, the limiting factor of woodlice distribution towards the north turned out to be the length of the warm period, expressed as number of days when the temperature was above 10°C. The highest species diversity was observed between isoclines of 180 and 210 days with temperature >10°C. Colder conditions slow down their physiological processes (Hopkin 1991) and limit their distribution. For a better understanding of distribution of woodlice, a Species Distribution Modeling (Elith and Leathwick 2009, Franklin 2009) should be applied, which is a next step in the analysis of the database of Russian isopods.