Lithuania is on the edge of the main distribution range of the root vole (Microtus oeconomus). It was found that the trappability of M. oeconomus did not depend on the trapping effort, the number of recorded species and individuals or the species diversity. All 13 localities with proportions of M. oeconomus constituting over 30% of the small mammal community are situated in the south and west of Lithuania. Analysis of data collected over 50 years confirms that the species has become more abundant, more widespread and its distribution range has spread north and east from the south and west of the country. The main habitats for M. oeconomus in Lithuania are reedbeds, meadows, shrubby areas and shores of water bodies. Abandoned former agricultural areas create suitable habitats for the species, thus promoting its increased abundance and wider distribution.
Introduction
The root vole (Microtus oeconomus) is a Holarctic species. In Europe, its main range extends from eastern Germany and northern Fennoscandia to Poland, Belarus and northern and central European Russia (IUCN 2007). Isolated relict populations are also found in the Netherlands, southern Scandinavia, on the Finnish coast, in Austria, Slovakia and Hungary (van Apeldoorn 1999, Shenbrot & Krasnov 2005). M. oeconomus is absent in Latvia and Estonia (van Apeldoorn 1999) and there are recent reports which mistakenly suggest an absence of the species in Lithuania too (Rácz et al. 2005). In Lithuania, M. oeconomus is a dominant species in some habitats, but 50 years ago it was considered the rarest vole species. It still remains a poorly documented species in publications.
This article analyses changes in distribution of M. oeconomus in Lithuania at the border of the restricted distribution of this species. The aim is to identify if species distribution and abundance are remaining stable over the long term period.
Material and Methods
Historical data on the findings of M. oeconomus have been obtained from publications and grey literature (reports, student theses) cited as personal communications. In addition, small mammals were trapped in various habitats in eleven districts between 1998 and 2008, including different types of forests, wetlands, meadows, reedbeds, shrubby areas, agricultural areas and ecotones of various habitat types. Trapping was done by snap trap lines, the traps being set at a distance of 5 m from each other, baited with bread and sunflower oil and, in most cases, exposed for three days and checked once or twice a day (Balčiauskas & Juškaitis 1997, Balčiauskas 2004). For older data such information may be missing.
The summarised data are presented in Appendix and mapped in Fig. 1. Where possible, the trapping effort in trap/days, number of trapped species and individuals, as well as diversity of the small mammal community (expressed as Shannon-Weaver diversity index, H′, on the base of log2 according Krebs 1999) are presented. An additional dataset was obtained by analysing food remains and pellets of tawny owls (Strix aluco).
Food remains from nestboxes were collected in five districts (Balčiauskienė et al. 2005) and pellets in eight districts of Lithuania (Balčiauskienė et al. 2006). To ensure compatibility with trapping results, non-mammalian food items, along with bats, moles and weasels, were excluded from diet analysis. The summarised data on M. oeconomus in S. aluco diet are presented in Table 1.
Table 1.
Data on the presence of M. oeconomus from food remains and pellets of S. aluco (n — number of samples, S — number of species, N — number of individuals recovered, H′ — Shannon-Weaver diversity index, Moe% — % of M. oeconomus.
Fig. 1.
Localization of analysed datasets in Lithuania, 1961–2008 (numbers of published data, trapping sites and sites where S. aluco prey remains from nest-boxes or pellets were collected correspond to data in Appendix and Table 1).
Results
Chronology of investigations
In Lithuania, M. oeconomus was first mentioned in 1964 (Ivanauskas et al. 1964). At that time, there were only five findings of M. oeconomus, all in the south and south-western parts of the country (Fig. 2A) and the species was identified as the rarest of the voles in Lithuania. In 1976, a single specimen was trapped in north Lithuania, near distrophic Lake Kilučiai, overgrown with shrubs and reedbeds (L. Balčiauskas, pers. comm.). Thus, before 1980 only one of six inhabited localities was situated in north Lithuania.
The 1980s and 1990s were characterized by increased small mammal trapping efforts in Lithuania, especially between the two editions of the Mammal Atlas (Balčiauskas et al. 1997, 1999). Only two of the 19 localities where root voles were trapped were in north and east Lithuania (Fig. 2B). In the most eastern locality, intensive trapping was conducted over ten years on the borders of Lithuania, Latvia and Belarus (see Appendix). Ten individuals were trapped, seven of which were in Belarus, the other in Lithuania. In Latvia, despite trapping less than 10 km from the border, no root voles were recorded. Additionally, small mammal investigations were conducted in south-east Latvia in 2007 and in several localities across the country in 2007–2008, but M. oeconomus was not registered. To date, M. oeconomus has not been recorded in Latvia, not being found prior to the end of the last millennium (van Apeldoorn 1999), nor since (Zorenko 2008). Thus, the north-easterly range of the species distribution ends in Lithuania.
In 2001–2004, M. oeconomus was successfully trapped in the south, west and central parts of Lithuania and additionally, for the first time, in the north-west (Fig. 2C). In the most recent period of trapping, 2005–2008, the only localities where M. oeconomus was not found were in north, north-east and east of Lithuania (Fig. 2D).
Proportions of M. oeconomus in small mammal community
Analysis of over one hundred trapping sessions with different trapping efforts shows that in the years 1976–2008 the proportion of M. oeconomus varied from 0 to over 50% (Appendix). On average, nearly 2000 traps per session yielded 7.6 ± 0.27 species and 222 ± 38 individuals trapped. The Shannon-Weaver diversity index varied from 0.59 to 3.03 (mean 2.025 ± 0.05) and the average proportion of M. oeconomus in small mammal communities in Lithuania was 8.45 ± 1.31%. Such proportion was not correlated with the trapping effort (Pearson's r, n = 91, r = -0.08, NS), number of species (Pearson's r, n = 111, r = -0.11, NS) or individuals trapped (Pearson's r, n = 111, r = -0.09, NS) nor with the diversity of a community (Pearson's r, n = 111, r = 0.03, NS).
The sites where the species was absent or contributed less than 10% to the small mammal community were scattered across the country (Fig. 3A). Though many trapping sessions were performed in the eastern and north-eastern parts of Lithuania, only five of them registered the presence of M. oeconomus. Three of 11 trapping sessions with the proportion of M. oeconomus in the range 10.0–19.9% were done in the north-western part of the country. All localities where the proportion of M. oeconomus was over 20% were located in the western, south-western and southern parts of Lithuania (Fig. 3B).
A similar proportion of M. oeconomus, varying from 0 to 10 percent of all recorded small mammals, was found in the food remains and pellets of tawny owl (S. aluco) (Table 1). As S. aluco catches rare species more effectively than snap-traps do (Balčiauskienė & Naruševičius 2006, Balčiauskienė & Balčiauskas 2008), the absence of M. oeconomus in owl food could be related to the absence of the species in a certain locality. The highest proportion of M. oeconomus in owl food was characteristic of the districts situated in west and south-west Lithuania and, to a lesser degree, in central and northern Lithuania.
Changes in M. oeconomus distribution
In west Lithuania, where M. oeconomus was first recorded (Ivanauskas et al. 1964), the presence of the species was subsequently confirmed in all later investigations, namely those conducted in 1999 (Juškaitis & Ulevičius 2003, 2004), 2005 (dominance) and 2007–2008 (moderate presence) (G. Medvikytė, pers. comm.). In the period from 1999 till 2008, M. oeconomus was, on the whole, dominant in small mammal communities, though recorded as a subdominant small mammal species on the northern side of the River Nemunas 1991–2000 (Balčiauskas & Juškaitis 1997, Juškaitis & Uselis 2005). In north-west Lithuania M. oeconomus was absent in 1991–1995, but has been registered since (Balčiauskas & Juškaitis 1997, EPA 2009, P. Alejūnas & V. Stirkė, pers. comm.). In south-west Lithuania (Žuvintas SNR), investigations in 1961–1962 showed the presence of M. oeconomus with an abundance of 1.1–2.6 ind. per 100 trap/nights (The Reservation of Žuvintas 1968). In 1981–1983, M. oeconomus was already a subdominant species in moist habitats and, in 1993–1997, a moderate abundance of M. oeconomus was confirmed (Baleišis et al. 1993, Balčiauskas & Juškaitis 1997). The species was absent in the far south-west of Lithuania in 1994 (Vištytis RP). In 1995, however, M. oeconomus was found to be dominant in deciduous forests near Vištytis RP. A decade later, M. oeconomus comprised 14.1% of all trapped small mammals in Vištytis RP. In the last ten years, M. oeconomus has comprised 10–15% of all trapped small mammals (Balčiauskas & Juškaitis 1997, Juškaitis & Ulevičius 2003, EPA 2009).
In south Lithuania, the proportion of M. oeconomus in small mammal communities is increasing: it quadrupled in the period between 1978–1980 to 1991–1993 (Balčiauskas & Juškaitis 1997, Mažeikytė 2003) and increased from 0.3% in 1999–2004 to 12.9% in 2002 (Ulevičius & Juškaitis 2003, EPA 2009). A high abundance of the species was found in 2007 during investigations of moist meadows in the area. A growing dominance of M. oeconomus was also found in the far south-east of Lithuania. In 1997, the proportion of the species was only 5% (Balčiauskas & Juškaitis 1997), but in 2005 and 2008, the species was dominant in small mammal communities (J. Vilkova, pers. comm.). In southern regions near to central Lithuania, M. oeconomus was absent during first investigations in 1991–1994 (Balčiauskas & Juškaitis 1997), but in 2006–2008, the proportion of the species already reached 6.5–30.0%. A few individuals were also registered in the city of Vilnius (O. Grincevičiūtė, pers. comm.).
In central Lithuania, M. oeconomus was absent in most localities in 1983–2003 (Balčiauskas & Juškaitis 1997, EPA 2009), even in very suitable habitats (Pakeltytė & Andriuškevičius 2004, EPA 2009), with the exception of one district where high trapping efforts confirmed the presence of M. oeconomus in low concentrations (Balčiauskienė 2006, Balčiauskienė & Naruševičius 2006, EPA 2009). The proportion of M. oeconomus in S. aluco food was also low (Table 1), reaching only 4.5% (Balčiauskienė et al. 2005) and just 0.2–1.4% in two neighbouring districts (Balčiauskienė et al. 2006). The presence of M. oeconomus westward from central Lithuania was confirmed, except in Šiauliai town (D. Prielaidas, pers. comm.). The increase in species dominance was almost fivefold (Balčiauskas & Juškaitis 1997, Zalunskaitė & Lopeta 2005, EPA 2009).
In the north, north-east and east Lithuania, the distribution of M. oeconomus is the most interesting. The species was absent in 1997 and its proportion was less than 1% in 1999–2004, but had reached 7–20% in 2008 (Maldžiūnaitė 1980, Balčiauskas & Juškaitis 1997, Mačiulis 2002, EPA 2009, P. Alejūnas & V. Stirkė, pers. comm., E. Ginkuvienė, pers. comm.). In the most northern districts, M. oeconomus appeared only after 1999 and its proportion has not increased. In north-east Lithuania, M. oeconomus has not been registered yet (Balčiauskas & Juškaitis 1997, Mažeikytė 2002, 2003, Šinkūnas 2006, EPA 2009, P. Alejūnas & V. Stirkė, pers. comm.) and in the very east of Lithuania, only a few individuals have been trapped on lakeshores, though most investigations have not revealed the presence of the species (Balčiauskas & Juškaitis 1997, Balčiauskas 2005, Balčiauskas & Gudaitė 2006, Šinkūnas & Balčiauskas 2006).
Habitats of M. oeconomus
As indicated in Appendix, the main habitats for the species in Lithuania are reedbeds, meadows, shrubby areas and shores of water bodies. The species was first recorded in wet meadows, on the shores of water basins overgrown with lush vegetation, in reedbeds and near swamps. Flooded meadows and reedbeds in the Nemunas Delta still are the habitats of primary importance for the species. Later, the species was also found in mowed pastures, sparse forests, forest openings and clear cuts (Balčiauskas et al. 1999).
The species was dominant in reedbeds (41.0% of all small mammals) and various meadows (82.6%), including very dry meadows and wastelands, but it was not found in forests of west Lithuania (Curonian Spit; Ulevičius et al. 2002, Juškaitis & Ulevičius 2003). In 2005–2008, root voles were trapped in flooded meadows, reedbeds and shrubby areas, river valleys, unmowed meadows, and near forests. In south-west Lithuania, the highest abundances were registered in reedbeds and moist meadows. In north Lithuania, the main habitat of the species were natural meadows. M. oeconomus was not found on lake islands (Balčiauskas & Juškaitis 1997, Šinkūnas 2006) and additionally, dry coniferous forests prevailing in south Lithuania are not suitable for root voles.
Discussion
Over recent decades, M. oeconomus has not only increased in numbers, but also spread through Lithuania in northerly and easterly directions. What were the driving forces of such changes in distribution? First of all, we suppose that changes in distribution have been conditioned by changes in land use since 1990, with abandoned agricultural areas overgrowing with grass, weeds and shrubs. According to CORINE database, changes in land class usage in Lithuania between 1990 and 2000 were as follows: the area of forest and pasture shrank by about 1%, while the area of arable land, heterogeneous agricultural land and scrub/herbaceous vegetation expanded by 0.5%, 0.3% and approximately 0.8%, respectively (EIONET 2008). The latter two land classes are suitable habitats for M. oeconomus. In north Lithuania, half of landuse changes that occurred between 1957 and 2000 were related to renaturalization: farmland decreased by 8.3% and renaturalised areas increased by 3.9% (Bauža & Baužienė 2008).
The most pronounced changes in land use, and most favourable to M. oeconomus, were in northeast Lithuania. Before World War II, all suitable land was utilized for arable purposes. After the War, land reclamation improved land quality, but slopes unsuitable for use with heavy machinery were left for renaturalisation. In the period 1970–1990, the percentage of arable land remained stable, but in the years following 1990 more than 20% of arable land in north-east Lithuania was abandoned, subsequently overgrowing with grasses and shrubs. Shrubby areas expanded from 1.8% to 6.5% of the total area (Ribokas & Milius 2007).
Additionally, reedbeds are rather frequent in the territories where the former land reclamation systems were abandoned. The renaturalization of the land reclamation systems is a significant factor in north Lithuania (Bauža & Baužienė 2008) and northeast Lithuania (Ribokas & Milius 2007). As such territories suit the ecological requirements of the species (damp sedge meadows and reedbeds with good cover and sufficient food resources, as stated by van Apeldoorn (1999) and Bieberich & Olson (2007)), former agricultural areas are suitable for the existence of M. oeconomus. Our results show that the distribution range of M. oeconomus has expanded from the south and west of the country in easterly and northerly directions.
The main distribution range of M. oeconomus in Europe occupies a warmer environment compared to that in Lithuania, but the species also inhabits much colder regions in Scandinavia, Siberia and Alaska (Bieberich & Olson 2007), thus temperature regime seems to be only a secondary factor. Though average annual temperatures have risen by 0.4–0.5°C in Lithuania in the 20th century (Bukantis 2007), an increase in temperature of 1.3°C had no influence on the M. oeconomus population inhabiting alpine meadows in China (Sun et al. 2005). Thus, climate warming as another possible reason for the expansion in the distribution range of M. oeconomus in recent decades requires clarification or a dedicated investigation.
Acknowledgements
Research into M. oeconomus in the period 2005–2008 was partly financed by the Lithuanian State Science and Studies Foundation. Re-structuring of the results chapter was aided by Professor K. Perzanowski and proof-reading and language editing was conducted by J. Stratford.
Literature
Appendices
Appendix.
Data on M. oeconomus from publications and trapping (n — trapping effort in trap/days, S — number of species, N — number of individuals trapped, H′ — Shannon-Weaver diversity index, Moe% — % of M. oeconomus, na — data not available, res. — nature reserve, RP — regional park, NP — national park, SNR — strict nature reserve, q. — quarry, l. — lake, f. — forest, r. — river, d. — district, t. — town; habitat: m — meadow, p — pasture, s — shrub, f — forest, r — reedbed, b — bog, w — shore of waterbody, a — agriculture, v — various, including ecotones).
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