Natrix natrix (Linneaus 1758)

Elmberg, Johan, 2023, Amphibians and reptiles in North Sweden: distribution, habitat affinities, and abundance (Classes: Amphibia and Reptilia), Zootaxa 5301 (3), pp. 301-335 : 319-322

Grass Snake Natrix natrix (Linneaus 1758)

Distribution ( Figure 9 ). Included records from Artportalen (N=200): all reports from Gästrikland, Hälsingland, and coastal Medelpad (where widespread). Reports from other areas have been included only if documentation was available. This species and black morph Vipera berus are widely confused, even among naturalists.

Occurs throughout the Southern Boreal. Widespread and fairly common in Gästrikland and coastal Hälsingland , local and scarce in interior Hälsingland and coastal Medelpad. There is a disjunct well-documented record at 480 m altitude in central Härjedalen, in the Northern Boreal region ( Figure 9 ). This is by far the highest known record, whereas the 2 nd to 4 th highest in North Sweden were all made at 180 m or lower. For the former extraordinary record there is no obvious suspicion of anthropogenic spread. However, well-documented records north of the present range (filled black circles in the map) can all be suspected to have anthropogenic origin, and none represents permanent presence or a reproducing population.

Although the species is an excellent swimmer there are just two records on offshore islands off the Baltic coast of North Sweden (Limön and Eggegrund, both in Gästrikland).

For the period 1900–1950, Gislén & Kauri (1959) listed many records in the Middle and Northern Boreal, far north and west of the known present range. Several were well documented, but it is not known whether those occurrences were relicts from a previously wider natural distribution, or the result of anthropogenic activities such as long-distance transport of hay and manure ( Elmberg 1995). As late as the late 1960’s there were confirmed records in several places around Umeå (Västerbotten, Middle Boreal; Stefan Ericsson personal communication), an area where the species has not been documented since. During the last 50 years, though, there have not been any indications of changes in distribution.

Habitat and movements. Found in open places providing shelter, such as tall field layer vegetation, cairns, stone walls, ditches, and heaps of plant debris. Typical habitats are shores of ponds and lakes, but also edges of fields and pastures. It is not known if habitat use varies over the annual activity period, but adult females must find suitable oviposition sites in early summer. This likely forces them to move considerable distances, probably through less typical habitats. All known oviposition sites in North Sweden were in composts, manure heaps or livestock fodder stacks ( Löwenborg 2009; Mattias Hagman & Simon Kärvemo, personal communication).

Although movements have not been studied in North Sweden, Natrix natrix are frequently seen crossing roads before and after oviposition (adult females) and after hatching in late summer (juveniles). This mobility sadly leads to many being killed by cars.

Nothing is known about hibernation habits in North Sweden, but communal hibernation at suitable south-facing sites is likely, as farther south in Sweden.

Abundance estimates and trends. There are not any data about abundance in North Sweden, nor any indications of changes in abundance over the last 50 years.

Figures

FIGURE 9. Distribution of Natrix natrix in North Sweden. The green circle denotes a disjunct record in Härjedalen, for which there is no suspicion of anthropogenic origin. Triangles show occurrence on far offshore islands in the Baltic. Filled black circles denote well-documented records where anthropogenic origin can be suspected (cf. Elmberg 1995). None of the latter represent permanent reproducing populations. A question mark shows area where presence is likely but has not been documented.