Rana temporaria 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 : 312-313

Common Frog Rana temporaria Linneaus 1758

Distribution ( Figure 5 ). Included records from Artportalen (N=2000): a ubiquitous species for which all reported records have been included.Any rare confusion with Rana arvalis would not change the overall picture of distribution or abundance.

Widespread and abundant throughout the Southern Boreal, Middle Boreal, Northern Boreal regions, and in the Subalpine life zone. Widespread and locally common in the Low-Alpine zone. Scattered occurrences in the Mid-Alpine zone ( Figures 15 , 16 ).

Breeding sites in the Low-Alpine zone reach at least 840 m altitude in Lycksele lappmark ( Elmberg 1991; Figure 17 ) and probably up to or above 1000 m further south in the Scandic Mountains. In other words, it is a widespread breeder well above treeline. In summertime adults have been observed at 1070 m in Åsele lappmark and Lycksele lappmark, and at up to 1400 m in Jämtland (Helagsfjället), all of which are in the upper part of the Mid-Alpine zone.

Occurrence and reproduction are known from several far offshore islands in the Baltic in the three northernmost coastal provinces (Norrbotten, Västerbotten, and Ångermanland), where distributed offshore equally widely as Bufo bufo . In the coastal provinces farther south, however, offshore occurrence is scarcer and concentrated to more nearshore islands. This may indicate a somewhat lower dispersal capacity over brackish water than in Bufo bufo .

There are no signs of changes in distribution during the last 50 years.

Habitat and movements. Spring migration of adult males to breeding ponds is largely nocturnal and often strongly synchronized, with up to 50% arriving within a few days ( Elmberg 1990). Adult females arrive more gradually to breeding sites ( Elmberg 1990). Juveniles emerge later than adults and are rarely seen before the spawning period is over.

Breeding habitats range from ponds and bogs above treeline ( Figure 17 ), through any type of wetland and lake in lower biotic regions, to sheltered shallow brackish bays of the Baltic ( Elmberg et al. 1979). They further range from wetlands in pristine forest to landfill swamps, flooded disused gravel pits and quarries, and man-made ponds in residential areas. The entire range from the most oligotrophic to the most eutrophic conditions is utilized for breeding ( Figures 11 , 14 ). This is the most frequently encountered anuran breeding in barren rock pools along the Baltic seashore ( Figure 12 ). Calling males and spawn clumps are usually found in shallow and well vegetated situations, that is, less exposed than eggs of Bufo bufo and Rana arvalis . Females remain in breeding wetlands only until they have deposited their eggs, whereas adult males remain as long as there are receptive females around, or longer ( Elmberg 1990).

Dispersal from breeding sites to nearby summer foraging habitats is gradual and not very conspicuous. Since Rana temporaria is so widespread and abundant, summer foraging habitats are extremely diverse, from alpine heath, open coniferous forest to dense bottomland woods along the major rivers ( Figures 15 , 18 ). The highest summer densities are observed in abandoned fields and grassy meadows along lakes, rivers, and seashore, but also in deciduous forests with well-developed undergrowth ( Figure 13 ). Close to nothing is known about movements and home range in summer. Autumn migration to hibernation sites is usually conspicuous and synchronized, triggered by rains or mild cloudy conditions.

Hibernation in North Sweden is aquatic. It may take place in breeding wetlands if not prone to developing anoxic conditions in winter. However, in most cases Rana temporaria migrate to spend the winter in nearby welloxygenated streams and rivers. There they seek shelter on the bottom in nearshore aquatic vegetation, but they will move to deeper water later in winter if the ice sheet expands downwards or if the water level recedes. Hibernation has also been observed in springs and natural wells in closed forest, as well as in brackish water in the Baltic ( Elmberg et al. 1979). Terrestrial hibernation in the boreal is known from Norway ( Frislid & Semb-Johansson 1981) and Finland ( Pasanen & Sorjonen 1994) and may occur in North Sweden, although it has not been documented. Experimental work in Finland strongly indicates that the species cannot survive more than a few days of freezing temperatures at the most ( Pasanen & Karhapää 1997), making it a much less likely terrestrial hibernator than Rana arvalis .

Abundance estimates and trends. Throughout the 1980’s I studied two populations in Umeå, Västerbotten, at breeding sites encircled by built-up urban areas (Tvärån: 63°49’50.9”N, 20°13’38.4”E and Bölesholmarna: 63°49’31.4”N, 20°14’6.7”E). The summer foraging habitat surrounding these ponds comprises mid- to late successional riparian deciduous woodland (2.5 and 15 hectares, respectively; Figure 18 ). Counts of calling males at both sites and drift fence catch data from the Tvärån site permitted accurate estimates of population size ( Elmberg 1990). Accordingly , the density of adult frogs in these summer habitats averaged 65–75/hectare (i.e., 6500–7500/ km 2). In the 1990’s the isolated and far offshore island Stora Fjäderägg in Västerbotten (63 o 48’N 21 o 00’E; area ca 170 hectares) was subject to a census of spawn clumps in all its wetlands (Stefan Andersson, personal communication). Supposing an even sex ratio and that each female laid one egg clump, the island’s population was 1200 adults (both sexes), that is, 7/hectare (700/km 2). The density of adults in summer habitat on Low-Alpine heath well above treeline (Kraipe, Lycksele lappmark; 65°50’4.0”N, 16°22’23.0”E; altitude 780 m; Figure 17 ) was estimated at 2–4/ hectare (200–400/km 2) in a population studied by me for several years in the late 1980’s.

The abundance estimates from the two Umeå sites are remarkably similar and probably close to the maximum occurring in North Sweden. Stora Fjäderägg and Kraipe, on the other hand, both represent suboptimal habitats. I propose that an abundance of 600 adults /km 2 is a realistic average value for North Sweden, from the Southern Boreal up to and including the Subalpine zone.

There are no indications of changes in abundance during the last 50 years.

Figures

FIGURE 5. Distribution of Rana temporaria in North Sweden. Although the entire Scandic Mountain range is colored, the species is absent from the High-Alpine zone in the Alpine region (see Figure 1 for outline of the latter). In other words, the species is missing from areas higher than 1400 m in Jämtland, decreasing to 1000 m in Torne lappmark. Colored parts of the Baltic are areas with widespread occurrence on offshore islands.

FIGURE 11. Mesotrophic lakes with extensive beds of grasses and sedges provide breeding habitat for Rana temporaria, R. arvalis, and Bufo bufo. If fishless, also Lissotriton vulgaris is likely to occur in lakes of this type. Spring aspect from Öster- Skivsjön, Västerbotten, Middle Boreal region. Photo: Johan Elmberg.

FIGURE 12. Ongoing post-glacial land uplift creates open habitats along the Baltic coast of North Sweden. Rock pools (foreground) serve as breeding sites for Rana temporaria and Bufo bufo, occasionally also for Lissotriton vulgaris. Brushy areas along the forest edge (background) are typical summer habitat for Zootoca vivipara and Vipera berus. Tjäruskär, Ångermanland, Middle Boreal region. Photo: Johan Elmberg.

FIGURE 13. Recently abandoned hayfields are prime summer habitat for Rana temporaria, R. arvalis, Bufo bufo, Anguis fragilis, and Vipera berus. The forest edge in the background is typical habitat also for Zootoca vivipara. Baggböle, Västerbotten, Middle Boreal region. Photo: Johan Elmberg.

FIGURE 14. Vast areas of North Sweden’s interior are covered by level open bogs with permanent water. Rana temporaria, R. arvalis, and Bufo bufo breed in the pool depressions (foreground) and come ashore to forage in grassy areas in summer. More elevated and drier string parts of the bogs offer protective vegetation and summer habitat för Zootoca vivipara and Vipera berus. Photo: Jonas Grahn.

FIGURE 15. Summer habitat for Rana temporaria in the transition between the Mid- and High-Alpine life zones. Here the annual activity period lasts three months or less, during which adults adopt a semi-aquatic lifestyle, spending much time in small creeks and seepage areas. Stekenjokk, Jämtland, 1070 m altitude. Photo: Johan Elmberg.

FIGURE 16. Rana temporaria is the hardiest amphibian in North Sweden, ranging up to the transition between the Mid- and High-Alpine life zones. Individuals in these uppermost populations often give a toad-like impression due to their dark color and short hindlimbs. Stekenjokk, Jämtland, 1070 m altitude. Photo: Johan Elmberg.

FIGURE 17. Rana temporaria is the only amphibian breeding in alpine heath habitats well above tree line in the Scandic Mountains of North Sweden. Among the variety of wetland types available, chorusing and spawning invariably take place in shallow ponds with moderate to sparse vegetation. Despite high altitude and latitude, a short annual activity period, and a cool overall climate, breeding ponds enjoy constant daylight, high insolation and thus warm up rapidly. Kraipe, Lycksele lappmark, 820 m altitude, Low-Alpine zone. Photo: Johan Elmberg.

FIGURE 18. Summer habitat for Rana temporaria and Rana arvalis in boreal riparian deciduous woodland. The canopy is dominated by Alnus incana, Prunus padus, and Sorbus aucuparia, with scattered Salix caprea and Betula pubescens. The lush herbaceous field layer provides shelter and abundant invertebrate food in summer. Population density at this site has been estimated at up to 7500 and 6000 adults/km2 of Rana temporaria and R. arvalis, respectively. Bölesholmarna, Umeå, Västerbotten, Middle Boreal region. Photo: Johan Elmberg.