Bufo bufo (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 : 309-312

Common Toad Bufo bufo (Linneaus 1758)

Distribution ( Figure 4 ). Included records from Artportalen (N=1325): all reports have been included from the Southern, Middle, and Northern Boreal. However, in the Alpine region inexperienced observers may confuse this species with locally dark and short-limbed Rana temporaria . Therefore, only records of calling males, those made by known experienced observers or documented by pictures have been included from the Alpine region and adjacent uppermost parts of the Northern Boreal.

Widespread and common throughout the Southern and Middle Boreal, scarce but widespread in lower altitudes of the Northern Boreal north to Lule lappmark. The documented northern distribution limit runs near the Arctic Circle but could be farther north ( Figure 4 ).

As expected, the highest records are from the southern part of the Scandic Mountains (580 m altitude; Tjovre, Jämtland). Farther north, the highest records in Lycksele lappmark are at 475 m, and at 270 m in Lule lappmark, where the northern range limit bends east towards Finland.

Summertime occurrence and reproduction are known from many far offshore islands along parts of North Sweden’s coast, indicating a very high dispersal capacity over brackish water. This is the only amphibian known from the tiny and extremely isolated islet Bonden (Ångermanland, 63°25’55.0”N, 20°2’19.4”E; Figure 4 ), more than 15 km from the mainland.

There are no indications of changes in distribution over the last 50 years. The more extensive occurrence to the north presented here (compared to in Gislén & Kauri 1959) and the first records from Härjedalen do not represent range expansion but are merely due to the species having been overlooked there before. Note, however, that all older and also recently claimed records in the Alpine region (e.g., Holm 1921, Gislén & Kauri 1959; Frislid & Semb-Johansson 1981; Artportalen) are likely due to confusion with Rana temporaria . Based on current knowledge, the species does not occur in the Alpine region in North Sweden.

Habitat and movements. Breeding habitats include virtually all types and sizes of lakes, tarns, bogs, mires, ponds, and stagnant parts of streams and rivers ( Figures 11 , 14 ). The entire range from oligotrophic to eutrophic conditions is utilized, including strongly humic dystrophic wetlands. In barren oligotrophic wetlands, this is the most widespread anuran. As a consequence, it often breeds where there are predatory fish, for example Perca aquatilis Linneaus and Esox lucius Linneaus. Calling males and amplectant pairs are mainly found in nearshore shallow areas with sparse vegetation, but often in more exposed situations than is the case in Rana arvalis and R. temporaria . Calling males can be observed in rock pools and shallow brackish bays along the Baltic ( Elmberg & Ericsson 1982; Figure 12 ).

After breeding, toads move to summer foraging habitats, which are equally diverse, ranging from dry pine forest, mesic coniferous forest, and lush deciduous bottomland forest to natural grasslands, seashores, meadows, agricultural land, gardens, and parks ( Figure 13 ).

In dry and warm summer weather this is the only amphibian commonly moving around in the open. Movements from summer habitats to hibernation sites can be noticeable on warm damp nights in early autumn but are not as synchronized and spectacular as in the Rana species.

Hibernation is usually aquatic, taking place in breeding wetlands or in nearby streams and rivers. Terrestrial hibernation has been documented in the Middle Boreal ( Elmberg 1995) but based on current knowledge it must be considered as an exception from the rule. Hibernation in brackish water has been documented from northern Ångermanland ( Elmberg & Ericsson 1982).

Abundance estimates and trends. There are not any published data on local abundance, but a large number of roadside surveys of chorusing anurans in Ångermanland and Västerbotten in the 1980’s showed that Bufo bufo is often less numerous at the wetland level, but more widespread at the landscape level compared to Rana temporaria and R. arvalis . This difference is largely because it occurs in more truly oligotrophic conditions ( Elmberg & Ericsson 1982). Abundance estimates based on extensive field work in the Umeå area (Västerbotten, Middle Boreal) 1975–1994 run in the neighborhood of 400– 600 adults /km 2 in representative landscapes near the coast (Elmberg, unpublished). Lower densities are likely in the Northern Boreal region.

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

Figures

FIGURE 4. Distribution of Bufo bufo in North Sweden. Colored areas of the Baltic are those with widespread occurrence on offshore islands. The extremely isolated offshore occurrence on the islet Bonden is shown by a triangle. Possibly disjunct occurrences in northern Norrbotten and close to the Alpine region in Jämtland are shown by a filled circle. Question marks show areas where presence is possible but has not been documented.

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.