The European tree frog

The family of the tree frogs (Hylidae) is highly diverse, currently containing 40 genera and 901 species (Frost, 2011). In Europe six species of the genus Hyla occur: Hyla arborea (Linnaeus, 1758; European tree frog), Hyla intermedia (Boulenger, 1882; Italian tree frog), Hyla meridionalis (Boettger, 1874; Mediterranean tree frog), Hyla molleri (Bedriaga, 1890;

Figure 1.1: Calling tree frog male (Foto: Michael Werner)

Iberian tree frog), Hyla orientalis (Bedriaga, 1890; Shelkovnikov's tree frog), and Hyla sarda (De Betta, 1853; Tyrrhenian tree frog ). In Germany only the European tree frog, also known as the Common tree frog, can be found.

The European tree frog is one of the smallest European anurans (Figure 1.1). The body length of adult individuals ranges from 27 mm to 50 mm (Tester, 1990; Friedl and Klump, 1997). Their dorsal skin is smooth and bright green. Depending on temperature,

“mood”, and substrate, the coloration varies from yellowish to green, grey, or dark brown. The ventral side and the inner surface of the limbs are whitish to light grey

with granular skin. On both sides a dark lateral stripe goes from the nostrils over the

tympanum to the inguinal region, where it forms the inguinal loop. Characteristic for the tree frogs are their finger- and toe tips expanded into microscopic structured discs, which enable them to climb smooth plants. Males can be detected by the yellowish to brownish subgular vocal sac, while females have a white and smooth throat.

The breeding season starts between late March and early May and ends between early June and mid-July (Schneider, 1966; Schneider, 1971; Tester, 1990; Grosse, 1994). The breeding ponds are characterized by rich submerged vegetation, shallow areas and exposure to the sun (Grosse and Nöllert, 1993). While males spend several nights at the breeding site, females typically stay for only one night. Friedl and Klump (2005) observed that the duration of male chorus attendance reflects male quality. Females deposit several clumps with a total clutch size of 150 to 450 eggs (Clausnitzer and Clausnitzer, 1984). In Eastern Europe clutch sizes up to 1000 eggs per female were observed (Bannikov et al., 1985). The majority of the tadpoles in Central Europe complete metamorphosis between June and August.

After the mating season the tree frogs migrate to their summer habitat, usually within the radius of 500 m of the breeding site. Single individuals, especially juveniles migrate greater distances up to 3400 m (Fog, 1993). They can be found in trees, bushes, perennial plants or riparian vegetation. Important are sunny places with a moist microclimate and a

complex vegetation structure (Stumpel, 1993). In the autumn the frogs migrate to their winter habitat. Deciduous and mixed forests with dense layers or piles of leaves and brushwood, copses, crevices and caves offer frost free places for hibernation (Nöllert and Nöllert, 1992;

Geiger, 1998; Grosse, 2009).

The lifespan of European tree frogs can reach in the wild 4 - 6 years (Stumpel and Hanekamp, 1986; Tester, 1990; Friedl and Klump, 1997). In captivity ages up to 22 years have been reported (Bannikov et al., 1985). Year-to-year survival rates were found to range between 20 and 44 % and in cold winters even lower (Tester, 1990; Friedl and Klump, 1997).

Tester (1990) determined a population turn over rate of only three years.

1.3.2 Distribution

Hyla arborea is widely distributed across the European continent (Figure 1.2). It occurs from North West Iberia and France eastwards to Western Russia and the Caucasian region, and southwards to the Balkans and Turkey. Except for southern and eastern Denmark and extreme southern parts of Sweden it is absent from Scandinavia (Kaya et al., 2009). It is a lowland species that has been recorded at a maximum altitude of 1,000 m a.s.l. in the Carpathian Mountains (Zavadil, 1993).

Figure 1.2: Distribution map of the European tree frog (Hyla arborea), modified from IUCN (2009).

1.3.3 Conservation status and major threats

The European tree frog is listed on Appendix II of the Berne Convention and on Annex IV of the EU Natural Habitats Directive. Therefore, it is subject to a strict protection system.

Although the species is listed in the IUCN Red List in the Least Concern category, the overall population shows a decreasing trend (Kaya et al., 2009). While the species is common in suitable habitats in parts of its range, it has been reported to be fragmented and in significant decline over large parts of its Western European distribution (e.g. Fog, 1995; Baker, 1997;

Gasc et al., 1997). The species is protected by national legislation in many countries. In the German Red Lists it is categorised in five states as Vulnerable, in six states as Endangered, in three states as Critically Endangered, and in one state (Berlin) as Extinct (Bast et al., 1992;

Podloucky and Fischer, 1994; Bitz and Simon, 1996; Jedicke, 1996; Laufer, 1999; Rau et al., 1999; Schlüpmann and Geiger, 1999; Nöllert et al., 2001; Beutler and Rudolph, 2003; Klinge,

2003; Brandt and Feuerriegel, 2004; Meyer and Buschendorf, 2004; Schneeweiß et al., 2004;

Kühnel et al., 2005; Flottmann et al., 2008)

The European tree frog is affected by habitat fragmentation and habitat degradation (Grosse, 1994; Tester and Flory, 1995; Pellet et al., 2004a; Pellet et al., 2004b). The loss of calling and breeding sites and the introduction of fish (Filoda, 1981; Clausnitzer, 1983;

Bronmark and Edenhamn, 1994) are the main reasons for population decline.

1.3.4 Conservation genetics in the European tree frog

Some conservation genetics studies on the European tree frog have already been conducted.

One of the first studies used allozymes to investigate the status tree frogs in Sweden (Edenhamn et al., 2000). Low genetic variation was found in comparison with continental populations. The development of species specific microsatellite-primers by Arens (2000) and Berset-Brändli (2008) prompted more studies that assessed the genetic status of tree frog populations in Denmark, the Netherlands, and Switzerland (Andersen et al., 2004; Arens et al., 2006; Angelone and Holderegger, 2009; Dubey et al., 2009; Angelone et al., 2011). Most of these populations suffered from habitat loss and habitat fragmentation which was apparent in the genetic data. The lowest genetic diversity was found in the Danish populations on Lolland and was associated with an increased larval mortality (Andersen et al., 2004).

However, in Switzerland the genetic analyses provided compelling evidence for the success of conservation and connectivity measures in the Reuss valley, leading to an enhanced tree frog migration among breeding sites within distances up to 4 km (Angelone and Holderegger, 2009). In a study in 2005 I started to shed light on the genetic situation of tree frogs in

Germany. Bayesian analyses indicated that the tree frog occurrences near Hannover were fragmented into four genetically distinct clusters. However, the genetic variation was relatively high compared to the values in the adjacent countries. Moreover, within the Hannover region, I identified a potential source population for an introduced and previously unknown population in southwest Hannover (Krug and Pröhl, submitted).

A phylogenetic study of the circum-Mediterranean Hyla species was carried out by Stöck et al. (2008). Their data suggest the Balkan region as a possible Pleistocene refugium with the subsequent colonisation of Middle and Western Europe by a single genetic lineage.