Amphibians

Written by Elsbeth Leighton, Coalface to Wildspace Midlands Project Assisstant

There are over 1000 species of frogs from the family Hylidae, the group we refer to colloquially as tree frogs. This is one of the largest families of frogs with species found on all continents in the world except for Antarctica, however they are particularly diverse in the tropics of South and Central America (AmphibiaWeb, 2025).

The species of tree frogs found within Europe all come from the genus Hyla, within this large family Hylidae. Until the late 1960’s it was believed that all tree frogs found across Europe were the same species, Hyla arborea, often referred to as the European Tree Frog or the Common Tree Frog. Following more research into their phylogeny, i.e. evolutionary history and species relationships, this has been refined and there are now seven full species described that are found in Europe (Speybroeck et al., 2020).

They form what is described as a species complex; a group of species that are closely related and may be hard to distinguish without the use of molecular tools. As a result of this, many of these species are cryptic; hard or impossible to identify by visible physical traits alone. This species complex extends beyond Europe and into Africa and Asia, including 2 other closely related species whose ranges partially overlap with some of these species outside of Europe (Dufresnes et al., 2020)

H.arborea is quite widespread across Europe with a range that spreads from France, up to Denmark, across to Poland and down to Greece. These frogs are typically green on top, with white or grey undersides and a dark stripe that runs along their sides, from their nostril down to their back leg, with an upward flick at the end. The spend much of their time on land in shrubby habitat, meadows and low down in the trees, whilst breeding in sunny freshwater pools, lakes and marshes. To attract mates, they produce what is often referred to as a ‘short call’, made up of repeated bursts of sound less than a second apart (Speybroeck et al., 2016).

H.intermedia is commonly called the Italian Tree Frog, and it is found within Italy and Southern Switzerland. Following a study on the DNA of these frogs in 2018, this species was divided into two, with individuals found within the north of the range reclassified as H. perrini (Dufresnes et al., 2018). However, during a more recent checklist of Euoprean species, H. perrini was reverted back to a subspecies of H. intermedia and is now referred to as H. intermedia perrini (Speybroeck et al., 2020). The species status of the frogs in this species complex are often up for debate as new molecular analyses take place or old data is revisited.

H.molleri, the Iberian Tree Frog, and H. orientalis, the Eastern Tree Frog, were formally classified as separate species to H. arborea in 2008 based on genetic analysis (Stöck et al., 2008). Within Europe, H. molleri is found in Portugal, Spain and Southwestern France, whilst H. orientalis range spreads from Poland, up to Lithuania, across to Ukraine and down to Greece. Both species’ ranges overlap with H. arborea at their margins, and, along with H. intermedia, none of these four species can be reliably differentiated from each other by appearance alone (Speybroeck et al., 2016). All four species have ‘short’ calls, but research analysing sound recordings indicates that there are subtle differences in H. orientalis calls compared to H. arborea, at least within their populations in Bulgaria (Lukanov, 2024). It is possible that with further study, more of these cryptic species will be found to differ in their calls despite their similarity to our ears.

H.Sarda, the Tyrrhenian Tree Frog, was considered a subspecies of H. arborea until 1983, when it was elevated to a full species (Frost, 2024). Its native range includes the Mediterranean islands of Sardinia and Corsica, among others. Unlike the other Hyla species mentioned above, it does display visible differences to H. arborea. The stripe along its body can be shorter and lacks the upwards tick at the base of the back leg, additionally some individuals have darker spots on their backs (Speybroeck et al., 2016).

The European range of H. savignyi, the Lemon Yellow Tree Frog, is limited to Cyprus, but most of its range extends beyond Europe (Zotos et al., 2024). This frog was classified as a separate species following research into its physical and mating call differences in 1974, supported by additional research since (Frost, 2024). The key physical differences include that the stripe along the body lacks the upward tick and is often broken up into spots along the side, they also have an ‘intermediate’ mating call (AmphibiaWeb, 1999; Frost, 2024).

H.meridionalis, the Mediterranean tree frog, is thought to be a naturalised species in Europe and has populations in Southern Spain and Portugal, the Baleartic and Canary Islands, as well as Southern France, Northern Spain and Northwest Italy. Molecular analysis shows that these populations originated from Morocco, but that they likely arrived through 3 different events. The population found in Southern Spain and Portugal is the oldest and the only one that may have colonised naturally without human intervention, but this is still up for debate (Dufresnes and Alard, 2020). This frog is slimmer than H. arborea and is sometimes referred to as the Stripeless Tree Frog, because its stripe is either missing or shorter, stopping at the shoulder. It’s different breeding call is distinctly different, referred to as a ‘long’ call; a single burst of sound lasting around 1 second (Speybroeck et al., 2016).

All seven of these species are listed as Least Concern in Europe according to the IUCN Red List, however some of them, including H. arborea, are noted as having declining populations (e.g. Agasyan et al., 2024). Threats to these species differ by location and circumstance but include climate change and the loss or degradation of habitat due to human impacts. Mediated by both the effects of humans moving species around and by potential range shifts due to changing climates in the future, these Hyla species have the potential to pose a risk to one another over time (Dufresnes et al., 2020).

Despite their status as separate species, some of these tree frogs are capable of hybridising. If this occurs too frequently it could erode their genetic distinctness leading to a loss of genetic diversity. In cases where a physical barrier, such as a mountain, or some aspect of their coevolution at a range boundary has limited interbreeding between species, when they are introduced to a new area this barrier could be removed resulting in hybridisation (Dufresnes et al., 2020). In the Netherlands for example, only H. arborea is native, but over the years 2 non-native tree frog species have been illegally introduced and formed populations (Kuijt et al., 2022). If these introduced tree frogs start to interbreed with the native H. arborea in the future, the study authors wonder what conservation action would or should be taken to protect these tree frogs from the threat of other tree frogs.

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