The introduction of three cryptic tree frog species in the Dutch coastal dunes challenges conservation paradigms

Abstract

Invasive species are considered one of the main drivers of the sixth mass extinction. Conservation solutions depend on whether a species is also indigenous to the country it invades (i.e., beyond its native range). In the case of invasive cryptic species, genetic tools are required to establish their identity. We illustrate these issues with the human-mediated colonization of the Dutch coastal dunes by Hyla tree frogs. Although previously assumed to concern the indigenous common tree frog H. arborea, European tree frogs comprise a complex of allopatric cryptic species, meaning the taxonomic identity of introduced Dutch populations warrants investigation. We sequence mtDNA for 173 individuals from native and introduced populations across the Netherlands and compare our dataset with hundreds of Hyla haplotypes previously barcoded in the Western Palearctic. Two of the dune populations carry an mtDNA haplotype of the native species H. arborea that occurs naturally elsewhere in the Netherlands. In contrast, mtDNA assigned to the eastern tree frog H. orientalis was detected in all three other dune populations. In one of these populations mtDNA of the Italian tree frog H. intermedia was also found. Not one, but three species of tree frogs have thus been introduced to the Dutch coastal dunes, only one of them being native to the Netherlands. This situation causes a conservation conundrum as some introduced populations are lawfully protected but could pose a threat to local biodiversity. Regarding the ‘true’ exotic tree frog species, all conservation options should be considered.

Piecing the barcoding puzzle of Palearctic water frogs (Pelophylax) sheds light on amphibian biogeography and global invasions

“…Amphibian translocations are increasingly reported worldwide (in link to the pet trade, as pest control, garden ornaments, or as food, medicine or research resources), generating a high socioeconomic cost when introduced species become a threat to autochthonous wildlife (Measey et al, 2020;Soto et al, 2022). Establishing the identity and origin of non-native amphibians is instrumental to inform wildlife managers and conservation scientists (Dubey, Lavanchy, et al, 2019;Koster et al, 2023;Kuijt et al, 2023), but it first requires a priori knowledge of their native diversity, which is rarely trivial (Robbemont et al, 2023). For example, infamous global invaders such as the cane toad (Rhinella marina), the clawed frog (Xenopus laevis) or the Asian common toad (Duttaphrynus melanostictus) all belong to species complexes composed of multiple lineages in their home range (Furman et al, 2015;Rivera et al, 2022;Wogan et al, 2016).…”

An isolated crested newt population in Dutch coastal dunes: distribution relict or introduction?

“…It is unclear if the isolated population in the coastal dunes is the remnant of a formerly larger natural distribution (Creemers and van Delft, 2009). Human-mediated establishment of the coastal dune population is a realistic scenario as well, considering that introductions of the common midwife toad Alytes obstetricans, naturally occurring only in the extreme southeast of the Netherlands (Vliegenthart et al, 2023), and of two non-native tree frog Hyla species (Kuijt et al, 2023), have been well documented in the same area (albeit these only took place in the 21st century).…”

The conservation paradox of an introduced population of a threatened species: spadefoot toads in the coastal dunes of the Netherlands

“…Paradoxically, the thriving Callantsoog spadefoot toads potentially poses a conservation threat to native biodiversity. Because P. fuscus is native to the Netherlands, where it is considered (severely) threatened, and is protected by both national and European law, it is not straightforward to determine what conservation action could potentially be taken for the Callantsoog population (Kuijt et al, 2023). We urge stakeholders to design a plan of action and for now suggest the impact of the Callantsoog spadefoot toads on native biodiversity is closely monitored.…”