Although over 30% of amphibians are threatened with extinction, there are some amphibians that are highly invasive aliens and are themselves drivers of the decline of native species. The introduction of the non-native marsh frog (Pelophylax ridibundus) into the counties of Kent and Sussex in south east England may have accelerated the common frog (Rana temporaria) decline in this region. The objective of this research is to identify whether marsh frogs are affecting the distribution of common frogs in Kent and Sussex.
Species distribution models revealed that the distribution patterns of the two species are largely non-overlapping. This pattern may be partly down to differences in habitat preferences between common frogs and marsh frogs rather than competitive or predatory exclusion. However, there was a negative correlation between common frog presence and high pond density which was inconsistent with habitat preferences.
A smaller-scale analysis in areas identified as suitable for both species showed a higher presence of common frogs in areas without marsh frogs, but more great crested newts (Triturus cristatus) - a predator of common frogs - in areas with marsh frogs. Great crested newts are correlated with high pond density. Low numbers of common frogs in marsh frog areas may therefore be due to high densities of great crested newts, rather than marsh frogs. Equally, grass snakes (Natrix natrix) may prey on – yet coexist – with marsh frogs, but negatively impact common frog populations. The interactions between marsh frogs and common frogs are therefore complex and involve indirect effects mediated through other species in the community.