Is Great Britain heading for a Ponto-Caspian invasional meltdown?
The outcome of multiple invasions from a common origin may lead to facilitative interactions because the invaders have co-evolved under similar environmental conditions. This outcome is often referred to as invasional meltdown, with a resultant increase in invasive species and a decline in native species richness and abundance. This study seeks to assess the full scope of the threat posed by a high-risk group of 23 freshwater invaders originating from the Ponto-Caspian region (south-east Europe) across Great Britain. Ponto-Caspian invaders constitute a group of special concern because they have recently caused a large-scale invasion into western Europe. According to a literature review, 76% of reported interactions between Ponto-Caspian invaders are positive (mostly provision of food and commensalism) or neutral (habitat partitioning). Negative interactions (mostly predation) were rare, thus highlighting the ability of Ponto-Caspian invaders to coexist. At least 14 out of the 23 Ponto-Caspian organisms investigated are well-established in the Rhine estuary and Dutch ports. Four of them (Hemimysis anomala, Dikerogammarus villosus, D. haemobaphes and Hypania invalida) have recently established in Great Britain. Regression models suggest the rest are under a critical risk of being transported, with four species predicted to have arrived already to Great Britain: Echinogammarus ischnus, Jaera istri, Limnomysis benedeni and D. bispinosus. According to species distribution models, the cumulative risk of invasion of multiple Ponto-Caspian species, thus invasional meltdown, is highest in the south-east of England and decreases to the north and west. Great Britain might be on the brink of invasional meltdown, and as a consequence, confronting the problem of Ponto-Caspian invasive species is a vital element for national biosecurity. Synthesis and applications. The predictive models and maps developed in this study provide a means for an evidence-based prioritization of species and habitats for the management of existing and future invasions of Ponto-Caspian species. This integrated approach can be easily applied to risk assess other groups of species and habitats.