Interspecific hybridisation between alien and native plant species in Germany and its consequences for native biodiversity

We explored the extent of interspecific hybridisation between alien and native plant species in Germany with a special focus on the potential threat for native biodiversity. In total we listed 134 hybrids which are interpreted as products of hybridisation between 81 alien and 109 native plant species (including 13 archeophytes) that occur in Germany. Seventy-five of these hybrids have been recorded in Germany, while the remaining 59 hybrids have not been detected in Germany yet, although both parental species currently occur in Germany. Interspecific hybridisation between abundant alien and rare native species can threaten populations of the native species through outbreeding depression and/or through high rates of gene flow swamping native populations. We identified 37 threatened native plant species which hybridise with aliens. Seventeen of these threatened plant species may suffer from outbreeding depression when hybridising with a more abundant alien invader (minority disadvantage). Using hybrid abundance as an indicator of hybrid fitness we argue that introgression of alien genes may affect the gene pool of eight threatened native plant species. Consequently, hybridisation with aliens has to be considered as an additional risk potentially leading to a loss of biodiversity and should be included in the repertoire of causes for rare species extinction in German Red Lists of threatened plant species.