Ash dieback risks an extinction cascade

Large-scale decline in populations of European ash (Fraxinus excelsior) are occurring throughout Europe due to the invasive fungus Hymenoscyphus fraxineus. This has grave ecological implications not only for ash trees, but also for the biodiversity supported by, and in some cases solely dependent on ash. Here we used data on the tree-species associations of biodiversity in Sweden, to predict extinction risks for ash-associated organisms, and the potential for combinations of other tree species to sustain ash-associated biodiversity. Of the 483 ash-associated species identified, 11% are exclusive to ash, and a further 23% prefer mainly ash. Notably, many ash-associated species are shared with wych elm (Ulmus glabra) which is similarly threatened by an invasive fungus. Considering the level of host association and the species' conservation status, 115 species were deemed at high risk of regional extinction. Using a mathematical optimization model we found that up to nine additional tree species would be needed to sustain all non-obligate ash dependent/preferring species in the absence of ash and elm. We discuss mitigation and adaption options to reduce the potential for an extinction cascade and conserve ash-associated biodiversity, but all pose unique challenges.