Cumulative impact assessments of multiple host species loss from plant diseases show disproportionate reductions in associated biodiversity

Non-native plant pests and pathogens are increasing exponentially, causing extirpation of foundation species. The impact large-scale declines in a single host on associated biodiversity is widely documented. However, the multiple loss whether these impacts multiplicative has not been assessed. Ecological theory suggests that systems with greater functional redundancy (alternative hosts) will be more resilient to sympatric hosts. We test this show its importance relation pest/pathogen assessments. assessed potential two occurring European tree species, Fraxinus excelsior Quercus petraea/robur, both which currently threatened by range pathogens. At UK scale, total number species at risk from diseases affecting hosts than sum either alone. F. 45 obligate (species only found host) Q. petraea/robur 326. decline trees would 512 across taxon groups, 38% increase. Assessments local 24 mixed excelsior–Q. woodlands revealed may even due lack redundancy. Only 21% sites were able provide for can use other In most woodlands, required present, although site conditions often suitable them grow. Synthesis. Understanding should applied assessments pests/pathogens biodiversity. assessments, higher scores given already impacted pests/pathogens. Current assessment approaches ignore cumulative, cascading effects shown study allow an insidious, mostly overlooked, driver continue.