Abstract
At least 10% of the world’s tree species are threatened with extinction and pathogens are increasingly implicated in tree threats. Coextinction and threats to affiliates as a consequence of the loss or decline of their host trees is a poorly understood phenomenon. Ash dieback is an emerging infectious disease causing severe dieback of common ash Fraxinus excelsior throughout Europe. We utilized available empirical data on affiliate epiphytic lichen diversity (174 species and 17,800 observations) among 20 ash dieback infected host tree populations of F. excelsior on the island Gotland in the Baltic Sea, Sweden. From this, we used structured scenario projections scaled with empirical data of ash dieback disease to generate probabilistic models for estimating local and regional lichen coextinction risks. Average coextinction probabilities (Ā) were 0.38 (95% CI ±0.09) for lichens occurring on F. excelsior and 0.14 (95% CI ±0.03) when considering lichen persistence on all tree species. Ā was strongly linked to local disease incidence levels and generally increasing with lichen host specificity to F. excelsior and decreasing population size. Coextinctions reduced affiliate community viability, with significant local reductions in species richness and shifts in lichen species composition. Affiliates were projected to become locally extirpated before their hosts, illuminating the need to also consider host tree declines. Traditionally managed open wooded meadows had the highest incidence of ash dieback disease and significantly higher proportions of affiliate species projected to go extinct, compared with unmanaged closed forests and semi-open grazed sites. Most cothreatened species were not previously red-listed, which suggest that tree epidemics cause many unforeseen threats to species. Our analysis shows that epidemic tree deaths represent an insidious, mostly overlooked, threat to sessile affiliate communities in forested environments. Current conservation and management strategies must account for secondary extinctions associated with epidemic tree death.
Highlights
Pathogens are individually, or in association with other factors, increasingly implicated in the decline, threats and extinction of a wide range of species and the degradation of ecological systems throughout the world [1,2,3,4,5,6,7]
The percentage of infected trees varied between study sites with an average of 71% of trees infected with the disease across sites (Table 1)
Low disease incidences were recorded in two localities with only 17% and 33% of F. excelsior infected
Summary
In association with other factors, increasingly implicated in the decline, threats and extinction of a wide range of species and the degradation of ecological systems throughout the world [1,2,3,4,5,6,7]. Throughout Europe, the fungal pathogen Hymenoscyphus pseudoalbidus (anamorph Chalara fraxinea) [8] has been causing severe dieback of common ash Fraxinus excelsior in wooded stands of all ages and [9,10,11,12]. Such emerging infectious diseases (EIDs) [1] threaten their immediate host and have serious, often unknown, cascade effects on species composition, structure and function of terrestrial ecosystems [6,13,14]. In this study we utilized available empirical data on affiliate epiphytic lichen diversity among 20 ash dieback infected host tree populations of F. excelsior
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