Ash Dieback disease, caused by the fungus Chalara fraxinea (Hymenoscyphus pseudoalbidus), arrived in the UK in 2012 and has caused a furore in the media. But what effect will it really have on wildlife?
Ecology of a disease
It is the latest of many pests and diseases to infect plants in Britain and elsewhere in the world. Most do not cause any serious harm. There are two typical scenarios where a newly arrived disease might cause ecosystem damage:
1) When the ecosystem in question has been isolated from the disease for a long period of time and the native species have no natural defence. This typically applies to remote islands, but can also apply when diseases cross between the main continental land masses.
An example of this occurring is the American chestnut blight of the early 20th century, which wiped out whole forests in the USA.
2) When the host is genetically weak or exceptionally abundant. This is mainly a problem for agricultural crops, where genetically identical plants are used over large areas. A small number of wild plants might also have a limited gene pool, typically when a genetic ‘bottleneck’ such as habitat fragmentation or colonisation of new territory has occurred.
An example of this is the Dutch Elm disease in Britain in the 1970s, where many English Elms died. The English Elm went through its own genetic bottleneck when it was introduced to Britain, possibly in Roman times. It is not a sexually reproducing species: the plants that died were essentially all clones.
Implications for British ash trees
These scenarios apply only in a limited extent to Chalara fraxinea and the British ash, Fraxinus excelsior:
1) The pest is newly-described fungus that is very closely related to one that is common in Europe, so we don’t know to what extent this is a new disease. Obviously, any disease could potentially mutate or adapt to become fatal, but this does not seem to be a situation where the host has no natural defence against a previously unknown disease.
2) Ash is a sexually outbreeding with a reasonable level of genetic diversity, although the recolonisation of Europe - and particularly Britain - after the last Ice Age may have left ours with less genetic diversity than might otherwise be expected. This has not yet been proven, but does suggest that there may be some vulnerability to a new disease.
Therefore, on the face of it, it seems unlikely that Chalara fraxinea will decimate wild populations of ash in Britain. It is most likely to infect a proportion of trees, simply forcing natural selection of the surviving, disease-tolerant varieties.
This is what botanists on the continent are reporting, where at at least 10% of trees are reported to be resistant to it. If this is so, then we can expect the resistant varieties to increase while the more vulnerable ones succumb or are weakened, eventually to be displaced from the ecosystem.