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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.
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Effect of diseases on ecosystems
Most ecosystems are highly tolerant of disease outbreaks. Diseases, pests and other types of predators are all part of what makes up an ecosystem, and the more different types there are, the more stable the ecosystem becomes.
An example of this is the Brazilian rainforest, where rubber trees, Hevea brasilensis, grow in small numbers as widely spaced individuals. If grown closely together in plantations they succumb to leaf blight - which defeated early attempts by Europeans to cultivate rubber in its native country. When moved to other countries, however, free from this particular form of blight, it was possible to create rubber plantations.
The ecosystems that are most vulnerable to disease are therefore ones that are dominated by one or a small number of species. Whenever this occurs, some sort of pest or predator eventually “learns” to exploit the dominant species, and a more balanced system is created. This is why ancient, semi-natural habitats tend to be more species-rich and more valued for nature conservation, than recent, secondary habitats are.
The effect of diseases, therefore, is to create diversity in the habitat as well as genetic diversity in the species.
Implications for ash woods
Ash woods in Britain are the typical climax community on base-rich soils throughout the country. The canopy is typically made up of a wide range of species, including oak, cherry, lime, sycamore, wych elm, alder, birch, beech, aspen and various types of willow.
If a disease like ash dieback affects trees in a wild wood, it is unlikely that the casual observer would even notice. The rate at which resistant varieties of ash, or other species such as sycamore and cherry, come to replace the dead trees is so quick that there are unlikely to even be gaps in the canopy for more than a few months. The woods that are most vulnerable are the ones with the least diversity in tree species.
Evidence from Europe supports this hypothesis. As far as I can ascertain, there is not yet any documented case of an ash tree dying from Chalara fraxinea infection. What happens is that seedlings die, and mature trees suffer some dieback of new growth in their crowns. If this continues, it is likely that eventually some ash-dominated woods will become more diverse. It seems altogether unlikely that there will be large scale loss of woodland ground flora, as some people have claimed.
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The importance of ash dieback
Ash dieback, like other diseases of wild plants, is probably just an addition to our ecosystem. It will settle in and eventually we will see that its main effect is likely to be to increase overall diversity and ultimately the stability of woods.
What alternative is there to studying the progress of the disease to see if this does indeed come to pass? Interventions such as felling diseased trees would probably cause more harm than good. When disease resistant varities are found, they could be planted in woods, but this creates the danger of having woods that are even less genetically diverse, and therefore even more vulnerable in future. The worst thing would be to produce cloned or grafted trees and plant these all over the countryside. As Peter Marren has said on his blog, one of the main lessons to learn from this is not to do too much planting. Best to leave it to nature.
Over the last 50 years the amount of ash, Fraxinus excelsior, in England and lowland parts of Wales and Scotland has increased greatly as a result of lower grazing pressure and the decline of elms. A slight reduction, if that happens, is unlikely to do much harm.
Alan Stubbs, working for Buglife, has identified 28 species of invertebrate that are monophagous on ash (they eat nothing else). It would be useful to get good quality data on these species in particular in case there is any change in the composition of ash woods.
The British Lichen Society has produced a list of lichens that live on ash. I am not sure whether it is yet known which of those species are restricted to ash, and whether any of those species live on the young shoots that are most affected by the disease, so more research into this would be timely.
But what can the ordinary person, or a conservation manager do? Well, the most practical thing seems to be to observe and monitor. The BSBI was found to be vital to the issue when the disease struck, as we were the only organisation in Britain recording the distribution of ash trees. Many of our county recorders also have detailed information on the composition of woods, and will be well placed to report on changes resulting from this and other changes to the ecology of the countryside in future. Feel free to join naturalists societies like this and make a serious contribution to conservation by participating in our valuable work.
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Speculation
This account is a cautious scientific viewpoint based on what we know about ash dieback and the ecology of woodlands. But speculation is more interesting, and is what has dominated the public debate on this subject for the last few weeks. So what do we think will actually happen? Well, having never seen ash dieback and not being particularly interested in it, here is what I suspect will happen, based on talking to people who know a lot more than I do...
+ A fair proportion of ash trees in Britain will probably get infected, mainly through airborne dispersal of the disease. Well over 10% probably. Possibly as many as 70%. Probably not more than 90% in any one wood.
+ Some of these infected trees will be weakened and may eventually die. The ones that die will take between a few and many years to succumb. Others will recover and will eventually die of other causes. At a guess, we might speculate that 50% of all the mature ash trees in Britain may die of this disease over the next 100 years if nothing else kills them first.
+ This is slightly higher than the natural rate of turnover of ash trees in woods. We don’t know the precise life expectancy of an ash tree in the wild or how this will be affected by the disease. Assuming resistant varieties appear within a few years, the overall effect is likely to be a small dip in the population and productivity of ashes (relative to other forest trees) over the next decade or two, followed by a complete recovery - al other things being equal.
+ Unless these estimates turn out to be wildly inaccurate, it is difficult to believe the claims in the press that (a) bluebells and other woodland ground flora will be severely affected by the disease; (b) any species will become extinct; or (c) Britain will in any way be devastated.
But this is mere speculation. It would be good to see a scientific process to measure these things as they happen and put in some accurate numbers.
Do you disagree with this? If so, send us your predictions of the effects of ash dieback by 2015 and if we get enough we shall run a competition and award a prize to the best answer. Choose something measurable, like (‘all bluebells will die” or “this insect will go extinct”). Here’s your chance to have your say!
Written by Alex Lockton with grateful thanks to Quentin Groom, Martin Godfrey, Sarah Whild, Richard Gornall, Fred Rumsey, Alan Stubbs, Keith Kirby, Roy Smith, Oliver Rackham and Oli Pescott for their contributions and information that I have borrowed. Any opinions or errors in the account above are purely my own. Updated to 25/11/2012. Do feel free to send us your feedback or contribute any new factual information.
Useful Links:
>> EU Briefing paper
>> Fraxback web site
>> Flora Locale’s briefing
>> Forestry Commissions facts page
What can you do and what does the BSBI do?
If you are a farmer, gardener or land manager, your crops, hedges, plantations and even livestock are at risk of disease if they lack genetic diversity. If you are a wildlife site manager, wild plants and animals on your land are similarly vulnerable if they are in small populations suffering from isolation. You can help to maintain genetic diversity by supporting rare breeds, growing traditional crops, creating larger nature reserves, and promoting woodland expansion through using locally collected tree seed or creating conditions for natural regeneration. The charity Flora Locale (http://www.floralocale.org) can offer advice and has a leaflet 'Creating Woodlands Naturally'. Flora locale also suggests that if any nursery-grown stock of native trees or shrubs are to be used, full checks should be undertaken to confirm that they have been grown by that same nursery from seed collected from one of the appropriate Forestry Commission Seed Zones.
The Botanical Society of the British Isles records and monitors wild plants in Britain, and we teach botany so people can competently tell apart not only the native wild plant species, but also the introduced non-native species that are widely planted. We also support research into subjects such as conserving the genetic diversity of wild crop relatives. Join the BSBI and support scientific research and education.
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