4.3 Dutch elm disease
Not all change is a direct result of human intervention. Sometimes changes can occur over which we have little control. One such example is the case of Dutch elm disease (so-called because most of the early studies of the disease were carried out in Holland, although the disease was first observed in France in 1918). The disease is caused by a fungus, Ceratocystis ulmi, that has the elm, Ulmus procera, as its only habitat and food source. Spores of the fungus are carried by the elm bark beetle, Scolytus scolytus, which feeds on the shoots and young bark of the elm. The beetles breed in the bark of elms, so there needs to be some damage already present to allow them to get under the bark. Once the spores get under the bark, the fungus can start to spread. As the fungus can grow alongside the developing beetle larvae, the newly emerged beetles will themselves carry spores and spread the disease as they fly to new trees. The fungus produces toxins and it also blocks vessels as it grows in the elm. The result of this is that the leaves wilt and the branches die back. Figure 5 shows the cycle of activity.
At first, Elm trees generally recovered from these attacks and the wood, which is used in the furniture industry, could be utilized without loss of value should the tree die. So you would be right in thinking that no serious efforts were made to eradicate the disease. Indeed, by the 1930s it had spread to the USA, probably carried by imported veneer-quality timber. But the elm has considerable amenity value: it is attractive in hedgerows and avenues and is particularly useful as a town tree because it is resistant to fumes and pollutants. For this reason some town councils did take steps to curb the spread of the disease.
However, during the 1970s the fungus suddenly and inexplicably became more virulent, causing the death of the trees. The elm has now been virtually wiped out in the UK. For example, in the whole of southern England the only places where you can still see mature elms are in Brighton and Hastings.
Can you suggest why the elm remains in these two places?
Their councils were prepared to put sufficient resources into efficient eradication of any outbreaks. They were probably further helped by the natural barrier of the treeless South Downs, and the prevalence of onshore winds tending to blow the beetles back across the downs if they started to swarm seaward.
In the north of England and in Scotland, the closely related species, the wych elm, Ulmus glabra, is surviving. One difference between the two species is that, whilst the elm usually propagates by means of suckers (these are, in effect, asexual clones of the parent plant), the wych elm is normally reared from seed and therefore has a broader genetic base. In other words, there is more genetic variation in the population.
Why is genetic variation important?
The more variation there is between members of a species, the better the chance that, in a changing environment, there will be some individuals that have the ability to thrive (or at least survive!) in the new conditions.