Red Squirrels on the Isle of Wight
The Isle of Wight is one of the few remaining strongholds of the native red squirrel (Sciurus vulgaris) in the south of England . Over much of the country it has been replaced by the grey squirrel (Sciurus carolinensis), which was first introduced from North America in 1876. The inability of the grey squirrel to cross the Solent has so far ensured the survival of the red squirrel on the Isle of Wight. This is a good illustration of the problems caused by the introduction of alien species.
As the two squirrels belong to different species, this is an example of interspecific competition. There appear to be two different mechanisms involved in this competition:
1 Diet. The grey squirrel has a wider diet. It can feed on acorns which the red squirrel cannot digest, and can digest seeds which are not fully ripe, enabling it to exploit food resources before the red squirrel. Where the two species coexist, the red squirrel tends to be confined to coniferous woodland. This is an example of the competitive exclusion principle. On the IOW, in the absence of competition from the grey squirrel, the red squirrel is able to thrive in coniferous and deciduous woodlands.
2 Squirrelpox. This is a viral disease similar to myxomatosis in rabbits. Grey squirrels are unharmed by the virus, but the disease will normally kill red squirrels within fifteen days. Until now, red squirrels could not make antibodies against the disease, but recent research gives hope that this may be changing. The blood of a small number of red squirrels which had died from other causes has been found to contain antibodies to squirrelpox, indicating that these animals have been exposed to the virus and have survived. It is possible that the ability to resist the disease will spread due to natural selection.
It is likely that the genetic mutation leading to the production of the antibody occurs at random in a small percentage of the red squirrel population. The mutation has not spread in the past, since, in the absence of the disease, there has been no selective advantage to the individuals who possess it. With the introduction of the disease into the squirrel’s environment, selection pressures have changed, and individuals which can produce the antibody are more likely to survive than those that cannot, and will pass this ability to their offspring. It is therefore likely that the mutation will become increasingly common over time.
This finding also opens up the possibility of vaccinating squirrels. Vaccination works by stimulating the production of the appropriate antibodies before the animal encounters the real disease. This will only work in animals which have the ability to make the antibody.
A’ Level links
This case study can be used to illustrate a number of important topics found in A level specifications:
Concept of niche, including fundamental and realized niche;
Problems of invasive alien species;
Management of ecosystems to achieve in situ conservation;
Immunity, antibodies and vaccination.