Occasionally the population of an animal or plant is drastically reduced to only a couple of people by a major calamity. This is known as a genetic bottleneck. A recent example of this is a study from the lion population from the Ngorongoro Crater, Tanzania. The study illustrates the dangers of reserves becoming ecological islands surrounded by agricultural and developed land. The crater was declared a nature reserve in the late 1920s and became a World Heritage Website in 1979. The crater floor covers 259 sq km and is over 61Omdeep, cutting it off from the surrounding savannah from the Serengeti Plain.

In 1962 there had been 65-70 lions. Exceptionally heavy rains that year allowed an explosion within the population from the biting fly Stomoxys calcitrans. Despite the lions’ legendary strength, there was nothing they could do against such a little adversary. The flies tormented the lions constantly, and they became covered in sores and too weak to hunt. By the end from the rains that year there were only 10 lions left – nine females and one male.

Sometime later an additional seven male lions migrated into the crater. Nevertheless, since that time no other lions have managed to get in. All from the 100 or so lions in the crater these days are descendants of the original few. A study was carried out to see just how inbred the crater lions really were. The results confirmed fears that they are almost all genetically identical. The males within the crater had been compared with those from the Serengeti Plain. The crater males showed a much high degree of sperm abnormalities and had a far lower reproductive success rate. Both of these problems are nearly certainly the result of inbreeding.

The main threat facing the lions of Ngorongoro and other genetically impoverished populations is not habitat destruction or poaching but the risk of an epidemic. Their lack of genetic variability means they have a severely retarded immune system and a single outbreak of disease could wipe out the lot.