The lions of Ngorongorro crater
Hazards of genetic isolation
Many visitors to Tanzania are attracted to the Serengeti Plain, with its breathtaking views and rich diversity of wildlife. But the magnetism is especially powerful along the eastern edge, with a range of high mountains and the spectacular Ngorongoro Crater. This natural feature is one of the World's largest calderas: the walls of the crater were originally part of a giant dome formed by volcanic activity and the top of the dome later collapsed to become the crater floor. The perimeter of the crater rises to about 2400 metres (8000 feet), the floor is at about 1700 metres (5600 feet), and it has a diameter of about 20 kilometres. It has become an ecological island, with the caldera walls forming a natural barrier to the movement of animals. Whereas migration is the norm on the Serengeti Plain, the crater animals are permanent residents. The soil is good; the rainfall is reliable; there are good numbers of animals - indeed, the crater has the highest density of large carnivores in Africa.
The lions of Ngorongoro are much photographed by tourists, and have been the subject of numerous detailed studies by zoologists. Few have suspected the sad story that lies behind their proud appearance! We are indebted to Craig Packer for providing a fascinating overview of the developing crisis (National Geographic, April 1992, 122-136). The lions have a problem of inbreeding: genetic isolation is an unseen enemy which weakens them as individuals and as a population.
Numerous factors are relevant to the inbreeding problem:
1. The crater walls inhibit movement: keeping the present population inside and making it easier for the patrolling males to deter the advances of would-be immigrants.
2. The high density of existing male lions, which further reduces the chances of invasion.
3. Big-game hunting prior to 1924 significantly reduced the lion population.
4. A plague of biting flies occurred in 1962: the lions faced disaster as they were so weakened that they could not hunt and numbers dropped from over 70 to about 10. An estimated 7 new males entered the undefended crater, but since then the lions have been genetically isolated.
The detailed studies of Packer and his wife show that the present 100 lions (comprising 6 prides) are all descended from 15 animals. Inbreeding has led to notable DNA similarities (representing a reduction in genetic diversity of 10% in 20 years), to the presence of defective sperm, to reductions in reproduction rates, and to reductions in the vitality of the immune system (making them susceptible to disease). Packer considers that the decimation caused by the biting flies may be linked to assaults on the immune system from inbreeding for several decades prior to 1962.
Packer, of course, has some thought-provoking comments to make about these findings. He reminds us that the protection of the World's wildlife requires a more sophisticated policy of conserving habitats. Unless care is taken, animals may be confined to small, isolated reserves which can result in the same problems for their inhabitants as Ngorongoro has for its lions. Such game reserves are not havens of rest, because the animals will be vulnerable and the different species may experience long, lingering deaths. For populations to be viable, they must not be `captives in the wild'.
It appears to me that Packer's article has implications, not only in the area of conservation, but also for evolutionary theory. One of the more popular ideas goes by the name `allopatric speciation'. The suggestion is made that evolution occurs, not in the main breeding population, but in small isolated communities: the so-called `peripheral isolates'. In these marginal groups, potential exists for genetic change which will not be diluted by contact with the main population. Now the allopatric speciation concept is sound and worthy of investigation. I have no doubts that it is relevant to speciation. However, the experience of the Ngorongoro lions makes me ask: can allopatric speciation achieve the transformations required by evolutionary theory?
Today, peripheral isolates are considered to be populations under threat. No one is excited by the thought that these populations might be evolving rapidly to something novel! The hard facts of defective sperm, reduced reproductive rates and weakened immune systems shout `Danger!' rather than `Potential'. It seems to me that the allopatric speciation model only works when the genetic diversity of populations is high: when this diversity is innate and not dependent on the incidence of chance mutations.
If this is so, allopatric speciation can be seen as one of the mechanisms for speciation after God created the Genesis Kinds. When they were created, they possessed an enormous potential for adaptation - offering potential for different species to occupy innumerable ecological niches. Such speciation was rapid after the Great Flood, as the Earth was recolonised and as there were plenty of opportunities for peripheral isolates. The result of any speciation event is that the genetic diversity of the species is reduced. As environments change, these organisms find it hard to adapt. So extinction, rather than speciation, looms large. This is the problem faced by the lions of Ngorongoro.
David J. Tyler (1992)
