One of the ways in which a healthy population of any species – in the wild – can survive is by sharing a sufficiently large gene pool. Studies have shown that many of the current captive tiger populations in zoos around the world suffer from severe diseases, weakened immune systems, abnormalities, and birth defects.
The Ranthambore free ranging tiger populations - resident to an approximate area of 400-500 square kilometers in the district of Sawai Madhopur in Rajasthan - have, in the past, been well connected with other tiger-bearing areas of Kota, Karuli, Jaipur, Bharatpur and Sariska. As a result, a large gene pool was available to the tigers. This genetic diversity is crucial in order to avoid the well-known hazards of inbreeding. However, due to several factors leading to higher demand for agricultural land, coupled with expanding urban clusters, the forests around Ranthambore have been fragmented. The movement of the tigers -- both inter-state and intra-state has thus been drastically restricted for the last several years. A self-sustaining population of tigers is now confined to about 20 to 25 breeding females and about 18 to 20 males. This population has seemingly grown from five tigers as reported in 1971, when the area was small (150sqkm) and surrounded by several villages. There is a need to develop and interpret a variety of genetic data in order to answer specific questions regarding tiger populations in Ranthambore.
Population fluctuations and the breeding of tigers in Ranthambore have been recorded over the last 40 years. Almost seven generations of tigers have so far not shown any morphological perturbations. Their populations crashed to ten females in 1992 and then again to fifteen in 2005. Remarkably, the total population bounced-back to 45 and 44 in 2003 and 2009 respectively. Fluctuating cycles naturally occur in wild populations of several species. From each generation to the next, weak genes are filtered out either by predation or fights.
This holds true for a normal and constraint free population group, where genetic variability is high and a large free-ranging habitat is available. However, a range of questions now arise. How large does the habitat need to be? What determines the breeding? What about inbreeding? What is the scale/index to identify its effects and measure it before it has expressed itself in recessive genes that show morphological variations? Why do population crashes occur? Such questions cannot be answered easily. However molecular genetics when applied to wild animal population groups points to possibilities. Breeding among wild animal populations has always remained an enigma. While some species survive population bottle-necks, others do not.
Negative effects
Inbreeding depression, as the term suggests, is a decrease in the fitness of the population as a result of breeding related individuals. It is of major concern in the management and conservation of endangered species. But its magnitude and specific effects are highly variable because it depends on the genetic constitution of the species or populations. Recent natural experiment shows that greater inbreeding depression is evident in stressful environments. In small populations of randomly mating individuals – which are characteristic of many endangered species - all individuals may suffer from inbreeding depression because of the cumulative effects of genetic drift.
Out breeding in species, on the other hand, has been considered by classical conservationists as a threat that crosses the path of evolution and is said to cause genetic pollution The contention of restricting inter-crossing in the wild exclusively for the purpose of maintaining genetic purity at the cost of losing the species never had credibility in scientific theory and practice…
With the rapid growth of human populations, large landscapes have been scissored by highways and canals making it more and more difficult for the survival of many species. The gene pools of several species have been continuously shrinking and inbreeding among some species has become imminent. Existing inbreeding provides vital clues to understanding how we can allow mixing and when we should intervene to stop.
In India, the rapidly changing landscape and fragmentation of wild habitats in the last decade has put tremendous pressure on defining the conservation strategy for large carnivores. The dangers of inbreeding in a meta-population (groups of the same species that have some contact with each other) of tigers have now put pressure on conservationists to make choices and facilitate breeding with other meta populations. This is generally believed to purge the recessive gene and produce healthy progeny. However, some scientists state that different meta-populations, separated in time, develop adequate resistance that shield them from certain diseases. The tigers that live in the Sunderbans are believed to be tailor-made to survive their environment as opposed to the tigers of arid regions of Ranthambore. The divergence of climate between arid and semi-arid regions is deterministic of the sub-species of that region. Outwardly we may not see any difference between the tigers of Rajasthan and those of east India as they belong to the same sub-species, but certainly a distinct variation has been found to occur in at least one such study.
The genetic analysis of the recently exterminated population of Sariska (Western India) suggests a potential connectivity to the Ranthambore population, a scenario further corroborated by historical information on a forest connection among these two areas until 100 years ago. Divergent features of the meta-populations found among Indian tigers certainly point out that there is merit in preserving these gene pools. Preserving these pure lines of tigers with some currently unknown beneficial properties of their DNAs may be of benefit in the future.
Last resort
While the extent of intervention in dwindling populations is debatable, it is certain that we need to adopt a guarded regime to allow limited inter-crossing for genetic restoration. It must be restored to when populations of some endangered species have become so small that they have lost genetic variation and appear to have become fixed for deleterious genetic variants. To avoid extinction from this genetic deterioration, some populations may be allowed to benefit from the introduction of individuals from related populations or subspecies for genetic restoration, i.e., elimination of deleterious variants and recovery to normal levels of genetic variation. We would thus have to collect a database marking the genetic distribution maps of several important and threatened species of wild animals.
