Tigers on a forest corridor

By Dave Armstrong - 31 Jul 2013 8:24:4 GMT
Tigers on a forest corridor

When Bengal tigers go walkabout, they can penetrate the four separate populations’ territory along some forest "corridors." This paper supports the maintenance of these vital corridors for breeding; Tiger image Credit: Shutterstock

Gene flow in endangered species is crucial to their survival. While habitats are fragmented, corridors have often been left to allow some inlets and outlets for breeding. Panthera tigris is among the most fragmented of all, with many sub-species on the verge of extinction and the total wild population at risk.

Sandeep Sharma from the Smithsonian Conservation Biology Institute and his co-workers have used 4 tiger populations (Panthera tigris tigris) from central India in a study covered in a newly published Proceedings of the Royal Society B paper; with data on their genetics under the title, "Forest corridors maintain historical gene flow in a tiger metapopulation in the highlands of central India."

The central Indian population represents 17% of the total population, using 12% of the habitat. Gene flow was estimated for historical populations back to 10,000 years ago and showed a kind of recent equilibrium with gee flow successfully operating where forest corridors exist. This conclusive proof of corridor use and function is really helpful in order to explain why these valuable forest paths must be maintained. Many have been “degraded” and lost throughout recent history.

Models of the scenarios of habitat loss, fragmentation and population dynamics were reduced from 7 to 4 and after all that the most appropriate with highest probability was the simplest. The 4 populations diverged from a single ancestral group and did not migrate after divergence.

This sounds relatively unspectacular but gives great credibility to other fragmented situations in which other rare species seem to have placed themselves. The additional background to tiger history is also relevant. Three points stand out. 10,000 years ago, tigers seem to have entered the sub-continent’s forests and hills. 700 years ago, forest fragmentation divided the population into two.

River valleys especially were cleared for agriculture and then the Raj period resulted in the loss of more forest for railroads and ships. 200 years ago, Central India was cleared and tiger hunting became more popular. This left our 4 populations in those areas that had not become arable fields or were too inaccessible. They could mix but were distinctive groups because of the geographic separation. As well as other factors, human population had increase by 1000% after 1700. No genetic mechanism can keep up with this kind of anthropogenic interference in the habitat. Presumably there has been a delayed response because of the lengthy generation time in tigers. There has been a reduction in the level of gene flow, but the authors suspect this will be reduced even more.

The protected areas for tigers now need a well-defined set of corridors between the 4 populations. Some corridors had no resident tigers at the time of the study. Their effectiveness seems to be proved by the study, however. Now it’s important to show people, policy makers and conservationists how crucial they are.