By Rachael A. Bay
For tigers, visiting your neighbor is just not as easy as it used to be. Centuries ago, tigers roamed freely across the landscape from India to Indonesia and even as far north as Russia. Today, tigers inhabit is just 7% of that historical range. And that 7% is distributed in tiny patches across thousands of kilometers.
Habitat destruction and poaching has caused serious declines in tiger populations – only about 3,000 tigers remain from a historical estimate of 100,000 just a century ago. Several organizations are concerned with conserving this endangered species. Currently, however, all conservation plans focus increasing the number of tigers. Our study shows that, if we are managing for a future with healthy tiger populations, we need to look beyond the numbers. We need to consider genetic diversity.
Genetic diversity is the raw material for evolution. Populations with low genetic diversity can have lower health and reproduction due to ‘inbreeding effects’. This was the case with the Florida panther – in the early 1990s there were just 30 Florida panthers left. Populations with low genetic diversity are also vulnerable when faced with challenges such as disease or changes in the environment.
Luckily for tigers, they don’t have low genetic diversity. They have very high genetic diversity, in fact. But the problem for tigers is that losing genetic diversity happens more quickly in many small, disconnected populations than in one large population. The question is: how can we keep that genetic diversity so that tigers never suffer the consequences of inbreeding effects.
We used computer simulations to predict how many tigers we would need in the future to keep the genetic diversity they already have. Our study shows that without connecting the small populations, the number of tigers necessary to maintain that variation is biologically impossible. However, if we can connect some of these populations – between tiger reserves or even between subspecies – the number of tigers needed to harbor all the genetic variation becomes much smaller and more feasible. Case studies have shown that introducing genetic material from distantly related populations can hugely benefit the health of a population in decline. In the case of the Florida panther, when individuals from another subspecies were introduced into the breeding population, the numbers began to rise.
We do need to increase the number of tigers in the wild. If we can’t stop poaching and habitat destruction, we will lose all wild tigers before we have a chance to worry about genetic diversity. But in planning to conserve this majestic animal for future generations we should make sure those future populations can thrive – and that means trying to keep genetic variation.
Rachael A. Bay is a PhD candidate at Stanford University, and co-author of the paper ‘A call for tiger management using “reserves” of genetic diversity’, which appears in the Journal of Heredity.
Journal of Heredity covers organismal genetics: conservation genetics of endangered species, population structure and phylogeography, molecular evolution and speciation, molecular genetics of disease resistance in plants and animals, genetic biodiversity and relevant computer programs.
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Image credit: Tigers at San Francisco Zoo. Photo by Rachael A. Bay. Do not reproduce without permission.