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Increas­ing tigers’ genetic diver­sity — via inter­breed­ing and other meth­ods — and not just their pop­u­la­tion num­bers may be the best solu­tion to sav­ing this endan­gered species, accord­ing to Stan­ford research.

In other words, increas­ing genetic diver­sity among the 3,200 or so tigers (Pan­thera tigris) left on the planet is the key to their sur­vival as a species.

Iconic sym­bols of power and beauty, wild tigers may roam only in sto­ries some­day soon. Their his­tor­i­cal range has been reduced by more than 90 per­cent. But con­ser­va­tion plans that focus only on increas­ing num­bers and pre­serv­ing dis­tinct sub­species ignore genetic diver­sity, accord­ing to the study. In fact, under that approach, the tiger could van­ish entirely.

The research, pub­lished on 17 April in the Jour­nal of Hered­ity, shows that the more gene flow there is among tiger pop­u­la­tions, the more genetic diver­sity is main­tained and the higher the chances of species sur­vival become. In fact, it might be pos­si­ble to main­tain tiger pop­u­la­tions that pre­serve about 90 per­cent of genetic diversity.

Rachael Bay, a grad­u­ate stu­dent in biol­ogy at Stanford’s Hop­kins Marine Sta­tion and the lead author of the study, said, “Genetic diver­sity is the basis for adap­ta­tion,” which reflects Darwin’s and Wallace’s orig­i­nal idea of nat­ural selection.

Loss of diver­sity
The research focused on the Indian sub­con­ti­nent, home to about 65 per­cent of the world’s wild tigers. The sci­en­tists found that as pop­u­la­tions become more frag­mented and the pools of each tiger sub­species shrink, so does genetic diver­sity. This loss of diver­sity can lead to lower repro­duc­tion rates, faster spread of dis­ease and more car­diac defects, among other problems.

The researchers used a novel frame­work, based on a method pre­vi­ously employed to analyse ancient DNA sam­ples, to pre­dict what pop­u­la­tion size would be nec­es­sary to main­tain cur­rent genetic diver­sity of tigers into the future. The authors believe this new approach could help in man­ag­ing pop­u­la­tions of other threat­ened species.

The results showed that for tiger pop­u­la­tions to main­tain their cur­rent genetic diver­sity 150 years from now, the tiger pop­u­la­tion would have to expand to about 98,000 indi­vid­u­als if gene flow across species were delayed 25 years. By com­par­i­son, the pop­u­la­tion would need to grow to about 60,000 if gene flowwere achieved immediately.

Nei­ther of these num­bers is real­is­tic, con­sid­er­ing the lim­ited size of pro­tected tiger habi­tat and avail­abil­ity of prey, among other fac­tors, accord­ing to the researchers. But “num­bers don’t tell the entire story,” said Eliz­a­beth Hadly.

Lim­ited habi­tat
“Since genetic vari­abil­ity is the raw mate­r­ial for future evo­lu­tion, our results sug­gest that with­out inter­breed­ing sub­pop­u­la­tions of tigers, the genetic future for tigers is not viable,” said co-​author Uma Ramakr­ish­nan, a for­mer Stan­ford post­doc­toral scholar in biol­ogy and cur­rent researcher at the National Cen­tre for Bio­log­i­cal Sci­ences in Ban­ga­lore, India.

Because migra­tion and inter­breed­ing among sub­species appear to be “much more impor­tant” for main­tain­ing genetic diver­sity than increas­ing pop­u­la­tion num­bers, the researchers rec­om­mend focus­ing con­ser­va­tion efforts on cre­at­ing ways for tigers to travel longer dis­tances, such as wildlife cor­ri­dors, and poten­tially cross­breed­ing wild and cap­tive tiger subspecies.

Under­stand­ing these fac­tors can help decision-​makers bet­ter address how devel­op­ment affects pop­u­la­tions of tigers and other ani­mals, the study noted.

Con­ser­va­tion efforts for other top preda­tors have shown the impor­tance of con­sid­er­ing genetic diver­sity and con­nec­tiv­ity among pop­u­la­tions, accord­ing to the report. One exam­ple is Florida pan­thers: since indi­vid­u­als from a closely related pan­ther sub­species were intro­duced to the pop­u­la­tion, Florida pan­thers have seen a mod­est rise in num­bers and fewer cases of genetic dis­or­ders and poor fitness.

The out­come of the research could be vital infor­ma­tion to save the tiger from going extinct, and could help to make the Global Tiger Recov­ery Pro­gram a suc­cess, and dou­ble wild tigers by the next ‘Year of the Tiger’ in 2022:

(Source: Stan­ford Uni­ver­sity news release, 15.04.2014; Global Tiger Ini­tia­tive; World Wildlife Fund YouTube channel)