Can Diversity Beat Adversity for Tigers?
I did my undergraduate degree at Auburn University, which is both a fantastic research institution and (in my exceedingly biased opinion), the crown jewel of Southern college football. I have spent many autumn Saturdays crammed in Jordan-Hare Stadium with 87,000 other people (keep in mind that the entire population of the town is around 40,000), and never ceased to be amazed at such a huge aggregation of humans, all there just to watch a game. The conservation biologist in me always felt some melancholy in contemplating the numbers, however, because the fans for just one game outnumber the population of some entire species by orders of magnitude. This is, ironically the case with Auburn’s mascot species, the tiger. If you took every single wild tiger left on the planet and put them in Jordan-Hare, they would fill little more than a single one of the 45 sections in the bleachers.
The tiger (which includes 6 extant and 2 extinct subspecies) is one of the most enigmatic and majestic species gracing our planet, and they have long been a flagship species for conservation efforts. A new PLoS Genetics paper by Mondol et al. brings news that may be seen as both ominous and auspicious for tiger conservation efforts.
The bad news: tiger populations have essentially been devastated to the point of near-extinction. The authors found that current tiger populations amount to only 1.7% of the tigers found historically, and are restricted to an almost insignificant 7% of their original range.
The good news: widespread sampling efforts showed that the Indian tigers retain 76% of the genetic diversity found in tigers worldwide. This indicates that the tigers’ genes are not disappearing as fast as their population numbers. For programs dedicated to preserving genetic diversity of declining species, this is a cause for celebration and hope.
These insights into the genetic structure of Indian tigers also yield clues to the tiger’s history. Mondol et al.’s analysis of the diversity patterns indicated that about 200 years ago, tigers in this region underwent a significant population crash, most probably human-induced.
This is indeed fascinating. I am becoming skeptical and jaded in my old age, however, and I am increasingly concerned that the public will get the impression that we can claim conservation success merely by preserving genetic diversity. Much has been made of “minimum viable populations,” “maximum sustainable yield,” and the like, with too little regard for the integrity and function of food-webs, and the resulting impacts on not only predators and prey but the ecosystem as a whole. Humans had been doing their best to eradicate large carnivores long before our historical and scientific records began. We would not know how large Indian tiger populations were several centuries ago if analyses like the ones in the current PLoS paper did not allow us to create estimates from molecular evidence. This makes it extremely hard to set appropriate goals for conservation and management plans.
Large carnivores are often the first species to go extinct or decline under stressful ecological conditions (whether anthropogenic or otherwise), and after they are gone their communities often shift from top-down regulated trophic structure to an alternate stable state with bottom-up regulation (Beisner et al. 2003, Steneck et al. 2002). In a sense, the only “natural” state we have ever observed has been one of depleted predator populations. Therefore, conservation efforts that seek to restore populations to a “minimum viable” number or to densities that match historical records may still be setting the bar far too low for predators to fill their ecological roles in regulating mesopredators and herbivores, which in turn affects smaller non-prey animals and plants, which affects water and soil nutrient content and the physical structure of the habitat itself. We might be able to preserve all of the genetic diversity of a species in a lab, and may even re-establish self-sustaining populations in the wild, but that does not mean that we have restored them to the densities and distributions required for them to perform the ecological roles that they played in their communities prior to relatively recent population crashes. Habitat loss and degradation is one of the most critical factors threatening biodiversity today, and as Mills (2003) points out:
“Biodiversity is a broad concept incorporating compositional, structureal, and functional attributes of ecosystems at four levels of organization—namely, landscapes, communities, species, and genes” . . . “the greater the range of ecosystems that can be conserved to accomodate large carnivores, the greater will be the number of opportunities for these variable interactions to be played out and for adaptations to changing conditions to evolve.”
Even if we had a complete tiger genome on hand, it would not do much good if the animals are relegated to zoo cages or small ecotourism resorts. Even if a token number of animals are allowed to roam in the wild, the species would simply be lingering as a present and yet enfeebled shade of its former self, with its role in community interactions and regulation essentially paralyzed.
Don’t get me wrong, genetic diversity is still an crucial factor, and the results of this paper are both important and fascinating. This information gives us further clues as to the size and distribution of historic tiger populations, which can lead to further analyses of predator-prey relationships and ecosystem interactions. The news about the remaining genetic diversity is encouraging; inbreeding depression can potentially prevent species from ever recovering from extremely low population numbers, even if their habitat is restored.
I suppose I just worry that we will lose sight of the forest for the trees, (or maybe lose sight of the tigers for the stripes, if I may put a spin on the metaphor?), and run the risk of congratulating ourselves for meeting artificially low bars due to shifting baselines of predator densities. The important thing is to keep in mind that conservation efforts cannot be broken down into parts; species they must be treated as integrated wholes, “package deals” including genes, physiology, behavior, and role in community interactions.
Mondol, S., Karanth, K., & Ramakrishnan, U. (2009). Why the Indian Subcontinent Holds the Key to Global Tiger Recovery PLoS Genetics, 5 (8) DOI: 10.1371/journal.pgen.1000585
Beisner, B.E., D.T. Haydon, and K. Cuddington. 2003. Alternative stable states in ecology. Frontiers in Ecology and the Environment 1(7):376-382.
Mills, M. G. L. 2003. Large carnivores and biodiversity in African Savanna Ecosystems. In Large Carnivores and the Conservation of Biodiversity, J. Ray, K. Redford, R. Steneck, and J. Berger, eds. Island Press, Washington. pp 208-229.
Steneck, R., M. Graham, b. Bourque, D. Corbett, J. Erlandson, J. Estes, and M. Tegner. 2002. Kelp forest ecosystem: biodiversity, stability, resilience, and future. Environmental Conservation 29: 436-459.
(For information on the Auburn chapter of the Society for Conservation Biology’s Tigers for Tigers program here.)