They say that a cat has nine lives but does it also have nine subspecies? This is a debate that has reopened recently in response to a paper published in the journal ‘Science Advances’ in which the authors claimed that, in reality, there may be only two extant subspecies. What does it matter? Well, claim the authors of the latest study, the current emphasis on taxonomy may be harming the practical application of effective conservation strategies across the tiger’s geographical range. It may be restricting opportunities to reinvigorate genetically-depleted populations and to develop and employ integrated strategies in efforts to ensure the long-term persistence of the species in the wild. More money is spent on the protection of tigers than for any other species, reflecting the global popularity of the largest of the big cats. But how effectively is this money being invested? At present, clinging to what may be an outdated understanding of genetic heterogeneity fragments the employment of such capital in responding to modern day realities affecting tiger protection. Before investigating this issue further it is first essential to understand the how and why of such taxonomic classification.
The concept of the subspecies is a way in which to recognise variability within a species, both in terms of its geographical distribution and its morphology. Not all geographical differences are deemed sufficient, however, for such a taxonomic departure as these may be viewed as being clinal in nature, or to put it simply, gradual changes to certain characteristics exhibited by adjacent populations of the same species. So, while geographical distance may be an indicator of taxonomic differentiation, it is not necessarily sufficient. Why? Well, tigers are a mobile species. They move around, they shift to other territories to enrich the genetic make-up. At least they do if the dispersal routes are free from obstruction, which sadly today few are. For this reason, scientists have estimated the maximum dispersal distance of tigers as being up to 1,000 kilometres. Tigers are, therefore recognised as a highly adaptable species and their historical range encompasses a variety of different habitat types. From the frozen taiga landscapes of the Russian far-east to the steaming jungles of the Indonesian islands, tigers have colonised a surprising number of ecotypes.
As these areas differ so much in terms of their physical and climatic structure, so tigers have developed traits to ensure success in their various homelands. The northern tigers, having to endure the sub-zero, snowy conditions of the higher latitudes, have assumed larger bodies, carrying more fat to insulate them from the severe winters. Their coats tend to be longer, their colour (or pelage) a paler tint and their stripe patterns wider apart than their southern counterparts. In contrast, the smaller, slighter Sumatran tigers are brighter in colour with their stripes set closer together, similar to the now extinct Balinese and Javan varieties. In terms of the other extant subspecies, similarities are so close that differentiation remains a difficult task. Similarly, some variation in morphological features, such as cranial size, do occur, but once more it is difficult to attribute this to the emergence of a subspecies. Indeed, critics of the resulting taxonomic classifications argue that the underlying science may be flawed due to the use of small data sets and, in some cases these date back over a hundred years. Hence, the whole foundation of the subspecies approach is open to question, with the authors of the latest study arguing that, instead of recognising nine subspecies this should be reduced to just two; the northern (mainland Asia) and southern (Sumatran) groups.
As science and technology continue to develop at a rapid pace, so too do the tools available to researchers and, recently, genetic studies have provided conservation with fresh insights as to the state and structure of the world’s remaining tiger populations. What has emerged from the resulting literature has been a recognition that the tiger is perhaps the least diverse of the big cats and that, genetically, there is an urgent need to replenish gene pools within its beleaguered numbers. This, it can be argued, is in direct contradiction of the taxonomic restrictions imposed by a proliferation of subspecies, described as ‘taxonomic inflation’. Part of this limited genetic base can be attributed to historical events which have led to a series of demographic contractions over time, reducing the breeding stock available to propagate the species. This situation has left tigers susceptible to the effects of inbreeding depression and, today, only an estimated 1,000 females remain to repopulate a diminishing range.
This situation has undoubted ramifications for the future of tiger conservation. It is perhaps inconceivable that the plight of the tiger will improve dramatically over the coming decades. Indeed, it is far more likely that, as human populations and economies expand even further in the foreseeable future, so too will threats to this charismatic species. The good news (albeit contentious) radiating from the recent tiger population estimations in both India and Russia, needs to be read with caution, not least in terms of apparently expanding numbers offset by a diminished habitat range. It is hard to escape the fact that the tiger is a conservation-dependent species and one that will struggle to survive without human intervention. Moving forward, it is likely that this required intervention will become increasingly pervasive and driven by scientific innovation. This may involve a range of reintroduction and relocation initiatives, perhaps ultimately from captive bred specimens. In this case, blind adherence to the notion of taxonomic purity may restrict rather than facilitate efforts to rejuvenate diversity in tiger populations, an unintended consequence of which may be its ultimate extinction in the wild.
To illustrate this concern, a recent report in the Washington Post has likened the situation to that affecting another big cat, the Florida panther.
During the early 1990s, it was becoming increasingly clear that the writing was on the wall for this species. Its numbers had declined alarmingly due to a combination of habitat degradation, development and human intolerance and, with less than 30 individuals remaining, its remaining population was insufficient to ensure its persistence in the wild. It had entered a genetic bottleneck that was difficult, if not impossible to reverse. It was accepted that, without radical remedial action, the Florida panther was likely to be relegated to the status of a ‘relic’ species, which in the words of Professor Stephen Meyer from MIT, are forced to occupy “the margins in ever-decreasing numbers and spatial distribution” before descending further to become ‘ghost’ species whose extinction is inevitable. Heavily blighted by inbreeding depression and its accompanying genetic defects, time was clearly running out and, in a last-ditch effort to save the Florida panther, the difficult decision was taken to introduce eight female Texas cougars to rejuvenate the gene pool.
But how difficult was the decision in reality? The truth remained that the Florida panther and the Texas cougar were very closely related and the decision came down to one key question. Do we simply let the Florida panther disappear or do we instead relax the taxonomic idealism and make the more pragmatic choice to intervene?
Unsurprisingly, pragmatism prevailed and, today, the panther (albeit with a little help from its friends) has rebounded to somewhere between 100-180 individuals. Whilst their future remains far from assured they have at least enjoyed a stay of execution from the permanence of extinction. This story has evident parallels with that of the tiger. The question, across Asia, is startlingly similar. Do we accept some degree of genetic pollution or do we instead cling valiantly to a naive belief in genetic purity at the expense of the species as a whole, based on what increasingly appears to be a flimsy base of evidence? To answer this question more fully, it is useful at this juncture to return briefly to the turnaround in the tiger’s fortunes in India. As discussed in my previous post, despite the controversy surrounding the operation of the latest All-India tiger estimation, tiger numbers are generally regarded to be on an upward trajectory. This apparent growth, however, appears to be occurring primarily in a series of strictly demarcated parks reserved for tiger conservation, while outside their boundaries increasing levels of human activity are having a detrimental effect of blocking dispersal routes, restricting their movement and compromising their ability to colonise new territories. Opportunities for tigers from adjacent communities to meet and replenish the gene pool are therefore increasingly difficult to facilitate. As with earlier demographic disruptions, leading to the identification of taxonomic departures, this situation depletes isolated communities of their diversity, increasing the risk of future extinction episodes. This becomes ever more likely as poaching crises emerge and indeed, during an era of increasing climate change, where conditions are likely to exert a fresh set of pressures and challenges on the big cat.
Returning to the ‘Science Advances’ paper, lead author Andreas Wilting claims that previous efforts to curtail taxonomic inflation in the case of tigers have largely failed to gain traction due to a lack of evidence in support of its reversal. As we know, research can move slowly, one small advance at a time. Even where research finally emerges to suggest a rethink in policy, this process can also be pedestrian. Wilting, however, believes that the evidence now exists and that it translates into a strong message for the conservation community to act now to better organise tiger conservation globally. Other critics support this stance and question the trend towards identifying species in terms of their ancestry, an issue largely ignored in this particular post, rather than through their current, physical traits. After all, following such a long period of contraction and repopulation it has become impossible today to understand the linkages between lineage and behaviour, making any discussion largely academic to the needs of the modern tiger. Still others emphasise the influence of technology that has enabled scientists to study wildlife at a molecular level, identifying minute differences in their morphology before translating this information into new taxonomic understandings. Wilting continues by arguing that this process provides “so many species concepts” that you could make a legitimate case to “distinguish each population separately”. Legitimate, perhaps but helpful, definitely not.
Why, then, is it so important to tiger conservation to abandon such restrictive categorisations? Well, the argument proceeds, a strict taxonomic approach brings with it a proliferation of management units, areas in which only prescribed genetical material can be added to the existing gene bank. As we have seen earlier, the current dearth of diversity within mainland Asian tiger populations at least, negates such a need in practice and, with many isolated communities rapidly nearing a bottleneck, only two real options remain. One is to reconnect currently isolated populations through a series of corridors to facilitate greater dispersal between remote, insular communities. This is the focus of my next post and, as we will see, this is particularly difficult to achieve in areas containing large human populations and their associated development and infrastructural support. Secondly, and perhaps more realistic over the shorter term, is the artificial injection of genetic material to challenge current uniformity. This may be based on relocation of tigers between communities and, indeed, within his recent announcement of a 30 percent increase in tigers in India over the past four years, the minister in charge of environment and forests proudly stated that India may soon be in the position to donate surplus tigers to other range states to help repopulate and regenerate their own declining tiger communities. To enable this to happen in the future, a relaxation of taxonomic diktat is vital. Finally, the artificial introduction of diversity may include elements of artificial insemination from captive bred sources and even, in the event that reintroduction methods are developed that enable captive-bred tigers to survive in the wild, the prospect of the repopulation of ranges following extinction episodes may become an increasingly attractive and indeed feasible option.
It will be interesting here to follow the success, or otherwise, of the ‘Save China’s Tigers’ initiative, in which two zoo bred tiger cubs were initially transported to South Africa in October, 2003 to prepare them for ultimate release into the wild in China.
The brainchild of London-based Chinese businesswoman Li Quan, two tiger cubs were initially released into the veldt landscape and, following further introductions and subsequent births, the stock of tigers currently stands at eighteen individuals. Contained within a fenced reserve to prevent ingress into the wild, these tigers learn to hunt and the project is described as a ‘rewilding’ programme. The ultimate aim of the exercise is to return the tigers to China following the necessary rehabilitation of appropriate habitat, enabling the repopulation of de-tigered environments. The project is of particular interest as it focuses on the South China tiger, one of the subspecies that is presumed to be functionally extinct in the wild and, indeed the stem species from which the other subspecies are thought to have radiated. It has proved to be a controversial programme to date but, as critics of the existing restrictive approach argue, the whole issue of tiger conservation remains contentious and disputed. But, as we have seen, in order to safeguard the tiger’s long-term persistence in the forests of Asia, tough decisions need to be made and these may be most effective if past regimes are investigated and adapted to reflect modern realities.
The genetic purity versus pollution debate must therefore be debated more fully, as the present focus on subspecies may be leading us down a path of no return – irreversible extinction. Perhaps it is time to discuss ways of more effectively replenishing the gene pools of endangered tiger populations and, whilst remaining vigilant that we don’t compromise diversity, allowing genetic refreshment from outside to boost the resilience of the species to persist in the future. The question is clear. How do we want to see our tigers in the future: in captivity or in the wild? While the answer for most of us veers unerringly towards the latter, how we achieve this is far less clear cut. What is evident, however, is that moving forward we may be forced to intervene more fully in our conservation endeavours and become a little less idealistic in the way we respond to the spectre of extinction. The clock is ticking.