Under pressure: how many tigers is too many?

A 60's board game - Tiger IslandIt is useful at this juncture to return briefly to the debate surrounding tiger numbers. It is evident that the metric currently being used to assess the success of india’s tiger conservation is based on a continual growth model. It is also clear that this growth is being achieved across a declining base of territory, where habitat loss and fragmentation is creating ‘islands’ in which tiger communities are becoming trapped, unable to disperse into outlying areas of viable habitat.

As tigers are territorial animals, this growth strategy may ultimately create significant problems and, as numbers continue to increase, so territorial disputes are also expected to rise in parallel. This situation is likely to create a higher incidence of injured tigers with the inevitable outcome of enhanced human-tiger conflict at the periphery of over-stocked reserves.

Imagine a pressure cooker with a broken valve. The ingredients have been carefully prepared and placed on the heat. The lid fits snugly and is sealed to prevent loss of energy, which is instead regulated by the valve that enables the steam to escape. Should this valve malfunction then the build up of excess energy has nowhere to go and the ultimate result is likely to involve some form of explosion. To prevent this from happening you have three main options. Firstly you can take no action at all and merely react to the inevitable, placing all in proximity to the cooker in immediate danger. Secondly, and more prudently, you can take the cooker off the heat, sacrificing its contents in favour of safety. Finally, you can change the valve to enable the safe and regulated escape of the excess energy that is building. The final option is obviously the most judicious. Now let us return to the tiger reserves and consider these same options. For the hermetically sealed lid of the pressure cooker let us substitute the reserve boundary, its porosity compromised in order to release the energy more efficiently. As we have seen previously, it is easier to manage tigers in tightly controlled protected areas that are kept separate from people for a number of reasons, not least to reduce the potential for conflict. In this case, the excess energy represents surplus tigers, the valve the dispersal corridors enabling their escape. If these dispersal routes become blocked then our options are similar to those available to us in dealing with our faulty pressure cooker, with the only feasible option being to replace the valve, which in this case entails the re opening of key corridors. In India at present, this option is viewed as the most difficult and rather than invest in improving the sink regions, emphasis has been placed on increasing the coverage of source areas as opposed to connecting them. In other words, more damaged pressure cookers. In recent years, tiger reserves have increased from just eight at the inception of Project Tiger to 48 today, with more in the pipeline. The question, therefore, is how many tigers can India accommodate before reaching its saturation point?

This is a difficult question to answer and one that necessitates a closer consideration of stocking densities. On the surface, there are a few simple calculations that can be undertaken to determine appropriate capacities. First, the availability of sufficient prey is central to the ability of existing reserves to keep pace with growing tiger numbers. Second, the existence of overspill areas to provide space for dispersing tigers is equally crucial. Finally, although not exhaustively, appropriate mechanisms to mitigate human-tiger conflicts will determine the extent to which these available spaces are habitable to growing tiger numbers. This may at first sight suggest that the issue of tiger numbers is relatively simple to reconcile, although in reality it clearly is not. Estimates of stocking densities range depending on the types of habitat comprising the national estate of existing tiger territory and a brief review of the figures acts to obscure rather than reveal any commonality.

Before considering this however, it Is first useful to ascertain what makes a viable and sustainable meta-population of tigers. Current estimates suggest that the figure consists of at least 75-100 tigers, including a minimum of 25 breeding females. This number is believed to be sufficient to offset annual population losses of up to 20 per cent, due to natural mortality and permanent emigration. If this minimum figure is accurate, it is then important to estimate the physical space required to accommodate such a population. Let us return then to our task of calculating the carrying capacity of existing tiger landscapes. As previously intimated, perhaps the most important indicator surrounds the extent of available prey species within habitat blocks. Tigers need abundant sources of food to survive and an adult needs to catch and consume around 50 large-bodied ungulates each year merely to sustain itself. If we consider that only one in ten hunts result in a successful kill, then a ballpark figure of some 500 prey animals need to be present in any individual tiger’s territory. So, for a viable meta-population of 100 tigers, a prey base of at least 50,000 large-hoofed ungulates is a minimum requirement. The relative abundance of prey, therefore, represents a limiting factor to the growth of tiger numbers in delineated reserves. This means that there can be no simple equation to calculate the real carrying capacity across the current network of protected areas, characterised as they are by significant differences in habitat size, type and connectivity.

Barasingha in Kanha Tiger Reserve in Madhya Pradesh.
Barasingha in Kanha Tiger Reserve in Madhya Pradesh.

Take, for example, the frozen, inhospitable landscapes of the Russian far-east, the domain of the Amur tiger, the largest of the world’s subspecies. Here, the expansive territory size reflects the paucity and dispersed nature of the prey base, with a single tiger’s territory covering as much as 500 square kilometres, meaning that tiger numbers will always remain lower in this region than in the more prey dense environments of India where tigers can exist in clusters of 10-20 adults per 100 square kilometres. If we scale this up, it means that an area containing a single tiger’s territory in Russia could hold between 50-100 individuals in parts of India. Small wonder, therefore, that many experts cling to the view that the long-term prospects for the survival of wild tigers are inextricably linked to decisions taken in the forested landscapes of the Indian subcontinent. But, even in India, these densities remain the exception as opposed to the rule. Only a handful of existing reserves contain such viable populations at present and, across much of the country’s protected area estate, densities remain conspicuously lower than the minimum standards prescribed to ensure sustainable populations. Furthermore, while considerable areas of currently unoccupied territory exists across the subcontinent, this is rarely contiguous with the existing reserve system and therefore requires significant human intervention to populate these spaces with big cats. The Tiger Task force, introduced in the wake of the Sariska extinction event, suggested a natural ceiling on tiger numbers in India of up to 3,700 individuals, calculating this on a mid-range density of between 8-10 tigers per 100 square kilometres. This, however, was based upon the current protected area network and, in particular, the inviolate spaces comprising the core, taking no account of future inclusions into the system.

Cover page of the report of the Tiger Task Force
The report of the Tiger Task Force – joining the dots

In the absence of functional corridors, natural dispersal becomes severely compromised, leaving wildlife managers little option than to manually move tigers between areas of existing and new habitat. This method has been labelled ‘assisted colonisation’ and enables excess tigers to be siphoned away from overstocked reserves to populate patches of available habitat elsewhere. This emphasis upon prime habitat has an effect of removing poorer quality, often degraded, sink areas from the management equation, allowing development potential to be realised within and beyond the immediate periphery of the reserves. This provides decision makers with an opportunity to manipulate the extent and spread of tiger populations without them having to deal with the more complex issue of corridor restoration. Critics of this approach emphasise the dangers inherent in a growing lack of connectivity and the resulting loss of ‘wildness’ due to increasingly intensive management interventions. Moreover, they point to the threats posed by overstocking of reserves as a direct consequence of a strategy emphasising the continual growth of tiger populations. 2015 census summary. 2226 tigers in 47 tiger reservesHere, as we have seen, increasing human-tiger conflict and a growing gender imbalance has further amplified calls for enhanced management intervention, the result being an increasing focus of funding towards the core, source areas at the expense of the peripheral sink regions. In short, it is more convenient for managers to restrict the movement of tigers beyond reserve boundaries in favour of a growing concentration strategy. As we have seen, however, this is difficult when dealing with such free-ranging, territorial predators. Recent evidence, however, points to certain adaptive behaviours that may be developing in tiger communities as conditions change.

The Wildlife Institute of India (WWI) has recently launched a research study surrounding the meta-population dynamics of wild tigers and initial reports suggest some interesting departures from conventionally understood social traits attributed to them. There is a suggestion that, in areas where male tiger numbers are increasing, that their territorial behaviour may be shifting. The underlying research, focused on the Terai region of northern India which contains over one-fifth of India’s stock of wild tigers, aims to study connectivity, gene flow and social dynamics within resident tiger populations and their habitats. The lead investigator explained that the Terai region has already been well researched, enabling the current researchers to access existing data surrounding ecological processes. Another WWI researcher, Bivash Pandav, expressed surprise at some of the details emerging from the early stages of the study. He explained that our established understanding of tigers’ social traits focuses primarily on males occupying and defending distinct territories with two or three females overlapping their range. The new findings, however, suggest that established social dynamics may be changing. Researchers are raising the possibility that, as numbers increase, tigers may be becoming more tolerant of territorial incursions and, where these lead to clandestine breeding episodes, the resident male has been recorded to increasingly accept the resulting progeny as his own. Whether this is a result of ignorance or design is difficult to tell at this stage but the matter is undoubtedly worthy of further investigation to help ascertain the effects of higher stocking densities within increasingly isolated reserves.

While this post has concentrated heavily upon the role of prey availability in determine tiger stocking densities, it does need to be pointed out that a range of other factors impact upon the number of tigers extant within India’s network of tiger reserves. In a recent study, Rajesh Gopal, former director of Project Tiger and one of the architects of the National Tiger Conservation Authority (NTCA) identified the main determinants of tiger density as including not only prey abundance but also related to levels of human disturbance, remoteness of habitat and source population connectivity.

Tiger territories vary greatly depending on location
Tiger territories vary greatly depending on location

The pressure cooker analogy, although far from perfect, is useful to illustrate the perils associated with any containment policy in which dispersal is compromised and tigers are forced to live in greater numbers across a declining base of territory. The inability to release excess pressure naturally leads inevitably to increasing levels of management intervention, typified by the removal of ‘problem’ tigers and their forced relocation to less inhabited reserves elsewhere. Indeed, the Union minister commented on the apparent rise in tiger numbers as an opportunity for India not only to lead tiger recovery in India but also to contribute to efforts across the wider range, donating surplus individuals to other range countries to boost their tiger conservation measures. To some, this notion of assisted colonisation provides an opportunity to increase the potential for wild tiger survival, to others there is a real fear that we are playing God and compromising the wildness of tigers and that future generations of the big cat will exist only as tightly managed populations, commodities to be moved around at the whim of politicians and wildlife managers. It is perhaps a sad reality that as wildlife habitats need to exist in greater proximity to human habitation and, as dispersal is further compromised, so the seal on our pressure cooker becomes tighter and tighter to prevent ingress and conflict. There is a real risk, moving forward, that our valve may become redundant in favour of a manual removal of the lid to relieve excess pressure each time it materialises. This approach would contribute to reserves becoming mere repositories of closely managed boutique tiger populations and as breeding centres able to produce surplus stock to populate and repopulate under-utilised habitat elsewhere. The main fear is that a continued emphasis upon perpetual growth in tiger numbers may ultimately compromise efforts to bring sink areas under more effective management and produce instead a series of unconnected, strictly demarcated reserves within which tigers can be better insulated from external threats.