IN RURAL INDIA, DOMESTIC DOGS ROAM WIDELY INTO WILD HABITAT, WITH OR WITHOUT HUMANS, AND INTERACT WITH WILDLIFE. IF BOTH PACK SIZE AND THE DISTANCE THEY TRAVEL FROM VILLAGES INCREASE, THESE DOGS CAN BECOME A RECIPE FOR MANY KINDS OF DISASTER.
Ecological edge-effects can be positive or negative. Edges between two different kinds of habitats create ecological conditions (called an ecotone) that, in some cases, can actually increase biodiversity at the edge. However, when edges are a result of human modifications of habitats, such as when forests are clear-felled for agricultural use or human-settlements, then the subsequent edge can have deleterious effects for the fragmented natural habitats. For example, the creation of roads in the Amazonian rainforest alters the micro-climate of the region, aids in the spread of invasive species and ultimately results in the loss of habitat for edge-intolerant species. It was thought that the negative effects of edges extended to within 10-15 metres inside natural habitats. However, recent evidence shows that some human commensal animals such as generalist predators that occur in high densities in human-altered landscapes can traverse several kilometres into natural habitats. Movement of these animals creates a large-scale edge-effect that can have severe consequences for species inside natural areas, especially when such natural habitats are in small fragments.
In India, most natural and protected habitats are fragmented and are either surrounded by human-settlements or even have settlements enclosed within them. As with other rural areas throughout India, these settlements also harbour high densities of domestic dogs. These dogs roam widely into wild habitats either with or without human accompaniment. The risk that these dogs pose to wildlife is primarily explained by two factors: the density of the dog populations and how far dogs roam from their homestead. Higher densities of dogs increase the probability of pack formation,which makes them more effective when preying on wildlife or confronting other carnivores. Large populations of dogs near villages or households are unlikely to negatively affect wildlife.
Therefore, how far dogs are allowed to range from human settlements is a critical consideration. Dogs that travel several kilometres into wildlife habitat are more likely to come into contact with wildlife and thus have a potentially deleterious effect. Combine large populations with a propensity to roam widely and you have a recipe for a lethal scenario for many species of endangered wildlife.
The outcome of an encounter between dogs and wildlife can also depend on the kind of wildlife they encounter. Dogs are not particularly good at hunting wild animals. Low densities of dogs will succeed in killing large prey such as deer or antelope, but when this happens it appears dramatic and grabs headlines. However, the real cause for concern is when dog densities are high and they are wide ranging. For critically endangered species, such as the great Indian bustard, where every egg and every chick represents a substantial contribution to an alarmingly dwindling population, the risk of predation from even a single dog is unacceptably high. Furthermore, sustained harassment from attempted predation can result in high levels of stress in prey species, which under chronic conditions, can cause lowered reproductive output.
Dogs are also particularly dangerous to other kinds of carnivores, irrespective of their size, so even large tigers are at risk. The nature of this interaction, though, is quite different. For species smaller than themselves, dogs dominate by interference competition: chasing, harassing and in many cases even killing the subordinate predator. Thus smaller carnivores tend to avoid areas frequented by dogs or even abandon food resources that may normally be available to them. For example, golden jackals don’t scavenge from carcasses when dogs are found dominating these scarce but rich food sources. Interactions with larger carnivores is more in the form of exploitative competition, where dogs, by virtue of greater population densities, use shared prey resources faster than the native carnivores. In either case, the higher the density of dogs and the wider their ranging behaviour, the stronger these effects are likely to be.
Few dogs in India are vaccinated against common disease-causing pathogens such as rabies, canine distemper virus and parvovirus. These pathogens can be deadly to a variety of wild carnivores, from foxes and wolves to tigers and leopards.
Several well-established cases of disease-spillover from dogs to carnivores have been documented worldwide. Among the most famous examples was an outbreak of canine distemper virus that killed over a thousand African lions in Serengeti National Park in 1994. Genetic studies confirmed that this virus had originated in the large domestic dog population that resided in the villages on the periphery of the park.
Because a minimum threshold population density is required for pathogens to remain active in unvaccinated dog populations, low densities of dogs are unlikely to have a large effect, irrespective of their ranging behaviour. However, once populations get large enough for pathogen reservoir status to be achieved, ranging behaviour becomes more important. An unvaccinated wide-ranging dog that is part of a high-density, infected population has a high chance of coming into contact with carnivores and leaving infective materials in the environment. This kind of contact can have the most deleterious and far-reaching effect on wildlife, extending beyond individuals to entire populations.
Imagine a scenario where an epidemic of canine distemper virus, similar to the Serengeti one, were to hit the lion population in Gir. Within a relatively short period of time, a large proportion of the only population of Asiatic lion in the world could succumb to this disease, bringing to nought decades of hard-fought conservation success. Thus, dogs constitute a large-scale edge-effect, extending human-induced disturbances deep into natural habitats and potentially reducing the efficacy of protected areas that are supposed to be inviolate from anthropogenic influences.
Abi Tamim Vanak is a Fellow at the Ashoka Trust for Research in Ecology and the Environment, India. firstname.lastname@example.org
Illustration: Prabha Mallya
Photographs: Vickey Chauhan
More images of dogs interacting with wildlife.
Photographs: Kalyan Varma