The Predators of Nagarhole.
Nagarhole has a large number of predators and the three major predators of the park are the tiger, wild dog and leopard. Based on scientific studies carried out in the park by Dr.K.Ullas Karanth it is now known that all three co-exist in high densities. Let us take a look at the reasons for this.
To fully understand the relationships between predators we first need to look at the herbivore density and the availability of prey species in any environment. Without prey it is impossible to have any predators and hence both the number and type of herbivores present play an important role. The type of prey in an environment is a very crucial factor, as predators cannot survive without adequate food. For example a forest inhabited only by hares cannot expect to hold tigers as hares make too small a meal for tigers regardless of the number of hares in that forest.
Nagarhole has a large number and variety of large herbivores (defined as weighing more than five kilos) which include seven species of ungulates and two species of primates. Together they occur at a mean estimated density of 108 animals per square kilometer. If the average body weight of these animals are taken it works out to a biomass density of 14746 kgs. per sq. km. This is comparable to the Mara in Africa, which has a biomass density of 19200 kgs.
Among the large herbivores in Nagarhole the following animals constitute over 90% of the predator's diet and can be thus called the principal prey species. These are the Chital, Sambar, Gaur, Wild pig, Muntjak and the langur. Studies show that they occur at a density of 91 animals per sq. km. And compromised of the following % of the total biomass killed by each predator.
Tiger -97.6%
Leopard - 89.2%
Wild dog - 96.8 %
It is evident for the above data that there is significant dietary overlap among the predators i.e. they more or less hunt the same species. Therefore the question that arises is how do they co-exist in high densities when they compete for the same prey?
The answer is quite simple. If we further sub-divide the principle prey by body weight into large, medium and small, this along with some measure of species selectivity facilitates co-existence.
Small prey are animals weighing between 5 to 30 kgs and comprise of langur, young chital and young pig.
Medium prey between 31 to 175 kgs, yearling and adult chital, adult pig, yearling and young sambar and young gaur.
Large prey over175 kgs, adult sambar and adult gaur.
It has been found that the predators killed the following proportion of small, medium and large prey
Tiger - 39% small, 44% medium, 17% large.
Leopard - 48% small, 51% medium, 1% large.
Wild dog - 43% small, 56% medium, 1% large.
This can be understood better when we examine the average weight of prey killed by all three predators respectively. Tiger 92 kgs, leopard 38 kgs and wild dog 43 kgs. The niche occupied by each predator is now clear.
The study also showed that there was species selectivity among the predators with gaur being preferred by tigers and wild pig being avoided by leopards and langurs being underrepresented among the wild dogs. Further tiger perdition also showed a bias towards adult male chital, sambar and wild pig. Wild dogs were found to prey selectively on adult male chital while leopards avoided them.
While all predators routinely killed prey weighing less than their body weight they did kill animals weighing more with the maximum prey body weight ratios being 1:5 for tigers, 1:3 for leopards and 1: 2 for a wild dog pack.
The study shows that as there is choice in Nagarhole , because of the availability of prey in the appropriate size class, the larger predators selectively kill larger prey which in turn facilitates co-existence.
Behavioral and hunting techniques and patterns also play an important role in the relationship between species and we now examine the role played by this in the relationship between the predators of Nagarhole.The study showed that wild dogs hunted almost always during the day probably because of their hunting method. The dog course or run down their prey and therefore find it easier to hunt during the day when visibility is good. Also they prey mainly on chital which are more active during the day. Tigers and leopards were found to be more nocturnal hunters thus there was some temporal ( time ) separation from the dogs. Leopards were also found to be more active during the day in comparison to tigers. This can be explained by the fact that leopards kill a relatively larger proportion of prey that are active during the day like the langur and chital. Even at night leopards were found to be more active than tigers possibly because they have spend more time searching for prey because of the higher proportion of smaller nocturnal animals in their diet. However their activity pattern does not show any active avoidance of tigers by hunting at different times of the day.
Studies have also showed that there is no separation on the basis of hunting areas selected by the three. All three attacked their prey close to habitat features where prey concentrated to feed or drink. However tigers killed their prey where cover was significantly denser in comparison to leopards, most gaur (potentially dangerous) were attacked in more open areas where visibility was higher. Leopards killed chital in more open areas than tiger as they have grater ability for concealment in sparse cover. In fact 81% of all tiger kills were made in moderate to dense cover while 41% of all leopard kills were made in the open.
Both leopards and tiger were found to drag their kills into cover sometimes covering 400meters if the prey was easy to move. The dogs however did not drag their kills and any drag seemed incidental to the feeding process. Tigers dragged their kills into dense to moderate cover but left 21% of their kills in the open, these being all gaur too heavy to carry. The leopard on the other hand dragged 13 % of their kills up into trees and also left a significant 16 % in the open. Most wild dog kills (71 %) were left in the open.
Tigers killed their prey mostly using throat bites which lead to strangulation ( 70%) , neck breakage while twisting on impact (14%) , nape bites were used only on smaller prey. Similarly leopards employed the neck bite leading to strangulation 90% of the time with the nape bite being used again only on small prey. They did not employ the neck breaking method. Wild dogs on the other hand do not use the killing bite and their prey die due to shock and loss of blood.
Field observations showed that leopards flee tigers and wild dogs by climbing trees. Tigers also appropriated wild dog and leopard kills. Wild dogs were observed scavenging tiger and leopard kills when they were away. In one instance the leopard returned to the kill later. On one occasion a tiger and leopard fed on kills less than 300 meters apart without being aware of each other. On another occasion a pack of wild dogs were 50 meters away from a resting tiger without either being aware of each other's presence. This shows that both tigers and wild dogs are socially dominant over leopards but does not result in spatial separation, all three share the same space. However tiger and leopards kill and eat wild dogs occasionally. The impression is that the need to defend kills does not play a significant role because of the dense cover and high tree density in Nagarhole.
Thus it can be summarized that ecological factors such as availability of appropriate size prey, dense cover and high tree density and not behavioral factors such as habitat preference, choice of hunting sites or social dominance are of primary importance in Nagarhole when it comes to predator densities.
( 1. Prey Selection by tiger, leopard and dhole in tropical forests by K.Ullas Karanth & Melvin E Sunquist.
2.Behavioural corelates of predation by tiger , leopard and dhole in Nagarhole , India by K.Ullas Karanth & Melvin E Sunquist.
3.Population structure, density and biomass of large herbivores in the tropical forests of Nagarhole, India by K.Ullas Karanth & Melvin E Sunquist. )
