Disturbances and consequences

Like any normal wildlife tourist, I’m happy to see a new place, take some photos, talk about the trip with friends and family, and then forget about it. Writing a blog made a difference, because it made me look again at the photos and think about them. After seeing the beautiful but arid grassland of Tal Chhapar with its utter lack of predators, I began to wonder whether it is a self-sustaining ecology. If it were not for human care, wouldn’t the place soon be overrun by deer which eat up all the vegetation and reduce it to an arid desert? Have the old Maharajas and their unsustainable love of shooting tigers and cheetahs spelt the doom of their beloved hunting ground?

I wondered idly about this question, until, on the flight home, I read about a method that naturalists use to study the ecology of ancient terrains where mammals first began to radiate into new forms. This method of drawing cenograms turned out to be easy. The simplest of all would be to list, for example, all the herbivorous mammals in Tal Chappar, rank them by their size, and plot the average weight of each species against the rank. This method was apparently first used by Legendre in 1986.

I was pretty sure I’d seen all the herbivores in Tal Chhapar. The Nilgai (Boselaphus tragocamelus) was definitely the largest, followed by wild boars (Sus scrofa), black bucks (Antilope cervicapra), and Chinkara (Gazella bennettii), in that order. I’d also seen a few black naped hare (Lepus nigricollis). A checklist from the Zoological survey assured me that I’d only missed the three smallest: gerbil (Tatera Indica), palm squirrels (Funambulus pennantii), and desert Jirds (Meriones hurrianae). A quick search gave me their average weights. The only surprise was that the hares weighed more than 2 Kgs in mean. You can see the resulting cenogram above. There are no species with weights between a couple of hundred grams and a couple of kilos, allowing a clear division between small and large mammals. Apparently this is common in almost all ecologies where migration is possible, but the reason is not clear to me. The nearly horizontal line which connects the small mammals is said to be characteristic of hot climates (in cold climates this line would slope down more steeply). All very nice, but I couldn’t use this to say anything about whether the grassland ecology I saw was stable or not.

Fortunately, there is a method for helping you think about that. It was developed by Valverde in 1964. You start by drawing another cenogram from the mammalian carnivores. I’d seen a desert cat (Felis lybica, photo above), a white-footed fox (Vulpes vulpes pusilla), and a grey mongoose (Urva edwardsii). I’d also seen pelts of tigers (Panthera tigris) and Asian cheetahs (Acinonyx jubatus) hunted to extinction from these grounds in the Maharaja’s palace in Bikaner. The checklist told me that I’d missed seeing Golden jackals (Canis aureus) and jungle cats (Felis chaus). Newspaper reports also told me that I should add stray dogs to this list, because they are responsible for large numbers of attacks on deer in this place.

Valverde tells us to put the two cenograms on one figure, and draw lines which join each predators with its prey. You can see that one half of the figure (the part in yellow) is full of criss-crossing lines joining predators and prey. This strong food web is likely to keep this part of the ecology in balance, even if one of the species goes extinct. The remainder of the food web (the part in pink) seems to have been altogether more sparse. The two extinct cats used to feed larely on three herbivores: Nilgai, wild boars and black buck. With the extinction of the tiger and the cheetah, a niche in the ecology was vacant. Stray dogs have moved into it, replacing the cheetah as the sole predator on black buck. Although there is a lot of consternation about it, perhaps it is the reason this ecosystem has not collapsed yet. There is space yet for a larger predator, but nothing apart from humans has appeared here.

Two species of herbivores are currently unchecked by predators. I suppose a census would tell us that Nilgai numbers have increased over the last hundred years. But in this arid land, the lack of food would perhaps always have been a major limitation on this species. This scarcity is now the only thing keeping Nilgai numbers in check. The wild boars are more problematic I think. In many parts of India where tigers have disappeared, wild boar are in constant warfare with farmers. I recalled meeting farmers keeping vigil on their fields in Maharashtra, to prevent wild boars from raiding their fields (photo above). Perhaps dogs will eventually check their numbers by preying on the squeakers, but if they don’t then conflict is inevitable. Instead of decrying the role of feral dogs in this ecology, we should be happy that they have moved into this food web and begun to stabilize what humans had nearly destroyed.

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