Returning for another guest post, James Glover is once again drawn to a beer coaster for some quick, if somewhat morbid, calculations. For those taking to the road over the Christmas period, this post should also serve as a reminder to drive carefully!
Why did the chicken cross the road? To get to The Other Side. This twist on the ending to the iconic joke was based on the observation of a single dead chicken on the road while I was returning by car from Sydney to Melbourne at the end of my recent touring holiday. The holiday in fact started two weeks earlier when I was driving around Tasmania. While there were no chickens, there was a dead sperm whale on the road.
In Tasmania I noted that in addition to its abundance of quality food, beer and wines, it has a remarkable supply of one other thing compared to the mainland: native fauna road kill. If you think that the occasional dead kangaroo (or more likely fox) you see driving in the country is plenty, then you haven’t been to Tassie. We are talking a dead possum or wallaby every kilometre or so, which means if you are driving at 100km/hr means you see one every 36 seconds. While it is unfortunate, particularly for the animals involved, to see so many native animals dead (but no foxes because they have been eradicated from the island), it is actually cause for joy because it indicates a very healthy population of native wildlife.
This got me thinking. Could you actually use the number of road kill to estimate the density of animals living in the bush? The answer is yes, and without any derivation or proof I present it here:
density = road kill ratio/kill zone area
road kill ratio = av. distance between cars/ av. distance between road kill kill zone area = 2 x car velocity x time x car width
It’s a surprisingly simple formula, and can also be handy for keeping children occupied on long car trips (at least in Tasmania). The model it is derived from is admittedly fairly simplistic – let’s just call it the “Frogger model of vehicle/animal interaction”. Here the time is important because clearly if we had an infinite amount of time and no method of disposal of road kill then the number would build up without limit. In practice the attendant carrion birds on each road kill and its, shall we say, “freshness” (blood, guts, brains you get the picture) suggests that they were all products of the previous nocturnal period’s collisions. In fact there are road signs indicating to drivers to be particularly careful between dusk and dawn to avoid animals so I take “time” to be 10 hours and assumed all carrion are fresh. Taking the average distance between road kill to be 1km and cars to be 10km gives a road kill ratio of 10. The average car speed was 100 km/hr and my car is about 2.5m wide so putting this all together gives an estimated bush density of 20 animals per square km. That’s about one per 5 hectares. That seems a little on the low side for a densely populated area but as “beer coaster” estimates go, it’s probably not a bad start.
This reasoning got me thinking that there probably true that there are no Tasmanian tigers left, because one would have shown up as road kill by now. Tasmanian rangers patrol the roads every morning looking for Tassie devils to monitor the spread of that awful face tumour disease they get. In fact, I saw two Tassie devils on the road and they were both alive and moving. I also saw three live echidnas: three more than I have seen in my life up to now. As mentioned I also saw a dead sperm whale on the road. Not some replica or whale skeleton either: it had died the day before. It was on the beach near Strahan which I was driving on at the time. So, yes, it was on my road and hence I feel justified classifying it as road kill. And no, I don’t know why the whale crossed the road. Perhaps it mis-heard someone say that in Tasmania there was an abundance of “road krill”.