I am a frequent listener to Pulse of the Planet and most often find the information provided to be useful and well presented. However, I must take issue with the logic and conclusions presented in your show of April 13th entitled “American Toads:Crossing the Road”. The naturalist quoted seemed to imply that because the toads tended to cross the road in only one or a few evenings, that put them at greater risk of being killed by automobile traffic than if that were not the case. Now, I am a physicist by profession and not a naturalist, but that assertion is almost certainly untrue. The likelihood of any individual toad being killed crossing the road is determined solely by the ratio of how long it takes to cross the road divided by the average time interval between vehicles. The presence or absence of other toads has no bearing on that ratio, unless they are present in such numbers that they increase the amount of time it takes to cross the road. If the average time it takes a toad to cross the road is the same, and the rate of vehicle traffic is the same, it doesn’t make one bit of difference whether they all cross in one evening or they are spread out equally over the 365 days of the year. This is simple mathematics. What is different, and it is only a difference in perception, is how many dead toads an observer sees in the road before they are removed by scavengers, not what fraction of the toads are actually killed.

This brings me to my second point. The naturalist describes that in a one mile section of road he saw nearly one hundred toads and “out of those hundred toads, sixty of them were killed by cars”. “You can imagine the mortality rate on these toads, and that wasn’t even a busy road.” The implication of the numbers presented is clearly that a large fraction of the toads were killed attempting to cross the road. While I can imagine any number I like, the information provided is not sufficient for either me or the naturalist to compute one. If he had seen only 61 toads, 60 of them would still have been dead - they weren’t going anywhere. If he had seen 1060 toads, the same 60 would still have been dead. The 60 dead toads are the sum of all those that had been killed from the time the migration started (and hadn’t yet been removed by scavengers) until he made his count. There happened to be about 40 other live toads still in the process of crossing. What is missing from the count is the number that had previously crossed successfully and which were now out of sight. Without knowing that number it is impossible to make any sensible estimate of mortality rates.

As I said at the outset, I do enjoy the show, but this particular episode was scientifically misleading at best and serves to give those who don’t care about environmental issues a valid argument in saying that information presented in the name of environmentalism is not scientifically valid.

James Amann Ph.D

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