2. TubeTrap Capture Weights

There has been quite a lot of interest over the last year in the “sensitivity” of BioEcoSS TubeTraps. Those of you who have been using TubeTraps will know how easy it is to adjust the trip weights. But just how sensitive are they – and will they capture very light animals such as pygmy shrews or harvest mice?

Over the last couple of years I’ve been carrying out field trials of TubeTraps versus “another well-known trap”, in a couple of sites in Norfolk and Wales. Unfortunately, I’ve not caught any pygmy shrews in TubeTraps, but I have caught eight harvest mice – four in each type of trap. In total, I have caught and weighed 88 animals, giving the weight distribution shown in Figure 1. As you can see, the median weight is around 19g, with exactly 50% in the 16g to 22g range. You can also clearly identify the four harvest mice – one was 3g and three were 4g in weight. So, TubeTraps can catch light weight animals, but this result is probably influenced by the relative scarcity of these small animals in the UK.

Well, back to my previous snippet – trapping in Nepal. Of the 228 animals captured in the first season, I managed to weigh 222 of them. As you can seen in Figure 2, the weight distribution was quite different from what you would expect in the UK. Notice that there is a scale-break at the top end – my heaviest animal was 125g. You will also see that I caught several animals between 40g and 70g in weight, which is quite a bit heavier than you would expect to catch in the UK – and I lost one that weighed considerably more than 75g!

But, the most interesting feature is the very large number of small animals. These were three species of shrews, which were the most abundant captures by far. Now, 103 animals (>46%) were less than 10g in weight and Figure 3 gives a further breakdown of these very light animals. You can see that I caught a couple of 3g animals and a large number at 4g or 5g.

I think this clearly shows that, when they are present, very small animals – even less than 5g, are easily trapped using BioEcoSS TubeTraps. Not only that, but very large animals are caught on occasion too!

Simon Poulton: 20th January 2014