There have been a few attempts at measuring monitor densities. We tried with both savannah monitors and Nile monitors in Ghana. There are a lot of problems, not least because the really important number is the density of adult females, how many babies there are at any one time isn't usually important because the numbers fluctuate massively over different seasons. We were interested in baby savannah monitors because we wanted to know how what proportion were removed by trappers for the wildlife trade. We did it by digging up the babies in the normal way, marking them and releasing them. The results were published in Amphibia-Reptilia I think and although I can't remember the numbers offhand I think trappers did well if they found more than 40% of the babies during a visit. Overall the animals were very common in good habitat (which was farmland) and there was no problem with the numbers taken. It didn't work as well for Nile monitors because the babies and subadults were all clustered along riverbanks, which being linear features makes it difficult to assign a meaningful area. Also made it very hard to capture enough to make a mark-recapture experiment work.

But that was ten years ago and things are very different now. Nobody catches babies anymore, they go for the GRAVID FEMALES. Not only in Ghana but all over Africa and Asia. This allows people to sell animals as "captive hatched" or "ranched" or some other misnomer, but what it means is that instead of removing the animals that would probably almost all die within a year anyway, they are removing the key animals that drive populations. I have said it before, but even the most ignorant hunters in the world know that killing pregnant females is foolish. In the pet trade however it makes the most profit so that is how it is done. It has the potential to be massively damaging, particularly for animals that only occur on small islands. You might read or be told that the females are released back into the wild afterwards, but that is absolute nonsense. They all die. Every one of them. As far as I am aware there haven't been any investigations into how this shift in techniques alters populations, but common sense suggests that if you specifically target reproductively active females year after year populations are bound to decline (either in numbers or in terms of diversity) eventually. This is why I don't agree with Frank that the only problem is habitat destruction, although overall that is the main cause of worldwide biodiversity loss. Targetting gravid females of rare species on small islands is incredibly destructive and something that people don't really like to talk about. But they should do because, in my opinion, it is the reason why the pet trade has the potential to be a real threat to many island endemic species, including many species of Indonesian monitor lizards. Captive hatched/ranched is much less sustainable than old fashioned wild caught. It's something I am going to devote a lot of energy to (and I expect make a lot of enemies at the same time) as soon as I can shake this butaan albatross off my back.....
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Mampam Conservation

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