Captive herp populations (part 1)

Many -- or perhaps most -- people who keep herps don't breed them.  If everyone who did keep herps actively bred them, we might be overrun by captive animals. Or would we?

Consider a species that reproduces regardless of almost anything the keeper does to prevent it: the mourning gecko (Lepidodactylus lugubris).  Mourning geckos lay two eggs at a time (usually), year round every month or two, and do so from about a year old. They're parthenogenic, so they lay eggs whether the keeper intends them to or not. The number of mourning geckos in a population should show exponential growth, unless something happens to remove geckos from the population. 

So, one gecko is one gecko for a year, and then after another year is 11 geckos (conservatively: a clutch every two months, of two eggs each except two of those clutches are singletons), then after another year is 121 geckos, then after another year is 1331 geckos, and so on.

Let's say two mourning geckos are kept captive (we could start with one, but since they're social that wouldn't be very nice, now would it?).  At the end of their lifespan of 10 years, we should expect there to be 4.7 million mourning geckos.  

Well, we might expect them not to breed up until death.  But at the end of five years (one year to become mature, and four years of offspring), there should be 29,000 geckos.  

Perhaps the initial numbers, in spite of looking fairly conservative, were too high.  Suppose each gecko produces half as many offspring as was assumed.  Even with those unrealistically low numbers, our initial two would be almost 2,600 geckos in five years.  Does the overall captive population of mourning geckos increase by 1300 times every five years? I think it does not.

Figuring out why there aren't nearly as many mourning geckos in captivity as expected might not be best done mathematically.  It might be done by considering whether we're keeping mourning geckos in ways that benefit them.  Do we tend to keep them at their optimal temperatures -- that is, to we allow them to bask to maintain a preferred 85F body temperature? Do they, in virtue of being basking thermoregulators, benefit from UVB?  Do they really benefit from being kept with dart frogs? (The answers here, of course, are: no, not usually; yes, possibly; no, definitely not, and neither do the frogs).  

Mourning geckos are just an easy example, since we know exactly how many of them reproduce.  But how many other captive herp species maintain low numbers in captivity for reasons other than their breeding rate?  That is, how much does neglect factor in to population numbers of captive herps?

Werner, Yehuda. Do gravid females of oviparous gekkonid lizards maintain elevated body temperatures? Hemidactylus frenatus and Lepidodactylus lugubris on Oahu, January 1990, Amphibia-Reptilia 11(2):200-204.