With this thread having concluded 9 months ago and American Zootierliste debuting less than 2 months after it, I’ve had it in my mind that at some point I would revisit this project and re-evaluate it now that there is another full dataset to compare with. A more scrupulous scientist would probably let it cook a while longer, but the growth rate for new US ZTL additions has slowed considerably… and if I don’t do it soon while the analysis is still up-to-date I sort of wonder whether I’ll ever get around to redoing the analyses.
My main focus for now will be 1) comparing ZTL holdings to survey holdings for US zoos, and 2) comparing the data from both to holdings in Europe (given its the largest and most well-documented zoo region globally). At the moment I don’t have the appetite for a comprehensive analysis of every taxon, and frankly there’s a good chance I never will – so for now I plan on doing broad summary posts for each group, looking mainly at the ~25 most common species for each, and you can all enjoy and engage with the 5 or so posts I have. If anyone is curious about certain species or comparisons that *wouldn’t* take me several hours to investigate, feel free to throw them out there and I may do follow-ups.
Amphibians
I’ve compiled the holdings for what I now believe to be the 22 most common amphibian species in US captive collections… probably.
Something to keep in mind: the numbers on ZTL are constantly changing, so please don’t @ me if some frog is actually 77 now and not 75. I will double-check most listings before I post, but for some of the more complicated ones (ex. listings spread across many subspecies) the analysis might be a couple/few weeks outdated.
1. Dyeing Poison Dart Frog (Dendrobates tinctorius) – 100 holdings (68 Blue, 66 other – with a good amount of overlap)
2. Axolotl (Ambystoma mexicanum) – 97 holdings
3. Green-and-black Poison Dart Frog (Dendrobates auratus) – 94 holdings
4. Yellow-banded Poison Dart Frog (Dendrobates leucomelas) – 76 holdings
5. Amazon Milk Frog (Trachycephalus resinifictrix) – 47 holdings
6. Hellbender (Cryptobranchus alleganiensis) – 46 holdings
7. (tie) Panamanian Golden Frog (Atelopus zeteki/varius) – 41 holdings
7. (tie) Cane Toad (Rhinella marina) – 41 holdings
9. African Bullfrog (Pyxis adspersus) – 37 holdings
10. Anthony’s (Phantasmal) Poison Dart Frog (Epipedobates anthonyi) – 36 holdings
11. Golden Poison Dart Frog (Phyllobates terribilis) – 35 holdings
12. American Bullfrog (Lithobates catesbeianus) – 34 holdings
13. (tie) White’s Tree Frog (Ranoidea caerulea) – 32 holdings
13. (tie) Barred Tiger Salamander (Ambystoma mavortium) – 32 holdings
15. (tie) American Toad (Anaxyrus americanus) – 31 holdings
15. (tie) American Green Tree Frog (Dryophytes cinereus) – 31 holdings
15. (tie) Gray Tree Frog (Dryophytes versicolor) – 31 holdings
18. Red-eyed Tree Frog (Agalychnis callidryas) – 29 holdings
19. Colorado River Toad (Incilius alvarius) – 27 holdings
20. Golfodulcean Poison Dart Frog (Phyllobates vittatus) – 26 holdings
21. Eastern Newt (Notophthalmus viridescens) – 25 holdings
One species – the Eastern Tiger Salamander (Ambystoma tigrinum) is officially only at 20 holdings, but given that the vast majority of ~75 Ambystoma holdings that can’t be verified to species level are probably Eastern, I’d expect they're easily in the top 5. I didn’t do any further analysis on A. tigrinum though, for this reason.
A true enigma, nobody knows their true identity…
Tiger Salamander; taken by
@Semioptera at Downtown Aquarium Houston
US Survey Count vs US ZTL Holdings
Comparing my survey holdings to ZTL holdings, my count rate varied widely. My most accurate count was for Panamanian Golden Frog, for which I apparently nailed the count exactly (41 vs 41); this makes sense given its an AZA breeding program only held at well-known, well-documented facilities. My worst rate was for Axolotl, with my survey capturing just under 30% of ZTL holdings; that being said, the
Ambystoma holdings were largely entered systematically by ZTL staff rather than piecemeal by random users like most species. My next worst capture rate was for Eastern Newt at 44%.
My average capture rate across 21 common species was 67%, which is slightly better than I expected. That breaks down to 58% for 8 native species and 73% for 13 exotic species; I already knew my survey undercounted native-focused facilities and this data seems to validate that. Native collections often make up a sizable block of holders for native species: half to two-thirds of holders for American Bullfrogs, American Toads, Eastern Newts and native tree frogs are native collections. On the flip side, most holders of Hellbender, Colorado River Toad and Barred Tiger Salamander are traditional zoos and aquariums – so it’s not a universal truth that native species are found in nature centers, apparently.
Perhaps the sweating of an illegal substance makes it hard for minor-league nature centers to get in the desert toad game?
Sonoran Desert Toad; taken by
@evilmonkey239 at Toledo Zoo
In my amphibian survey recap, I found 5 species to be Abundant (40+ holders) and 6 species to be Very Common (30-39 holders). Based on current ZTL data, there are actually 9 Abundant and 9 to 10 Very Common species, so my survey did underestimate abundance by a sizable clip. Of the 22 common species I looked at, 13 (more than half) should have had their abundance category adjusted up by one (Common to Very Common, Very Common to Abundant, etc) while 9 had been estimated accurately – but 5 of those 9 had already been estimated as Abundant (the highest category possible) and I still numerically underestimated some of these so I think it's questionable to even count them as successful.
Overall, my feelings are mixed on whether the abundance was a useful or accurate metric – I wasn’t *dramatically* wrong for any species, but I wasn’t spot on for most of them either. Of course, I’m guessing my estimates were better for the vast majority of rarer species – so maybe it’s not that big a deal.
Counting can be hard, but it’s easier when you only have to count to one.
Lehmann's Poison Frog; taken by
@jayjds2 at Dallas World Aquarium
Something else interesting was looking at AZA accredited vs non-accredited facilities, since the two frequently diverge on types of species held. I didn’t do a species-by-species analysis for this stat, but I did look into it generally. Panamanian Golden Frog (an AZA breeding program) was 100% accredited holdings. Amazon Milk Frog was also highly concentrated in AZA facilities at ~95%, which might explain why my survey count was so accurate for that species too (only off by a couple holders). White’s Tree Frog was notably more common in non-accredited facilities, with roughly 2 in 3 holders being outside the AZA – and unsurprisingly my survey count undershot their holdings by at least 40%.
I also decided to test how much my survey captured holdings overall. I compared the overall holdings between the survey and ZTL for 8 amphibian families:
Phyllomedusidae (Leaf Tree Frogs) = 74/78 (95%)
Lungless Salamanders = 47/50 (94%)
Mantellas = 52/57 (91%)
Clawed Frogs = 27/32 (84%)
Poison Dart Frogs = 340/454 (75%)
Toads = 208/291 (72%)
Horned Frogs = 21/33 (64%)
Ranidae (True Frogs) = 50/94 (53%)
So a pretty wide range, with no clearly discernable pattern I can make out. My assumption would be that native groups would have lower effective capture rates on the whole than exotic groups, but with Plethodontidae being 94% and Ranidae being 53% that turned out to be too broad a generalization to make.
US ZTL Holdings vs Europe ZTL Holdings
Note: this comparison will not be a strong focus for these posts in general - partly because it's a lot of work to do in-depth, and partly because I get the feeling someone might attempt their own thread on this at some point.
For the comparison between Europe and the US, I analyzed native and exotic species separately. Unsurprisingly, of the 8 native species I looked at the US has on average 8.5x more captive holdings than Europe; the only native species I looked at that isn’t rare in European collections is the Colorado River Toad, which has about equal holdings in Europe as we do here. For exotic species, the US on average has 47% of Europe’s holding per species for the 11* species I looked at. This ranged from 69% for Green-and-black PDF to 25% for White’s Tree Frog. Main takeaway is, for common exotic amphibians Europe has twice as many holders as the US on average. Whether this holds true for rarer species, I can’t say for sure – but based on anecdotal perusing it seems likely that’s the case.
*I excluded Dyeing Poison Frog because I didn’t have the mental energy to compile Europe’s holdings across types. I also excluded Panamanian Golden Frog because it is absent from European collections.
The total European holdings for these species don’t reveal too much surprising. White’s Tree Frogs are notably more common there than here; whether this is reality or an artifact of America’s non-accredited facilities (where White’s are encountered more often) being less well-documented than Europe’s remains a question for the future, I guess.
You’d think having more amphibians than us would make Europe feel pretty secure about their holdings... so why the need for so many of the world’s most indestructible frogs?
Australian Green Tree Frog; taken by
@WhistlingKite24 at Alexandra Park Zoo (Australia)
And that’s it for the first analysis. Not sure when the next post (lizards) will go up, but I'm hoping to have it ready by at least next week if not earlier.