I have seen off-hand comments in a few threads over the years about the decline of the Detroit Zoo, director Ron Kagan's role in it, and losses within the collection.
I have not been able to find relevant details about the situation, as non-zoochat sources tend to bring up the elephant situation (which I know enough about) and often otherwise follow the same themes as many zoo director profiles, crediting Kagan with moving from museum-style exhibits to naturalistic exhibits modeled on animal's natural habitats.
It is not uncommon for zoochatters to complain about losses of species diversity in facilities in the age of homogenization and it is not uncommon for even fairly recent zoo directors to be credited for pioneering naturalistic exhibits in the mainstream press, so almost none of this tells me much of interest about the truth of the matter.
For someone who isn't as familiar with Detroit in specific, could someone educate me on where Detroit Zoo stood before Ron Kagan and how it changed under his tenure to better understand the situation?
Change in focus is probably a more accurate descriptor than decline. It's quite clear that the former director's focus was on maintaining high levels of animal welfare. The biggest downsides from a visitor perspective are a significantly smaller collection and potentially limited viewing options. On the upside, infrastructure is well maintained and the zoo has been able to complete construction projects on a regular basis. The collection reduction is mainly attributable to merging smaller exhibits into bigger ones, along with discontinuing mixes. Nothing like the closures of big exhibit complexes like at Baltimore in the early 2000s. I'm certain that attendance increased during Kagan's tenure, and I'd guess a casual zoogoer would be impressed by the new penguin center and wouldn't notice the decline in species with the exception of the well-publicized elephant move.
As for specifics, the zoo grounds look much as they would have in the 1990s. The general layout is a suburban park style with broad paths, lots of lawns, and open, dry moated exhibits. Sort of like Brookfield with fewer buildings. In the interests of welfare some exhibits have been merged, others have been extended into adjacent open space. A few new or heavily modified exhibits have been constructed as well. Collection-wise, mammals are probably easiest to keep track of. Here's a probably incomplete but representative list of mammal losses vs gains over the last ~15 years.
Losses: Ruffed Lemur, Drill, Lion-tailed Macaque, Capybara, Przewalski's Horse, Brazilian Tapir, Guanaco, Collared Peccary, Hippopotamus, Fallow Deer, White-lipped Deer, Southern Pudu, Pronghorn, Takin, Lesser Kudu, Painted Dog, Bush Dog, Arctic Fox, American Black Bear, Syrian Brown Bear, Coati, Binturong, Meerkat, Harbor Seal, Harp Seal, Gray Seal. Note that a few of these, like Pronghorn and Takin, were in and out in fairly short order.
Gains: Allen's Swamp Monkey (replaced Macaque), Beaver (new exhibit), Straw-colored Fruit Bat (replaced Meerkat), Wildebeest (new mix with Zebra), Gray Wolf (replaced Wapiti and Przewalski Horse), Sea Otter (replaced pinnipeds).
A fair number of losses result from discontinuation of former mixes. The camel paddock formerly held up to ~4 hoofstock species, now is down to camels and maybe one individual
Cervus sp. The guanaco paddock formerly held peccary, pronghorn, tapir, and capybara at various times, is now depopulated in preparation for conversion to a children's zoo. Drill used to be mixed with gorillas. The sea otter exhibit formerly had a mix of the three pinnipeds.
In terms of exhibit changes, several examples exist of expansions of existing exhibits into unused space (lawns) - lion, tiger, red panda.
Many other examples are of exhibit mergers - former Binturong is now part of the Giant Anteater exhibit; Black and Syrian bear exhibits are now part of Grizzly exhibit, Hippo exhibit is now part of Aardvark exhibit, Arctic Fox exhibit is in process of becoming a Polar Bear viewing area. I think the former Pudu exhibit is now a garden.
There is a range of variation in how effective for viewing the exhibits are. Great Ape exhibits have awkward, limited viewing where you can only see a small portion of the exhibit from any one viewing area. As a result it is difficult to watch them outdoors for any length of time. Polar Bears are often sleeping in unviewable locations. Aardvark and Beaver are virtually impossible to see. In contrast, other exhibits like the Tigers, Red Pandas, and River Otters work really well. Overall, from a zoo visitor's perspective, it's not really the lack of species that is frustrating, it's that there's big spaces between exhibits and then in half of them the animals are invisible because they're behind a hill or asleep in their indoor holding.