IUCN Red List, which is probably the most widely used catalogue of conservation statuses, roughly a quarter of all mammal species are "threatened with extinction". That's a broader category than "endangered species", but, either way, it's a proportion that's growing. (As it is, of course for non-mammalian species; amphibians, for instance, seem to be doing particularly badly).
Partly that's because we're getting better at evaluating such things, and identifying species as being at risk when we previously knew little about them - or, in many cases, didn't even know they were separate species. But it's also a consequence of mankind's ever-expanding ecological footprint, as some species become threatened that weren't previously.
Fortunately, that's by no means a one-way street. A number of mammal species (among others) have recovered in recent times as a result of successful conservation efforts. Notable examples include snow leopards and, yes, giant pandas, both of which lost their formal status as endangered species in 2016. True, they are both still listed as "threatened with extinction", but it's a step in the right direction and one that's cautiously to be welcomed.
Although the Red List was established in 1964, it really only began categorising species in this way in the 1990s. Had it existed earlier, and been as comprehensive at is now (which is to say, very much so for mammals and birds, if much less for some organisms) there would likely be many more examples of this sort of recovery that we could see. One such case is the grey seal (Halichoerus grypus).
Grey seals live in the Baltic Sea and on both sides of the North Atlantic. They were first named as a distinct species in 1791, around the same time that local bounties for hunting seals were first awarded in the United States. Prior to this, grey seals had long been hunted anyway, and remains of the animals have been discovered at archaeological sites in Massachusetts dating back as far as 2000 BC, continuing on and off ever since.
Truly large scale hunting, however, did not begin until the late 19th century. This was initially for fur and oil, but, towards the end of the century, that became less valuable. Unfortunately, this didn't really help the seals, since instead they began to be seen as competitors to fishermen, and campaigns were launched to intentionally drive their numbers down. Official bounties for killing grey and harbour seals were first awarded state-wide in Massachusetts in 1888, and in Maine from 1895. Estimates suggest that, by the time the bounties were cancelled in 1962, over 72,000 seals had been killed, a number sufficient to have a significant effect on their overall population.
By the 1950s, the animals were all but non-existent along the New England coast, and were rare in Canada. In 1972, however, the United States passed the Marine Mammal Protection Act, banning the hunting and capture of all marine mammals in US waters. Which, of course, includes seals.
The Canadian population had already started to recover by this point, perhaps due to a decline in demand, combined with a lower population and more isolated breeding sites than in New England. One site in particular, a remote sandspit known as Sable Island, is reported to have experienced an average 12% growth in grey seal population per year over a 40 year period, becoming the largest such colony in the world by 2003. It seems to have slowed since, but that's still a remarkable growth rate for such a prolonged period.
This colony, along with some smaller ones in the Gulf of St Lawrence, seems to have been largely responsible for the recovery of the grey seal population in US waters since the MMPA came into force. Genetic analysis of grey seals from the region shows no distinction between those living in Muskeget Island, off the coast of Nantucket, and those at Sable Island, implying that the latter population has recently been the origin of the former. However, there is enough diversity between individual seals living at the same colonies to suggest that their more distant ancestors came from a larger, and more widespread population than we have today.
By the time grey seals were first evaluated for the Red List in 1996, they were no longer at any plausible risk of extinction, if, indeed, due to their populations in the eastern Atlantic, they ever had been as an entire species. But, nonetheless, their US populations were still recovering from their almost total elimination in the mid 20th century. It's an example of how, at least given a potential refuge to hide out, species can recover from severe blows, given effective conservation measures.
Photographic surveys along the coasts of Massachusetts and Maine conducted from aircraft between 1988 and 2019 have shown that grey seals seem to have been particularly effective at recolonising the area. At the beginning of the study, they had just recolonised Muskeget Island, where a total of five newborn pups were observed in the 1988 survey.
Six were seen the following year, and, in 1990, a similar number were seen at Monomoy, off Cape Cod. The next year, the number of pups at Muskeget had doubled, and, in 1993, 28 were seen. A similarly sized breeding colony was discovered off Maine in 1994, and since then, more have opened up, and some of the existing ones have continued to expand.
By 2019, there were at least eight breeding colonies off the coasts of the two states surveyed. Some are on very small islets where there is no real room for growth. But Muskeget, which had recorded five pups in 1988, last year alone supported 3,532 known births, out of a total of 6,253 across the eight colonies. In fact, some of the breeding sites have shown an average growth rate of over 11% per year in the last three decades. And this is ignoring the fact that the survey can't possibly have photographed and identified every pup that was born at the larger colonies.
11% is a significant proportion, because studies conducted on grey seal populations in the Baltic (which had similar issues to the US, with an estimated 98% population decline between 1900 and 1970) have calculated that an 11% annual rise is about the maximum that can possibly be achieved, given the reproductive rates of the animals. So the fact that it's the average at four out of the eight breeding sites, with the highest being a whopping 26%, tells us that many of the breeding seals must be coming from elsewhere.
In short, they have to be immigrants from Canada, setting up a long-term home in US waters.
The MMPA is unlikely to be the only reason for this rapid expansion, although it must surely be a large part of it. Another factor is that people in the developing world have tended to make less use of small islands in recent years than they did 50 to 100 years ago. One of the breeding colonies, for instance, was used as a bombing range by the US Navy until 1996. Not only did they stop, but the fact that they didn't remove all their unexploded ordinance after doing so has tended to discourage human visitors. Other islands have lighthouses, which can now be effectively automated, or have otherwise been abandoned.
There are reasons why grey seal populations might recover more quickly than those of other large animals. For example, they start breeding relatively young, at around five years, and, being long-lived, they continue to do so for many years thereafter. Conservation of depleted populations may also face other challenges; in this case, a population managed to shelter on a very remote Canadian island and the original coastal habitat on the US islands to the south had managed to survive relatively unaltered. That doesn't work if the habitat the animal needs has, say, been converted into farmland and no longer exists.
But the downward spiral of previously hunted species to extinction is evidently one that can be avoided, in the right circumstances. Sometimes they just need time and space, once the hunting is relaxed, and can provide their own lasting success stories.
[Photo by Alistair Rae, from Wikimedia Commons.]