In August 1944, a US Coast Guard barge unloaded twenty-nine yearling reindeer onto a remote island in the Bering Sea, roughly two hundred miles from the nearest inhabited place. The animals, twenty-four females and five males, were meant as an emergency meat supply for the small crew running a wartime long-range navigation station on St Matthew Island. The station was abandoned within a year of the war ending, and no one thought to remove the reindeer. There was nothing on the island large enough to eat them, since the only resident predator was a subspecies of Arctic fox too small to take anything bigger than a newborn calf. The reindeer had 137 square miles of unbrowsed lichen mat, twelve hundred years of accumulated forage, and no reason not to expand.
The nineteen years of exponential growth
By the time a field biologist named David Klein counted them in the summer of 1957, the original twenty-nine had become 1,350. By summer 1963 they were around six thousand, at an average density of about forty-seven animals per square mile, which is high for reindeer anywhere. The population had been increasing at a compound annual rate of roughly 32 per cent for nineteen years, meaning it was doubling roughly every two and a half years, which is close to the maximum biological rate the species can sustain when nothing at all is holding it back. What was happening in the lichen mat while this went on is also documented: the slow-growing forage that reindeer depend on through the winter was being consumed faster than it could regrow, particularly the crustose lichens that take decades to establish. The animals were, on any reasonable long-term projection, heading for a food crisis.
One brutal season rewrote the reindeer story. On Easter Island it took rats eating the palm seeds, Peruvian slave raiders and an epidemic, none of which make the version everyone remembers. We made a video about it, watch below:
The standard interpretation of what happened next
What happened next is that in the summer of 1966, a return survey found forty-two reindeer alive on the island, forty-one of them female and one of them a male too old to reproduce. Ninety-nine per cent of the herd had died over the intervening period, most of it in one winter. Klein published his interpretation in 1968 and it became one of the foundational case studies of population biology: a herbivore population had erupted, exceeded the carrying capacity of its habitat, and collapsed under its own weight when the food supply gave out. Garrett Hardin cited it as a paradigmatic example of overpopulation. It has appeared in ecology textbooks and environmental-science curricula for the past sixty years as proof of what happens when animals breed past what the land can support.
What the 2005 reanalysis of the data found
In 2005, three Canadian Wildlife Service biologists went back through the surviving records and published a paper in the journal Rangifer arguing that the standard interpretation didn’t fit the numbers. Miller, Barry and Calvert’s reanalysis pointed out that if the reindeer had been dying from density-dependent food limitation, the population should have shown the standard warning signs in the years leading up to the crash: declining calf-to-cow ratios, distorted age distributions skewed toward older animals, dropping body weights. None of that was there. The calf:cow ratio was about 60 calves per 100 cows in 1957 and still about 60 per 100 in 1963. The age distribution of the animals that died was 72 per cent aged one to three years, which is what a fast-growing population looks like, not a collapsing one. Mean body weights had declined between 1957 and 1963, but only to the levels typical of mainland reindeer. And the crash itself, they noted, wasn’t the multi-year decline you’d expect from progressive starvation. It was a single winter. Ninety-nine per cent of a large mammal population had disappeared in a matter of months.
What that winter was actually like
The reason the crash was compressed into one season is that the winter of 1963-64 in the Bering Sea was, by the meteorological record, extraordinary. A NASA Earth Observatory analysis of the reanalysed meteorological data describes it as one of the harshest winters ever recorded in the region. Wind chills reached minus 71.5 degrees Fahrenheit, which is around minus 57 Celsius. Storms brought hurricane-force gusts. Snowfall was at a record for the island, and the snow that fell was reworked by the wind into a dense compressed pack with a hard icy crust on top. Reindeer and caribou feed in winter by using their hooves to break through the surface of the snow and reach the lichens underneath, which is a marginal proposition even in a normal year. When the crust is too hard to break, they cannot get to the food that is physically present a few centimetres below them, and they starve rapidly. Something similar happened to a caribou herd on Prince Charles Island in the Canadian Arctic in the winter of 2015-16, and the passive-microwave satellite data showed the same signature of an unusually dense, wind-packed snowpack.
What the story is actually about
None of this makes the textbook interpretation wrong exactly. The St Matthew reindeer had overgrazed their range and were heading for trouble on any long-term projection, and Miller and colleagues aren’t disputing that. What they’re arguing is that the specific event that appears in the textbooks, the 99 per cent single-year collapse, needs the extraordinary winter to explain it. Without that winter, the population would probably have continued to grow for a season or two more and then declined gradually as the lichen mat ran down, in the pattern that food-limitation actually produces. What appears in the textbooks as a clean demonstration of density-dependent population regulation is really a demonstration of what happens when a population under moderate food stress meets a weather event that would have killed a lot of them regardless of density. It’s a slightly less satisfying story than the one that gets taught, and probably a more useful one for thinking about what climate variability does to populations that are already close to their limits.
