Natural History

One of the most interesting biological stories takes place on the island of Newfoundland.  Before settlement, only Arctic hares could be found on Newfoundland. Its predators included the now extinct Newfoundland wolves and a very small population of Canada lynx.

Its population was small, mainly because Arctic hares use open habitats and they are always somewhat vulnerable to predation.

The small population of lynx that lived in Newfoundland were always at a bit of disadvantage. They are mostly adapted to eating snowshoe hares, which are creatures of  the dense forest. However, before the 1860’s, there were no snowshoe hares on Newfoundland.

The Canada lynx that lived on the island had to live like bobcats– eating what prey species availed themselves. Bobcats and Eurasian lynx are better at hunting deer species than the Canada lynx, but the Canada lynx on Newfoundland occasionally hunted caribou, especially the young of the year.

But because there were no easily captured snowshoe hares for the Canada lynx to eat, their numbers remained quite small. The Canada lynx doesn’t do well as a bobcat.

In the 1860’s, the government of Newfoundland discovered it had a problem. Lots of people were going hungry. The forests and sea were not producing enough to feed them.

To rectify this problem, the Newfoundland government introduced the snowshoe hare, which is staple in the diet of many rural residents of the mainland. The hares fed the people, and they adapted well to Newfoundland’s environment.

And they spread. In the early 1900’s, there were tons of them on the island. They soon reached what ecologists call the “carrying capacity” and then many of them starved.

Then something else happened.

Arctic hares began to disappear, and the caribou numbers began to drop.

What caused the numbers of those species to drop?

Well, it has something to do with the Canada lynx.

What?

Well, as I said before, the Canada lynx is a snowshoe hare specialist. On the mainland, its population is directly linked to snowshoe hare populations. It lives almost exclusively on them, and it is very well adapted to hunting them.

When the  population of snowshoe hares began to take off in Newfoundland, the native Canada lynx population could stop living like bobcats. They could return to their ancestral habits of hunting the snowshoes, the species they evolved to eat.

Things were fine until the snowshoe hares reached their carrying capacity and their population dropped off.

Then, the larger population of Canada lynx that had developed from eating those large number of snowshoe hares had to find something else to eat.

They slaughtered the Arctic hares, even though Arctic hares are much harder for the Canada lynx to hunt. With so many Canada lynx in Newfoundland looking for food, the poor Arctic hares had no respite from the predation. The predation was so intense that Arctic hares can be found only in remote areas the northern part of the island, where one cannot find Canada lynx or snowshoe hares.

On the mainland, Canada lynx, snowshoe hares, and Arctic hares are not found in the same spots. Arctic hares are always found to the north of prime Canada lynx and snowshoe hare habitat. It is likely that Canada lynx are the main reason why Arctic hares have a rather clearly demarcated southern limit to their range. They simply cannot live where Canada lynx and snowshoe hares do, because the Canada lynx will eat the Arctic hares when the snowshoe hares have their population crash.

Yes, snowshoe hares have a ten year cycle in which the population hits its carry capacity within ten years and then has a massive die off. Then it rebuilds after that die off until it hits its carry capacity ten years later. The Canada lynx is at the mercy of these ten year cycles. And so, it seems, is the Arctic hare.

The introduction of the snowshoe hare in Newfoundland had been a major disaster for the Arctic hare, even though the two species do not necessarily conflict with each other. They don’t even live in the same habitats, with Arctic hares preferring the open tundra and snowshoes preferring the forest. It is the rather strong predator-prey relationship that exists between the snowshoe hares and the Canada lynx that ultimately affected the Arctic hare.

Now, that is only part of the story.

Why did the caribou drop off?

Well, it is a very similar story.

When the Canada lynx population exploded with the introduced snowshoe hares, they generally left the moose and caribou alone. Canada lynx will eat snowshoe hares before they’ll touch any species of deer.

When the snowshoe hare population collapsed, the caribou and moose population began to suffer almost as badly as the Arctic hares.

The caribou population collapsed through the 1950s until there were just a few hundred caribou on the island.

It turned out that many of these caribou were dying as calves from a bacterial infection. Large numbers of calves were found dead. They had strange puss-filled marks on their throats, which were cultured and found to have the Pasturella multocida bacteria in those puss-filled marks. It was this bacteria that was killing them.

The caribou of Newfoundland prefer to calve in low-lying swampy areas on the island. They try to keep their calves out of the elements so they do not succumb to illnesses or the elements.

So why were they getting this bacterial infection? And what about the strange marks on the caribou calves’ throats?

Well, remember the earlier story about the Canada lynx and the snowshoe hares in Newfoundland?

It turns out that the Canada lynx were not only preying on Arctic hares when the snowshoe population crashed. They were also preying caribou calves. However, as I said before, Canada lynx are pikers when it comes to hunting any species of deer.

They often made a mess of it.

As you are aware, cats often kill by a bite to the throat. Canada lynx kill biting the throats of their prey. However, when they tried to kill caribou calves, they really didn’t do too well. They really don’t have the teeth of a big cat to really suffocate a large prey species like a young caribou.  When they would have a young caribou on the ground biting its throat, the mother caribou would have time to run back and drive the lynx off its calf.

With that many lynx making failed attempts to kill young caribou, it didn’t take that long for lots of calves to get infected with nasty bacteria. And thus, they died.

Now, the discovery that Canada lynx were causing Arctic hare and caribou populations to drop was a major revelation in population ecology. The biologist who made this discovery was A.T. Bergerud.

Bergerud’s discoveries were a major afront to the accepted theory in wildlife management at the time.  Before Bergerud, the accepted theory was that of Paul Errington. Errington’s theory is the classical predator-prey relationship. Prey species produce many offspring, usually far more than the habitat can handle, but these prey species are kept in check because they are eaten by the predators. The ones the predators catch are called the “doomed surplus.” Predators play a vital role keeping these prey species at healthy numbers.  Because natural predators take the animals that are part of this doomed surplus, natural predators do not make prey species go extinct or make their populations drop precipitously.

Bergerud’s theory is quite different from that. It suggest that there are conditions in which predators actually can make a population drop really quickly.

I don’t think that it entirely negates the classical wildlife management theory on predator-prey relationships. However, there are exceptions to every rule, and the Canada lynx and snowshoe hare are pretty exceptional species. Not very many predators are so closely linked with a single prey species. It is also rather unusual to find a prey species with such clearly defined cycle to its population dynamics as the snowshoe hare.

And Newfoundland is a pretty strange place. It is an island that never had snowshoe hares on it. When prey species are introduced to an environment where they don’t have many predators, they will reproduce at an astounding rate. The doomed surplus doesn’t become doomed, and the population explodes until the ecosystem can handle no more. The small population of Canada lynx had been eking out an existence as a generalist predator until the snowshoe hares appeared like manna from heaven.

Yes, it is an unusual situation, but it proves that exceptions exist to every rule. And that’s why predators sometimes need to be managed to protect the prey species.