Treating poisoned wolves with poison:
Isle Royale is an island in Lake Superior. (Christine Mlot Are Isle Royale’s Wolves Chasing Extinction? SCIENCE vol. 340 no. 6135 May 24, 2013 page 919) It is the home of wolves and moose. The wolves are the only predators of the moose and the moose the only prey of the wolves. The situation appears intrinsically unstable and the island animals have been studied thoroughly and regularly since 1959.
The time line is more or less that some moose swam over from Canada about a hundred years ago and then a pair of wolves crossed an ice bridge from Canada in 1949. The time course of the two populations has been recorded in the SCIENCE article. Of course given an extended population history I have looked at it eagerly to see what might be going on in terms of outbreeding depression. The experts see only inbreeding depression and to a substantial degree they are right.
The moose in 1959 numbered something like 500 and the wolves numbered about 20. Both populations rose steadily for about 15 years. At that point the moose, numbering almost 1500 went into decline. That many moose, assuming random mating, is in the range where their numbers should have been expected to decline. But the situation is not clear because at the same time the wolves were increasing rapidly; maybe they were just taking down too many moose. And since moose tend to limit their genetic diversity by having harems so that only a minority of males reproduce, it is not clear that the moose had reached unsupportable numbers. By 1980 the wolves, numbering some 50 or more, began to decline rapidly. The caption says that they had reached the highest density found in nature. 50 doesn’t seem like a terribly high number. But while the moose are limiting mating pool size by their harem strategy, wolves maximize mating pool size by avoiding mating with near kin. So the wolves may have been mating with fifth cousins; that is more distant than the third or fourth cousin maximum fertility shown by the Iceland study; so maybe it was time for their numbers to decline.
I am assuming that on the small island both the moose population and the wolf population functioned as one undivided mating pool.
Alas it is not clear by any means that the wolves suffered a population decline for genetic reasons at that time. Their numbers underwent the double peak that seems to be a hallmark of inbreeding depression in humans and mice but not, bafflingly, in voles; they follow a pattern like fruit flies. So it seems that we have four patterns of population history to deal with. In fruit flies there is an oscillating pattern fingered by computer simulation as being straight post-zygotic infertility. In mammals there is the double peaked pattern, they can look a whole like flies, and there is the single very high peak and collapse that is actually the most common for mice during mouse plagues. Maybe the different patterns reflect different maximum population limits caused by other mechanisms.
Alas we shall never know. The wolf population crash coincided with the arrival of canine parvovirus, a disease brought in by a sick dog, and that is assumed to be the sole cause of the crash. I’m not sure either way.
Given the low wolf numbers and the release from predation, the moose population really went up, far higher than it appears to have been before any wolves arrived at all. The population reached 2,500 and then really crashed. Well that was to be expected, right? Not clear again. There was a severe winter, so the experts are again spared having to deal with the idea of outbreeding depression. Meanwhile the wolf population had stabilized around 10 to 20. I think it fair to say that there was significant inbreeding depression at that level.
But, pardon me, what’s wrong with that? It was working.
Then came doom in the shape of a single male that wandered in from Canada over an ice bridge. Old Grey Guy (OGG), they call him. And now the facts are clear, but there are two interpretations (again, dash it.) The fact is that OGG sired a lot of pups. The wolf population went up. His Y chromosome drove the previous Y chromosome to extinction. OGG died. The whole pack went into what appears to be terminal decline.
The experts say, “He saved them for a while.” I say, “He killed them.” Sure he produced a lot of pups. He had not been subject to the same inbreeding depression that held on the island. But by adding all those pups from a single parent he actually reduced the already marginal genetic (and epigenetic) diversity of the island. Yes, inbreeding can wipe you out. But in this case it appears that the smoking gun was the intruder.
So they are trying to decide whether to bring in more outsiders to beef up the wolf population. The past year or two it seems that there have been no viable wolf pups. Their hand may be forced. If the wolves do recover, I imagine that they will be coming back from such low numbers that there will be a wolf population explosion and another collapse this time without benefit of virus. If they do bring in more outside wolves to replace OGG, then they are going to have to do the same thing periodically as long as they want wolves above the minimum level the island was seeking on its own.
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