The Ten-Year Puzzle: Hares, Lynx and a Pattern That Almost Repeats

The Ten-Year Puzzle

[1]For more than a century, fur-trading companies in northern Canada kept records of the pelts trappers brought in each year. The ledgers were meant for business, but ecologists later realized they were a treasure: a year-by-year count, stretching back into the 1800s, of how many Canada lynx were being caught. Plotted on a graph, the numbers did not wander randomly. They rose and fell in waves, with a peak roughly every ten years. The animal the lynx depends on, the snowshoe hare, showed the same kind of waves.

[2]The two species are tightly linked. A snowshoe hare is a rabbit-like animal of the northern forests that eats twigs, buds and bark through the long winter. Hares breed fast: a female can raise several litters in one summer. A Canada lynx is a medium-sized cat whose huge furry paws work like snowshoes. Hares make up most of its diet. When hares are everywhere, lynx eat well and raise more kittens. When hares are scarce, lynx kittens starve and adult lynx may not breed at all.

[3]Put those facts together and the waves make sense. Hares increase first. A year or two later the lynx population catches up, because it takes time for well-fed lynx to raise kittens and for those kittens to grow into hunters. Now there are many lynx and they eat hares faster than hares can replace themselves, so the hare population crashes. With little food, the lynx population crashes next, again a year or two behind. With few lynx left, the surviving hares begin to multiply, and the whole sequence starts over. Each peak in the predator follows a peak in its prey. Ecologists call that delay a lag.

[4]It would be tidy to call it a law, but the waves are not identical. Some peaks are tall and some are small. The spacing is 'about' ten years: sometimes eight, sometimes eleven. Lynx are not the only thing pushing hares down. At the top of a wave, hares have eaten so many twigs and buds that food runs short, and hares that live in constant fear of predators raise fewer young. Hard winters, disease and other predators such as great horned owls and coyotes all add their push. The cycle is a pattern produced by several causes acting together, not a clock.

[5]In the 1980s and 1990s, a team of ecologists in the Kluane region of the Yukon tested which causes mattered most. On some large forest plots they built fences that kept out lynx and other mammal predators. On other plots they put out extra food for hares all year. On a few plots they did both. Extra food alone roughly tripled the number of hares. Fencing out predators alone roughly doubled it. Doing both together raised hare numbers to about eleven times the level on untreated plots. Yet on every plot, even with fences and feeders, the hare population still cycled. Food and predation together drive the waves, and neither one alone explains them.

[6]This story matters far beyond the Yukon because the shape keeps turning up. In the meadows of northern Europe, voles and the owls that hunt them rise and fall in waves a few years apart, with the owls lagging behind. On an island in Lake Superior, wolves and moose have been tracked for decades, and the wolf numbers tend to follow the moose numbers, though disease, ticks and harsh winters bend the pattern. The species are different, the cycle lengths are different, but the lagged rise-and-fall is the same.

[7]That is why ecologists treat the predator-prey pattern as a tool. If they know a predator depends mostly on one prey, they can predict that the predator's numbers will trail the prey's, and they can warn a park manager that a prey crash will be followed by a predator crash. What they cannot do is promise the exact year or the exact size of the next peak, because weather, food and disease always have a vote. A pattern that repeats across ecosystems is powerful evidence that the same cause-and-effect links are at work everywhere. It is not a guarantee that any single ecosystem will behave on schedule.