I am often asked what causes certain species to be abundant one year and scarce the next. “Why aren’t there any blue jays at my feeder this year?” “How come there were so many chipmunks and red squirrels last year and so few this year?” “I haven’t seen a rabbit around here in 20 years!”
I usually respond that, while making these observations is usually straightforward, identifying their underlying causes rarely is. Changes in animal abundance come in response to a variety of ecological factors including weather, food quality and availability, and disease and predators. The populations of some wild animals, such as snowshoe hare and meadow voles, fluctuate naturally from year to year. Some follow distinct cycles, while others, such as many songbird populations, fluctuate apparently randomly.
Often when an observer notes a decline in a common species, they attribute it to a predator: “Did the blue jays disappear because of the hawk I saw last week?” “Since I saw that coyote a few years ago, I haven’t seen any rabbits.” While making these connections between predator and prey is logical, population dynamics are rarely so simple. In fact, for more than 80 years, population ecologists have been preoccupied with the rise and fall of prey species, but most still cannot agree on the reasons for these cyclic variations in abundance.
In general, most ecologists agree that predator-prey relationships can be divided into “top-down” and “bottom-up” processes. In top-down systems, predators “control” the abundance of prey species directly through predation, which in turn may indirectly affect plant communities. Wolf predation on elk in Yellowstone is an example of this type of system. In the absence of wolves, elk populations increased, overgrazing much of the park.
In a bottom-up system, the abundance of prey species is controlled by available resources, such as habitat or food, and predators simply follow the fluctuations of their prey. An example of this would be how the biennial cycle of balsam fir cones affects the abundance of red squirrels, red-backed voles, and saw-whet owls in New England’s montane-fir forests.
Separating top-down from bottom-up systems is itself no easy feat. In the early 1800s, fur trappers noticed that snowshoe hare populations in the boreal forests of North America followed a roughly 10-year cycle. What makes these cycles all the more spectacular is that they are virtually synchronized over a vast area of boreal forest from Alaska to Newfoundland.
Over a period of 5-6 years, hare populations will steadily rise 30- or 40-fold, reaching a peak. Then, over the next 3-5 years, their numbers will steadily drop off before stabilizing and increasing again. The trappers also noticed that Canada lynx and ruffed grouse populations followed similar cycles. Putting two and two together, they assumed lynx were controlling the number of hares and grouse through predation – a top-down process.
But in the late1970s, scientists discovered that, as hare populations declined, they also experienced low birth rates, low juvenile survivorship, increased weight loss, and reduced growth rates, all of which could be experimentally induced not by predation but by simple food shortages. The scientists then discovered that, as hare populations increased, the plants they fed upon responded to heavy grazing by producing high levels of toxins, making them unpalatable to the hares. These chemicals protected the plants from hares for 2-3 years after heavy grazing, leading to a time-lag between the decline in hare numbers and the recovery of their food supply. Therefore, the hare-plant interaction generates the hare’s decline, and the time-lag generates the cycle – a bottom-up system.
So where do the lynx and grouse fit in? Lynx simply track the snowshoe hare cycle. As hare populations explode, lynx enjoy 4 or 5 years with an abundance of food, and their populations increase as well. With the bounty of hare, predation pressure on grouse drops off, and grouse populations rebound. But, as hares begin to starve and their numbers decline, lynx focus their attention on the now-abundant grouse, resulting in a drop of both predator and prey populations, completing the cycle.
Compared to wildlife, human populations just keep rising, thanks to technological advances in health care and agriculture. But imagine how human populations must have fluctuated centuries ago, when we lived closer to the land, when there were no vaccines, no Shaw’s, and no Price Chopper, and when the only available food was hunted, gathered or grown. Perhaps an understanding of predator/prey population dynamics was more intuitively obvious for them than it is for modern biologists.
Discussion *