The squirrel looked lost as it scurried in the shade of the old oak trees, searching for sustenance in stumps, beneath fallen leaves, along a grassy bank of the Muddy River that had been like an open buffet in recent years.
But there was not a single acorn to be found.
A year after a bumper crop of acorns littered streets and parks throughout the region, these nuts seem to have vanished this autumn in New England, a little-understood phenomenon that scientists say will ripple throughout the ecosystem and devastate the population of everything from chipmunks to owls.
“The difference between this year and last year is huge,’’ said Mark Ashton, professor of forest ecology at the Yale School of Forestry & Environmental Studies.
In a typical fall, he said, a mature oak tree will produce about 250 pounds of acorns, which serve as seeds for saplings and a vital source of food for rodents, turkeys, deer, bears, and many other animals. At his research station in a forest along the border between Massachusetts and Connecticut, Ashton said the oaks this year have each produced less than half a pound of acorns.
“I’m not sure when we last had so few acorns in our region,’’ he said.
Ashton, one of the nation’s leading specialists on acorns, and other scientists said the cycles of acorn production are erratic. Some years there is a bounty, as there was over the past two years; other seasons there is a smattering, as was last seen in New England seven years ago, but rarely do oaks produce next to none.
One theory for why there could be such a drastic change in acorn production from one year to the next involves the genetics of the trees. They may have evolved to produce a massive number of acorns some years, but then cut off production in other years, to reduce the population of animals that feed on their seeds. Such trees would be more likely to produce saplings that grow into new trees.
Weather may also play a role, and scientists said it could have an increasing impact as the region’s climate becomes warmer and moister, as most climate change models predict.
Heavy rains when oaks blossom in the spring can hinder pollination and reduce the likelihood the trees will produce acorns.
John O’Keefe, a forest ecologist and retired coordinator of the Fisher Museum at Harvard Forest in Petersham, said it is possible the record rains and heavy flooding that hit much of the region in spring 2010 played a role in this fall’s absence of acorns. It takes about 18 months for the effects to appear in oaks.
“The weather back then was not conducive for pollen movement during the flowering period,’’ he said.
Another possible cause is that the oaks are essentially tired from the large crops they produced over the past two years.
“We don’t really know what causes the cycles, but in this case, it may be that they were just depleted after all the acorns produced,’’ said Matt Kelty, a professor of forestry at the University of Massachusetts Amherst.
Whatever the cause, the impact will be felt throughout the food chain. Even people may be affected.
The most likely victims are the rodents on the bottom rung: mice, chipmunks, squirrels.
Richard S. Ostfeld, a disease ecologist and senior scientist at the Cary Institute of Ecosystem Studies in Millbrook, N.Y., said up to 90 percent of the current population of rodents could die by next spring as a result of the lack of food. That would mean a massive death toll, as the number of rodents has spiked over the past two years, given all the acorns.
“I expect to see a severe crash in the number of mice and chipmunks, and we could be looking at a loss of squirrels of as much as 50 percent,’’ said Ostfeld, who has studied acorns for more than a decade.
He said predators of rodents - such as hawks, owls, weasels, and foxes - will also experience a significant loss, although they will find other sources of food and should fare better. Songbirds and ground-dwelling birds, such as veeries and woodthrushes, are likely to feel the brunt of the competition for food with hawks and owls.
He said deer, raccoons, and bears should also survive without too many losses, but they will be more likely to come into contact with people, as they search for other food sources.
“We can expect to see them raiding garbage cans and gardens more often,’’ he said.
Marion Larson, a biologist at the Massachusetts Division of Fisheries and Wildlife who is based in Westborough, said the surge in the number of chipmunks and squirrels over the past two years has made walks through the woods feel crowded.
Without acorns, she expects those that remain will be scavenging bulbs and other vegetation. “They’ll be looking harder for things to eat - and that means they’ll be eating from our gardens,’’ she said.
There are, however, some perks of an autumn without acorns.
Fewer windshields are being cracked, and fewer pates thwacked. Joggers are less likely to take spills on what in past seasons have been like marbles spread across running paths, and raking has been less of a chore.
Bob Hebeler, owner of Acorn Alpaca Ranch on Acorn Street in Millis, said he is looking forward to having to chase fewer squirrels from his barns.
But it has been eerie to work in his metal hay storage building, where he has become accustomed to hearing the pings of falling acorns, their hard leathery shells dropping from the surrounding oaks.
“I can’t remember a previous year when there have been so few of them,’’ he said. “They’re fairly nonexistent this year. It’s odd.’’