30 November 2008

Acorn shortage? Maybe ...

ResearchBlogging.orgThe Washington Post reports that this fall's acorn crop is apparently very poor, at least in parts of the Eastern Seaboard. There are lots of interviews with naturalists concerned about starving squirrels:
For 2 1/2 miles, Simmons and other naturalists hiked through Northern Virginia oak and hickory forests. They sifted through leaves on the ground, dug in the dirt and peered into the tree canopies. Nothing.

"I'm used to seeing so many acorns around and out in the field, it's something I just didn't believe," he said. "But this is not just not a good year for oaks. It's a zero year. There's zero production. I've never seen anything like this before."

Photo by Martin LaBar.
Accompanying the article is a photo of a northern flying squirrel, a species so cute that Disney is probably trying to copyright their genome. There is, however, no reference to a systematic survey of acorn production in any of the areas affected. As the saying goes, the plural of anecdote is not data.

That might sound like a picky thing to ask for, but without hard numbers we have no way of knowing whether this really is an unusually bad year for acorns. Oaks are masting species, meaning that their seed production varies a lot from year to year. It's been suggested that this is actually a defense against seed predators [$-a], such as squirrels. In "mast" years, trees produce a huge seed crop, and seed predators cache more seeds than they will eventually eat, so that some seeds survive to sprout. Masting works for long-lived trees because an oak that lives for decades can afford to take a year off from reproduction every so often, if it means that when it masts a larger fraction of its seeds survive to adulthood.

So it's hard to say whether this acorn shortage is unusual. If it continues two years in a row in the same regions, that would be surprising. Of course, by that time, populations of cute seed predators may have already declined precipitously. Common squirrel species are probably in no real danger, but critters like flying squirrels and anything else that can't make a living on suburban bird feeders could be in trouble.

Reference

D.H. Janzen (1971). Seed predation by animals. Annual Review of Ecology and Systematics, 2 (1), 465-92 DOI: 10.1146/annurev.es.02.110171.002341

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