Thursday, September 25, 2014

An ecological look at acorns

An ecological look at acorns

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Last week I found a walnut on my windowsill, a big fat one with an unblemished bright green husk. Soon after, another appeared stuck in the wheel of my car. Now they’re cached everywhere around the outside of our house, from the shelf on the grill to the flowerpots on the deck.

Acorns are everywhere now, too, as anyone who bicycles where there are oaks can attest. A person’s got to keep both hands on the handlebars to avoid having them wrenched sideways.   

While these seeds may be present me with minor annoyances, they’re much more interesting and important from an ecological perspective.

Scientists group walnuts and acorns together with hickory nuts and beechnuts in the category of hard mast. This they distinguish from soft mast, which generally refers to fruits like crabapples and blueberries but can also apply to other parts of plants that serve as food for wildlife.

[Photo by author. Gray squirrel eating an acorn in a Chinkapin oak.]

According to Ed Heske, a mammal ecologist with the Illinois Natural History Survey at the U of I Prairie Research Institute, “The most important thing about hard mast from the perspective of wild animals is that it’s storable. Without hard mast many mammals that don’t hibernate in winter would have little to eat.”

Of course, while it’s a good thing for squirrels that acorns can be stored for eating over the winter, it is not in the interest of oak trees to expend all of the resources needed to produce such wonderful seeds if all of them wind up as squirrel food.

Evolution has provided oaks with a clever reproductive strategy to avert that outcome, referred to as masting cycles.

 In most years, oaks produce a sort of baseline quantity of acorns, and populations of animals that depend on them become calibrated to that. But every few years or so, depending on weather and other factors, the oaks of a local area synchronize their energy and produce a bumper crop—up to a hundred times the baseline quantity of seeds in some species. With populations of acorn eaters limited by the leaner years, chances are that some portion of acorns from the bumper crop will go uneaten and grow into the next generation of oaks.

There is another wrinkle to this story, though. Some years back Heske and a colleague conducted a study that found acorns would result in new oak seedlings only if some of them were buried by squirrels and then never recovered, a situation expected primarily when acorns are superabundant in mast years. Otherwise something—whether it was a deer, turkey, mouse or weevil—always ate them up from the soil surface before they had a chance to germinate.

In addition to promoting new generations of oaks, Heske explained to me, bumper crops of acorns initiate a cascade of other ecosystem effects. Extra acorns, for example, enable forest-dwelling mice to reproduce especially well; during mast years they can add an extra litter or two, and add to the size of their litters as well.

Good for the mice, right?

But what’s good for mice is, in turn, good for great horned owls and the other predators that eat mice. They generally experience a bump in reproductive success in the year following a mast year.

Thursday, September 04, 2014

Annual cicadas enliven dog days with love song

Annual cicadas enliven dog days with love song

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Even if the heat and humidity of recent weeks have limited your time outdoors, I bet you’ve been hearing a familiar insect song. It’s the mating call of dog day cicadas, loud enough to rise above the drone of air conditioners and so persistent and widespread that people who hear can hardly miss it.

I say, “dog day cicada” I mean the insect (pictured right) that goes by the two-part scientific name, “Tibicen canicularis,” which is the most common species of forest dwelling cicada that occurs in the eastern U.S. and Canada, one that has also adapted well to life in urban and suburban settings where enough trees grow to support it. These cicadas have bulky, bodies about an inch and a half long that are dark on top with green and white markings, and entirely white below. At rest their clear, heavily veined wings close over the back like a pitched roof and add another half inch to their length.

Some people call these and other cicadas locusts, a name that was first applied to them by settlers of European extraction for whom the emergence of large broods called to mind the plagues of the Bible. But the name locust is more properly applied to certain grasshoppers. Other people know cicadas by the name “harvestfly,” which derives from the fact that they emerge as adults at the same time crops are maturing.

The singing of dog day cicadas is one of the loudest insect noises on earth, sometimes exceeding 110 decibels up close. This means the song of a cicada perched your shoulder would be plenty loud to damage your hearing. The song is often compared to the whirring of a circular saw, although I think that comparison ought to be reversed, since cicadas have been around far longer than power tools. The earliest fossil record of a cicada dates back 65 million years.

[Photos by author: above, cicada "singing" on a tree trunk; below, newly captured and paralyzed cicada in the grip of a cicada killer wasp.]

The fact that dog day cicadas are also called annual cicadas sometimes generates confusion about their life cycle, but do they live for more than a year. They start out as tiny nymphs, which hatch from eggs laid in tree branches. These nymphs drop to the ground and burrow down to find a root they can latch onto for nourishment, and there they remain, probably for something like two to five years. (Scientists aren’t sure exactly how long, and the span probably varies according to conditions affecting the cicada’s development.) Some annual cicadas emerge as adults each year because their generations are staggered, which sets them apart from periodical cicadas, generations of which mature in synch, on 13- and 17-year cycles.

It is a common misperception that adult cicadas do not feed, but the fact is they have all the mouthparts needed to extract liquid from plants, and they’re not afraid to use them. Dog day cicadas do no damage to trees as they feed, and no measures to control them are warranted.

The only warm-blooded predators that pose a significant threat to cicadas are birds, but there’s another insect that specializes in them, the cicada killer wasp. A female cicada killer stings a cicada to paralyze it, then carries it back to her burrow still alive. There she seals the unlucky creature in a chamber with one of her eggs, to become nourishment for the grub that hatches.

But that’s not where I want to leave you. Let’s get back to cicadas singing their love song in trees, and appreciate how that enriches our summer.