Urban Jungle

When you're standing in a blizzard, it's hard to imagine that no two snowflakes are exactly alike. Surely at least one duplicate is possible.

"The odds of it happening within the lifetime of the Universe is indistinguishable from zero," writes Caltech physicist Kenneth G. Libbrecht on his Web site SnowCrystals.com.

Water can crystallize out of a snow cloud in 35 forms, including needles, bullets, cups, columns, graupel and flat, lacey snowflakes. But all crystals start out as microscopic hexagonal prisms (figure 1). If you looked at enough of these tiny crystals under a microscope, you would probably find some that look alike.

The big, intricate flakes, however, are each visibly unique. If conditions in the snow cloud are favorable, the tiny hexagonal starter crystals will develop elaborate branches and textures (figure 2), varying with wind, humidity, temperature and chemical impurities. A small percentage of the flake's water molecules will be composed of different isotopes of oxygen and hydrogen, which also affect the design.

Each branch of a snowflake grows independently of the others, but identical conditions will make branches grow in the same manner. However, "the vast majority of snow crystals are not very symmetrical," Libbrecht writes. "Don't be fooled by the pictures."

The images at right picture fairly symmetrical snowflakes. Libbrecht suggests that an observer might discern 100 different features on each snowflake. "Since all those features could have grown differently, or ended up in slightly different places," the number of possible arrangements for each snowflake approaches 10¹⁵⁸, or a 1 followed by 158 zeros. "That number," he says, "is about 10⁷⁰ times larger than the total number of atoms in the entire universe!"

SOURCES: National Solar Observatory, "The Physics Factbook"

In 1885 Vermont farmer Wilson A. Bentley pioneered a process for photographing snowflakes. The patterns at left are derived from a few of the more than 5,000 images he recorded.

Images courtesy of The Jericho Historical Society, Jericho, Vermont.

SOURCES: Mid-Atlantic Exotic Pest Plant Council; Maryland Native Plant Society;
American Journal of Botany; Biological Invasions

Commuters driving east in the mornings may have begun to notice that the sun has climbed slightly higher and is tracking to the north again.

If you take a photo of the sun from the same spot at the same time every week for a year and merge the photos, the sun will trace a figure 8 in the sky, a path known as an analemma.

That path is the result of two factors: Earth is tilted on its axis, and it orbits the sun on an elliptical path.

If our planet were in a perfectly circular orbit and had no tilt to its axis, we would always find the sun in the same spot at 8 a.m.. Yawn. (Figure 1)

Add the elliptical orbit -- where the Earth's orbital speed changes with its distance from the sun's

gravitational pull -- and the sun lags behind the clock for part of the year, then races ahead of the clock for another. The analemma for that would be a sun tracking back and forth on a straight line. (Figure 2)

It's our tilt that gives the analemma its figure. If Earth had a circular orbit but was tilted at its current angle, the sun's analemma would be a figure 8 with equal lobes. (Figure 3)

SOURCES: Neave planetarium; Bob Urschel, Analemma.com; John Oliver, University of Florida; Steve P. Brown, Woodson High School; timeanddate.com; NOAA solar position calculator

John Bowman cuts an ivy vine from the base of a tree not far from his home in Washington's North Portal Estates. Bowman is a volunteer with Friends of Rock Creek's Environment, which helps Rock Creek Park with stream cleanup and invasive plant removal.

In January, tree-climbing English ivy bears black-purple fruit that lures hungry birds.

That's good for the ivy but sometimes bad for the bird. Glycosides in the fruit can cause some birds to vomit, which further spreads the seed of the weed. Starlings and house sparrows -- two other British imports run amok -- aren't sickened by the berries, so their droppings may carry the seeds even farther.

Noxiously invasive ivy starts out as a ground cover, too often planted as an ornamental. It can relentlessly swarm over and replace native herbaceous plants. Thick mats of ivy will also thwart the growth of tree seedlings.

Worse trouble happens when the vine climbs into trees: The plant becomes a beast, its vine swelling into a woody stem as fat as your ankles. The stem sprouts branches bearing a spearhead-shaped leaf, quite different in shape from that of the ground-bound vine. Flowers appear, fruits ripen, the tree suffers.

"Ivy can kill trees," says Ken Ferebee of Rock Creek Park. It can envelop branches and twigs, suppressing tree growth and "causing a decline in the tree that can last for many years," says Ferebee. The weed adds extra weight to its host tree, and its dense growth catches the wind like a sail, increasing the likelihood that a tree will be blown over.

Because English ivy has no natural predators, competition or diseases, the task of controlling the weed falls to people.

Armed with pruners, loppers and saws, park volunteers liberate trees from ivy by cutting the weed off at its knees, removing ivy stems from a zone around the base each tree. Park specialist Joe Kish cautions his volunteer crew not to peel off the tree's bark while removing ivy. "That can introduce diseases into the tree," he explains, "Instead of helping the tree, you've damaged it."

After the cut, "ivy will stay green for a while," says Ferebee, "but it will eventually turn brown and fall off."

SOURCES: National Park Service, Friends of Rock Creek's Environment, noivyleague.com

Before and after: Ivy-infested trees in a corner of Whitehaven Park, left, and a year after treatment, right.

February 15, 2011

Snow swarms:

Winter crane flies

Mild days unleash

cold-hardy flies

After weeks of freezing weather, a sunny, calm afternoon in the 40s will prompt an emergence of winter crane flies in the woods. About a third of an inch long, the delicate insects resemble mosquitoes -- but without the blood-sucking beak.

Silvery swarms of rambunctious males hover a couple of feet above the ground, each fly bobbing up and down, stitching the air with a bouncy mating dance.

Females spend most of their time on the ground but will briefly join a swarm of males to couple with one of the bouncers. After mating, females return to the forest floor to lay their fertilized eggs.

Winter crane fly larvae live under leaf litter, in caves and in animal burrows. They feed on decaying plants, fungi, dung, even carrion.

The dark brown, short-lived adults can sometimes be seen sitting on the snow, which makes them a much more visible target for insectivorous birds such as wrens.

SOURCES: International Journal of Speleology, Insectsofwestvirginia.net, "Forensic Entomology"

March 1, 2011

Health care for wild things

What help is available for injured animals?

You find an injured cardinal struggling in the snow. After gently placing the bird in a box with some shredded paper towels (keeping it warm is crucial), the next step will depend on where you are.

NORTHERN VIRGINIA

The Wildlife Rescue League operates a hotline (703-440-0800) that connects people with independent wildlife rehabilitators.

The Wildlife Center of Virginia, a wildlife hospital and training center, is at least two hours away, in Waynesboro.

MARYLAND

Friskys Wildlife and Primate Center and Rescue in Woodstock accepts small native wildlife, domestic farm animals and primates.

Second Chance Wildlife Center in Gaithersburg accepts sick and injured wildlife, but because of a shortage of space, the center has had to limit the number of animals it accepts from the District.

THE DISTRICT

Although many creatures live in the city's extensive parklands, Washington has no wildlife rehabilitation center.

The Washington Humane Society has a contract to run the District's Animal Care and Control Facility, which requires its officers to pick up sick and injured wildlife, but the law permits only licensed wildlife rehabilitators to care for wildlife after 24 hours in captivity. The DCAC has no rehabilitators on staff.

The city must either release your injured cardinal (if it can survive), euthanize it (requiring a permit from a licensed rehabilitator) or send it to a rehab center across state lines. Second Chance is the closest, an hour away.

"Even if the District wanted to fund wildlife rehabilitation as part of its animal control program, the facilities at the D.C. animal shelter are so cramped and outdated that there would be no place there to house a wildlife care component," says Anne Lewis, president of City Wildlife.

The nonprofit wants to open a wildlife rehab center in the District but faces a steep financial hurdle. Operating such a center would cost about $300,000 a year, which would fund the space (probably in a warehouse) and the salaries of a staff of four to five, including two licensed rehabilitators.

"Volunteers will be critical to our success," says Lewis. "Fortunately, volunteering for wildlife is popular, and sometimes students can get credit for their services." The center would typically need six or seven volunteers working every day. Initially, the center would "accept all forms of native wildlife except deer [they require too much space] and rabies vector species" (foxes, raccoons, bats, skunks and coyotes), says Lewis, but many of those might be accepted after the center has been operating for several years.

"Each species has very specific handling and care requirements," she says, "and each animal has its own personality."

SOURCES: Jim Monsma, City Wildlife; Brittany Davis, Second Chance WIldlife Center

Even though winter isn't quite finished with us yet, two invasive plants have been getting a head start on spring, responding to brief warm spells by erupting with shoots of new leaves.

Japanese honeysuckle, which still bears some of last year's leaves,

and multiflora rose are well ahead of native plants in sprouting. They will soak up as much direct sunlight as they can before overhanging trees leaf out and pull down the shades.

When spring arrives, the thick and early growth will attract robins and

cardinals -- but the birds make a poor choice if they build their nests in these nonnative weeds. In urban areas, predators are twice as likely to disturb nests built in multiflora rose and Japanese honeysuckle as they are to attack nests built in native vegetation.

That may be because the two invasive plants tend to grow lower to the ground and are in some way structurally easier for predators to access. Also, predators may have learned to check honeysuckle and rose more often for hapless nesters.

SOURCES: Ecological Applications, Biological Invasions

The early March sun warms exposed soil, triggering the emergence of male polyester bees, which swarm the ground, waiting for females to dig to the surface. The half-inch bees -- also known as plasterer bees -- mate while rolling on the ground or while flying, joined to each other in midair.

Unlike social honeybees, polyester bees are solitary. After mating, males fly off to finish their short lives sipping from freshly opened tree blooms. Each female works alone on her own nest, a foot-and-a-half-deep tunnel as wide as a pencil, dug straight down into the ground. Eggs are laid in pockets, or brood cells, dug into the sides of the tunnel.

Every night, the female digs out a new brood cell and lines the cell with polyester secreted from her abdomen. "She spreads it on the cell wall with her paintbrush-shaped tongue," says Suzanne W.T. Batra, a retired USDA entomologist, who began studying solitary bees in the 1960s.

A still-unknown agent -- maybe something in the bee's saliva -- reacts with the polyester, causing it to harden into a flexible waterproof plastic resembling cellophane." "During the day, the female collects nectar and pollen and packs it into the cell along with some glandular material. She lays an egg, suspended over the food, and seals the cell with more polyester. "Closes it like a zip-lock bag," says Batra. The bee plugs the cell entrance with soil, packing it down with the tip of her abdomen before starting to dig another cell.

Some people might be alarmed to find polyester bees swarming the grounds of their property.

Fear not, says Batra. "The bees rarely sting. You'd have to sit on one to get it to sting you." Her advice: "Wait a month and they'll go away on their own." By mid-April, any remaining bees will be limping about on tattered wings. They won't be seen again until larvae go through metamorphosis and emerge late next winter.