The changing natural world at our doorsteps | By Patterson Clark
January 4, 2011
Snowflakes: Certainly, absolutely, positively unique
Groping for a number to describe the inimitable.
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
January 11, 2011
January's golden bouquet
While most of the plant world sleeps, a few shrubs are blooming.
Even the dead of winter has its uplifting blossoms, and the most prominent ones tend to be yellow.
Witch hazel is a native shrub that is just concluding its flowering season, which began in early autumn. Its sweet-scented flowers bear long, strap-like petals.
Spikes of fragrant leatherleaf mahonia flowers began to appear in late December and will continue blooming through early spring. The ornamental plant's bright blue fruits will ripen in early summer. Birds and mammals eat them and spread the seeds. Deer won't eat mahonia's tough, spiny leaves, which probably contributes to its proliferation as an exotic invasive species.
The first flowers of winter jasmine open in January on sun-drenched bushes backed by south-facing, heat-radiating walls. Winter jasmine is one of the few jasmines offering no scent.
Yellow flowers tend to attract flies and bees, which home in on the ultraviolet light (invisible to us) reflected by the petals.
For witch hazel, cool-weather pollination happens in the fall when the shrub's flowers are one of the few sources of nectar available for pollinators. Even so, only about 1 percent of the flowers produce seed.
Although it hails from China, leatherleaf mahonia draws enough local ants and winged insects during winter warm spells and early spring weather to accomplish fertilization.
American insects, however, are apparently inadequate for winter jasmine. The bush bears fruit only in its native China.
January 18, 2011
Rounding the analemma on a fast track to longer days
The sun's path across the sky reveals the Earth's tilt and orbital eccentricity.
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)
January 25, 2011
Volunteers help control English ivy
In winter, ivy-covered trees stand out as stark green pillars of exotic invasion.
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 1, 2011
Mixed-species foraging: Birds of different feathers flock together
In the winter woods, you may suddenly find yourself surrounded by a slow-moving gang of birds, a hunting party of several small species foraging for food together.
Tufted timice and Carolina chickadees are ringleaders of the bunch, which may also include kinglets, brown creepers, downy woodpeckers and white-breasted nuthatches.
Mixed-species foraging is less common when food is abundant, which supports the notion that the behavior increases feeding efficiency. In such flocks, birds can learn about food sources from other species, and the foraging is less competitive than in a flock made up of only one species.
With the mixed flock's variety of eyes and ears on alert, the birds may be safer from predators. They can spend more time searching for food and less time watching their backs.
Woodpeckers and nuthatches clearly benefit from the coalition. Outside the mixed flock they are more vigilant and less likely to leave their comfort zones, resulting in their eating less and suffering higher mortality rates.
But not everybody on the team is a winner: Chickadees pay price when they join such flocks. They fare better when foraging on their own, because in mixed flocks titmice steal food from them.
February 8, 2011
Only rarely will a snowstorm rumble
The heavy, icy snow that hit the Washington area on Jan. 26 was accompanied by muffled thunder claps
-- an unusual soundtrack for a winter storm.
Thunder and lightning are normally associated with warm-weather storms: Hot, moist air rises quickly and cools at
The thundersnow storm of Jan. 26.
high altitudes, forming rain, ice pellets and ice crystals. The heavier rain and larger ice pellets fall, but the tiny ice crystals rise on the strong updrafts. When crystals and pellets collide, electrons are stripped away from the crystals and rapidly form positively
and negatively charged layers in the clouds. Lightning flashes neutralize the charges, causing the surrounding air to violently expand and create thunder.
In winter, moist air starts off colder, so
in a storm it doesn't rise as quickly or generate much of a charge.
But an extra-strong low-pressure area
at a high altitude can suck up enough
air to keep ice crystals (in the form of snow) aloft, allowing clouds to build up enough static to pop.
The boom of the thunder, however, is damped by the snow-filled sky.
A cross-section through a thundersnow storm.
February 15, 2011
Winter crane flies
Mild days unleash
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.
February 22, 2011
Long-lasting shelf life
A fungus hangs tough through the winter.
Harsh February weather seems to have little effect on thick, leathery shelf fungi, which protrude from the broken branches and fallen trees brought down by storms of years past.
A single shelf fungus can release trillions of spores, some of which drift through the air and settle onto damp, exposed wood. Those fortunate spores produce tiny fungal threads that creep into the wood to digest it. A growing "white rot" will slowly soften a hardwood log, encouraging insects to enter and hasten decay of the timber.
As it exploits the wood, the fungus sends out a hard, durable structure called a conk, which has on its underside tens of thousands of pores. Each pore is the open end of a tube that can generate and drop millions of spores.
Unlike soft, perishable toadstools, conks can survive for years, producing concentric annual growth rings on the conk's top surface.
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.
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.
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.
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
March 8, 2011
March of the nest traps
| The first plants to leaf out may not be the best for birds to lay eggs in.
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.
March 15, 2011
Polyester bees: Born in a plastic bag
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.
A native alternative to honeybees
North America has 4,000 species of bees. Many lead solitary lives similar to that of the polyester bee. "Some are much better pollinators than honeybees," says Batra, "and native bees aren't affected by the parasites and diseases that are killing honeybees."
But modern agriculture, with its vast fields, pesticides and scarce natural areas, doesn't encourage fertilization by native bees. "You would need undisturbed areas nearby," says Batra, "so that the bees could nest and fly out to the fields to pollinate."
Unlike some synthetic plastics, bee plastic is biodegradable. Batra tested that by burying a bunch of brood-cell linings, which disintegrated after five years.
A research group at Olin College of Engineering has been studying polyester bee plastic for several years: "Bio-plastics are only in the early stages of development," says student researcher Shannon Taylor. "Our goal is to understand [bee plastic] well enough to create something similar ourselves."