June 26, 2012
Common milkweed: Bloom and doom
By late June, common milkweed blooms have reached their fragrant peak. People who find them on sunny roadsides and
lean in to inhale a deep draft of perfume from the nose-high, flower-crowded umbels will find the blossoms covered with a multitude of insects.
Bees, flies and butterflies sipping the abundant nectar can sometimes get caught in a tricky pollen-dispersal mechanism that snags insect legs if they step into slits in the side of each flower. If the insect is strong enough, its leg will yank out a pollinarium, a set of tiny saddlebags filled with pollen. Weaker insects can lose a leg or, if they can't pull free, will die stuck to the flower.
Although numerous insects are visitors, several species are dedicated exclusively to milkweeds, including the larvae of monarch butterflies, red milkweed beetles, and large and small milkweed bugs, all of which concentrate bitter milkweed toxins in their tissue — and all of which are conspicuously colored in black-and-orange or black-and-yellow patterns.
The loud colors say to would-be predators, "WARNING: Eat me at your peril."
But that potent threat could evolve into a bluff as atmospheric carbon dioxide levels continue to climb, altering the physiology of milkweed.
Scientists at the University of Michigan discovered that, as levels rise, milkweed tends to react by boosting its physical defenses while reducing the manufacture of chemical defenses. Rather than making itself less palatable, milkweed generally uses excess greenhouse gas to grow more biomass (outrunning insect damage) and to develop tougher leaves (barricading against insect attack). The plant sometimes even steps up production of the sticky white latex that bleeds from the plant when an insect bites, prompting the bug to eat elsewhere.
But there are exceptions. Milkweed is genetically diverse, and some populations buck the trend, raising production of toxins when CO2 rises. That genetic flexibility should help milkweed — and the organisms that depend on it — adapt to a rapidly changing environment.
Sources: Rachel L. Vannette, Stanford University; Global Change Biology; Craig Holdrege, The Nature Institute
Small milkweed bug
July 3, 2012
Robin fledglings face independence with the help of their dad
"PEEEK, chuck, tuck, tuck . . . PEEEK!"
Sounding the potential-predator alarm, a father robin alerts his fledgling to an approaching human.
Squat in the middle of a lawn, the baby bird freezes, its head cocked back, scanning for danger.
When the human draws too close, the chick panics, using its half-grown wings to propel it on a frantic, turf-top flight for cover in an overgrown fence row.
This fledgling might be the last of its siblings to survive. Only a quarter of American robin chicks make it to adulthood, successfully escaping the jeopardy of cats, crows, squirrels, snakes and lawn pesticides.
In June, while this chick was being incubated by its mother, its father was busily tending to fledglings from the brood that hatched a few weeks earlier.
That duty gave the father less time to protect his mate from the advances of other male robins. But a female robin is less likely to stray if she has what scientists call "confidence in paternity" — that is, when she sees that her mate is effective in helping raise the young ones.
Paternal care of fledglings lasts for about two weeks after they leave the nest. Dad feeds them while they learn how to fly, groom, hunt for earthworms and ripe fruits — and make warning calls.
Although young birds can take care of themselves about a month after hatching, they continue to beg for food from their parents for a little longer. With more luck, they will be protecting their own needy offspring this time next year.
Sources: Animal Behavior, Encyclopedia of Life, Messinger Woods, learner.org, Cornell Laboratory of Ornithology
July 10, 2012
The upside of downed trees
The violent derecho that ripped through Washington may have created human misery, with smashed roofs, power outages and spoiled food, but it generated a windfall for the organisms that live in and feed on fallen limbs and logs.
The rate at which woody debris is consumed depends on the species of tree (pines break down faster than oaks) and where and how the tree falls.
Logs suspended above the forest floor by side branches decay more slowly than logs lying in contact with the ground, where they absorb more moisture.
Compared with logs falling upslope or downslope, logs falling parallel to the contour of a slope will rot faster because they are more effective at catching and holding debris and water, which also helps slow erosion.
After debris falls, the wood initially dries out and cracks, creating thoroughfares for insects to enter. As beetles, ants and termites traipse into the log, they introduce microbes and fungal spores that contribute to the breakdown of the wood.
A log's nutrient-rich inner bark and sapwood are the first to be exploited, causing the naturally resistant outer bark to slough off. The log's hard, nutrient-poor heartwood is the last to disintegrate.
Persistent gnawing and decay eventually creates spaces in the log large enough for vertebrates to use as shelter. During hot, dry summers, rotting logs provide cool, damp havens for myriad animals and serve as a moisture reservoir for plant roots.
The log is finally rendered into a fertile mound of dark, spongy, mineral-rich, carbon-sequestering humus. Much of its fertility is due to the biologically diverse activity that deposited nitrogen-rich waste in and around the collapsed log, turning it into an ideal nursery for tree seedlings.
Flow of water
Parallel with slope contour
Pileated woodpecker feeds on carpenter ants.
Ambrosia beetles cultivate fungi in chambers.
Bark beetle larvae chew galleries along surface of inner bark.
July 17, 2012
The eastern cottontail way to chill
A heat wave may inspire some of us to crank up the AC and take a nap while the temperature spikes. Eastern cottontails do the same, but they use their own built-in cooling systems.
The solitary mammal restricts most of its activity to dawn and dusk, when it feeds on grasses, plantains, clover and garden vegetables. During the heat of the day, cottontails retreat to thickets, brush piles and hollow logs.
Rabbits use several strategies to regulate their body temperature, which becomes elevated on hot days. They have very few sweat glands (the fur coat prevents evaporation from the skin), so they use their ears as radiators.
Blood vessels in the ears dilate, allowing excess heat to dissipate from their large surface area. On very hot days, if the rabbit's body temperature approaches that of the surrounding air, its ears are no longer able to radiate heat, so the rabbit stretches out on the ground to release as much heat as possible from its body, and it begins to pant through its nose.
A panting rabbit increases its heart rate and breathing rate as blood is pumped through large, complex nasal passages, where inhaled air evaporates water from mucous membranes, cooling the area as the humid air is pulled into the lungs. Warmer exhaled air condenses some of its moisture onto the relatively cooler nasal passages, helping to conserve the rabbit's water.
Although rabbit ears may lose their ability to dump heat on record-hot days, they remain sensitive to the sounds of the animal's many predators, the advance of which inspires a bounding, zigzag escape into a nearby briar patch.
Sources: University of Michigan Museum of Zoology, Acta Neurobiologiae Experimentalis, ciheam.org
July 24, 2012
White mulberry's simpler leaves reflect maturity
The yard-long white mulberry twig below has finished growing for the year. The leaves have fully expanded and have hardened off, becoming tough and dark green. Next spring's buds are already forming at the base of each leaf stalk.
The twig and its first leaves emerged from a bud
in the spring. Subsequent leaves reveal the tree's tendency to produce leaves in a wide variety of shapes.
Leaves formed in early spring developed five to seven lobes each. As the season progressed, new leaves assumed a mitten shape. By summer, leaves had mostly abandoned the tendency to form lobes.
Simpler leaves reflect maturity. The majority of leaves on older mulberry trees have one lobe or
none at all. Younger trees, such as the one that
produced these leaves, grow plenty of simple leaves but support a higher percentage of multi-lobed leaves than do mature trees.
Mulberry leaves are rich in calcium and protein; good fodder for livestock and even for people, who sometimes cook the leaves in the spring while they are still tender. The tree's berries are also edible.
Sources: Castanea, Plants for a Future
August 7, 2012
The Asiatic clam: A double-edged guest
Sandbars on the Potomac River are often topped with heavy deposits of Asiatic clam shells, which are dime- to silver-dollar-size with concentric ridges and an olive-yellow coating that turns black and flakes off after the clam dies.
Although the exotic mollusk creates environmental problems in some parts of the country, it appears to improve aquatic habitats in some local waters without displacing native clams or mussels.
Asiatic clams are very effective at removing sediment and algae from the water, allowing deeper penetration of light. That encourages the return of submerged aquatic vegetation, which provides cover for young fish and other organisms.
Birds, fish and river otters feast on the little clams, which people can also eat, provided the clams are living in unpolluted water. In some parts of the world, the "good luck clam" is a major food source.
"It has a high glycogen content, like oysters," says Harriette L. Phelps, a University of the District of Columbia emeritus professor who has been studying the clam for decades. Although the clams are found in areas closed to shellfish harvest, Phelps says that Asians have been seen taking the clams from shallow Potomac waters using sieves.
The clams rest on or burrow slightly into silt, sand or gravel. When scooped up with river gravel used as aggregate at concrete plants, the tough little clams can wriggle their way to the surface of freshly poured cement, ruining concrete projects.
Although the Washington Aqueduct, which supplies water to the District and parts of Virginia, has never had problems with the clam, tiny juvenile Asiatic clams have been reported to clog industrial water-intake pipes farther downstream.
When they're not annoying industry, Asiatic clams can be used to monitor it. Phelps, who revealed the clam's environmental benefits to the Potomac, has for almost 20 years used Asiatic clams to measure pollutants in the Anacostia River and its tributaries. The clams, suspended in mesh bags for two weeks, concentrate toxins in their tissue, which is then tested in the laboratory to pinpoint contaminated areas in the troubled watershed. "In the Anacostia, many sites had high toxics and little life, but Corbicula fluminea can survive over a month," says Phelps. "We are fortunate in having local Corbicula available."
Sources: Estuaries, Army Corps of Engineers, Water Resources Research Institute, National Exotic Marine and Estuarine Species Information System, USGS, Potomac River Fisheries Commission
August 14, 2012
Reoccupying McPherson Square
By the time Occupy D.C.'s resident protesters were evicted from McPherson Square in February, the four-month-long encampment had compacted the park's soil and worn away the turf that had been restored in 2010.
In April, the National Park Service began fencing off vacated sections of the park, aerating and fertilizing the ground, and sowing tall fescue, a perennial
grass that helps improve contaminated soil.
The camp suppressed the emergence of yellow tulips that normally flank General McPherson's statue. During this year's restoration, the bulbs were dug up and donated to nonprofits, said Park Service spokeswoman Carol B. Johnson. Banks of begonias now occupying the flower beds "will be replaced in the spring" Johnson said, "but not sure with what."
The turf is slowly recovering, but many new uninvited occupants have arrived: Some weeds will be tolerated while grass takes hold, but the Park Service will discourage the burrowing brown rats making a stand in the park.
Sources: National Park Service, staff reports
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August 28, 2012
Tulip trees fly the first flags of fall
As summer vacation evaporates, a dry spell, such as the one we had early this month, triggers tulip trees to shut down some of their leaves, which become bright yellow flags signaling the final month of the tree's growing season.
Yellow leaves are sometimes visible on tulip trees during early summer droughts, but those fresh, young leaves are much less apt to surrender than are the aging leaves of August.
Old leaves lose some of their ability to stabilize water in their cells, rendering them more vulnerable during rainless stretches. Even a brief drop in soil moisture can trigger leaf loss in late summer.
Tulip trees have several other weaknesses that are often typical of fast-growing trees. Seedlings can perish in the winter when rabbits chew off the succulent bark and buds. Saplings are sensitive to late frosts, can be killed by even small fires and often fall prey to deer that devour their leaves, twigs and branches. Frail limbs can succumb to hail, heavy snow and vines, inviting fungi to invade the exposed creamy white wood. Scale insects suck the sap.
But rapid growth has it advantages: Tulip trees can outpace their predators to become the tallest hardwoods on the East Coast. Two of them planted at Mount Vernon by George Washington in 1785 now stand more than 130 feet high.
Sources: Oecologia, U.S. Forest Service, "Remarkable Trees of the World"
September 4, 2012
Beach plum: A late-summer tart from the dunes
If you spent Labor Day weekend on a Northeast beach, you may have stumbled into a beach plum, a small, scrappy tree with yellow-to-purple, grape-size fruits. They ripen in early September.
Plums from trees growing on sand dunes are edible but are mostly pit, which is covered by a stingy layer of sweet flesh wrapped in a bitter skin; the poignant taste of summer's end.
In a Washington back yard, beach plums can grow well if they are provided with well-drained soil and full sun. The rewards: Fragrant white
flowers in the spring and fruit — more succulent than
those plums from the dunes — for back-to-
school jellies, jams, cobblers, even
cordials and wine.
But simply planting a plum
pit can lead to disappointment
if you are not patient.
It can take as long as
two winters before a
sprout emerges — if
Efficient germination involves cracking the pit to remove the seed and refrigerating it in moist sand for two or three months to break the embryo's dormancy.
An easier way to start a beach plum is to begin with pencil-size root cuttings from young trees. Best results sprout from cuttings treated with rooting hormone and planted several inches into sandy soil in the fall.
Beach-plum trees are very effective at stabilizing sand, so leave them alone if you find them growing on dunes or in protected areas. Instead, ask a nursery to order two for you (about $10 a tree). Since they can't self-pollinate, they'll need a companion to be fruitful.
Sources: Arnoldia, Cornell University Department of Horticulture, University of Maryland Extension
September 11, 2012
Dragonfly migration: A one-way ticket
In mid-September, winds coming from the north create good conditions for observing dragonflies migrating south. Watch for them on light breezy days in the early afternoon, before thermal updrafts lift the insects out of sight. Even when floating just above the treetops, a big dragonfly presents only a subtle profile, that of a darning needle held aloft by four transparent wings.
Young adults, which recently emerged from their stage as underwater nymphs, prepared for migration by packing on fat — a third of their one-gram weight.
Migrating only during daylight hours, darners can fly more than 60 miles a day, often following shorelines. They regularly stop over for several days at a time to prey upon mosquitoes, gnats, flies and bees until falling temperatures urge them on.
But it's not all dining and sailing for the big-eyed insects. They have predators to reckon with; American kestrels, small falcons that follow the dragonfly swarms, picking them off to fuel their own migration.
If falcons don't get them, the clock will. Adult darners live for only a few months. The lucky ones making it to Florida, Texas, Mexico or the Caribbean will enjoy a few weeks of breeding before they die. Their progeny will be the ones heading north in the spring.
September 18, 2012
When barred owls attack
In the black hour before dawn on Sept. 4, Peter J. Grace, 41, stepped out of his house in Bethesda to begin his daily four-mile run. His first quarter-mile included Glen Cove Parkway, a seemingly peaceful street following a heavily wooded tributary of Little Falls Creek. But the tree-lined lane that early morning was merely a sylvan facade. Hiding in the folds of darkness was an assailant.
It was a stealth attack; no sound, no warning, just a sudden thump to Grace's head.
"I've been quite literally attacked from above by a large owl," Grace wrote to his e-mail group of fellow runners, "I had on a reflective armband and thought the owl might have mistaken it for an animal's reflective eyes."
But surely an owl can tell the difference. Grace is about 60 times heavier than the average rabbit nibbling on the grass at Westbrook Elementary, where Glen Cove takes a hard left to become Allan Terrace — the site of the next day's assault.
Morning of Sept. 5: Grace ditched the armband and chose instead to wear a white runner's cap. His mistake, however, was to run the same route.
"This time it swooped down on my head with its talons," wrote Grace, "three separate times!"
Did the owl mistake Grace's hat for a monstrous cottontail?
Peter J. Grace, left, and suspect, right.
Scene of attack: 5:30 a.m., Glen Cove Parkway at Allan Terrace, Bethesda.
No blood was drawn, but the mugger landed in a large pin oak illuminated by the sick amber glow of a sodium vapor lamp. Grace got a good look and later fingered his attacker in a Google lineup: a barred owl, 20 inches tall, two pounds; commonly heard and seen in the Washington area.
"Attacks like these are not uncommon," says Rob Bierregaard of the University of North Carolina at Charlotte, where he studies suburban barred owls. If people bother an owl nest or come upon a young owl that was bumped out of its nest prematurely, it would make sense that a parent owl would attack. (When he studies owl nestlings, Bierregaard wears safety glasses and a lacrosse helmet.)
But "strangely enough, these explicable attacks are less frequent than what to me are really inexplicable strikes in the fall," confesses Bierregaard. Young owls have dispersed by now and are wandering into adult territories, "which may explain the spike in
territorial calling by adults often reported at this time of year," he says.
"Barred owls are so used to humans that they've pretty much lost all fear of them. But I can't stretch that to explain why an owl would pop a jogger on the back of the head," he says. "Using Sherlock's strategy that after you've eliminated the impossible, whatever remains, however improbable, must be true, the only thing I can come up with is these are playful young." (Ever see the YouTube video of the young barn owl playing with a cat?)
The Bierregaard Hypothesis: September owl attacks are perpetrated by rambunctious teenage owls.
Small comfort to Grace, who isn't keen on being the target of juvenile raptor horseplay. He has since rerouted his predawn run.