Environmental Engineering Reference
In-Depth Information
After a reprieve from hydrologists, tamarisk is also getting a makeover from ecologists. While the
environmental orthodoxy still holds that tamarisk crowds out native species, the recent evidence sug-
gests that it simply moves in if others disappear. Interestingly, it does best on river courses downstream
of large dams that eliminate spring floods. Where the floods on untamed rivers persist, cottonwood and
willow continue to dominate. Below dams, tamarisk prevails. On this reading, it emerges not as a men-
ace to other species but as a colonist able to step in where others fail.
Meanwhile, ornithologists have discovered that in many places, tamarisk is the preferred nesting
site for a wide range of birds, most notably the endangered southwestern willow flycatcher (
Empidonax
traillii extimus
). Tamarisk and the flycatcher share similar territory in the West, which probably explains
why trigger-happy conservationists were quick to blame it for the bird's decline. But in some areas, 75
percent of the endangered subspecies nest in tamarisk bushes. Downstream of Glen Canyon Dam on the
Colorado is a notable instance. Overall a quarter of the flycatchers there nest in tamarisk.
16
With the
birds down to fewer than five hundred breeding pairs, this is a big deal—especially as conservationists
say the main threat to its survival is loss of habitat. Nor can tamarisk be accused of being the only option
for birds that would prefer native trees to nest in. Mark Sogge at the USGS found that the flycatchers
fed as well, reproduced as well, and survived as well on tamarisk as other trees.
17
Right now tamarisk
looks like the best habitat for the bird. Conducting a war against it doesn't sound smart. Soon they may
be planting tamarisk again.
Since the first Europeans showed up, it has always been morning in America for alien species. Like hu-
mans, they just keep coming. In 1672, English traveler John Josselyn cataloged forty European weeds
that he said had “sprung up since the English planted and kept cattle in New England.” Early and gen-
erally benign arrivals included buttercups and ox-eye daisies. By the nineteenth century, New England
was known as “the garden of European weeds.”
18
Botanists quickly learned where to root around looking for the most interesting newcomers. The best
botanical Ellis Islands included the dumps where ships from Europe emptied their rock and soil ballast.
Purple loosestrife (
Lythrum salicaria
) probably arrived this way two centuries ago. The pretty flower
likes damp places and watersides, and it spread far and wide along the developing network of navigation
canals in the East and irrigation systems in the West. Now it can be found on riverbanks and in wetlands
from the Hudson River to Alaska.
19
It is widely seen as a threat to biodiversity, especially in wetlands,
though the evidence is thin, says Michael Treberg of the University of British Columbia.
20
And native
bees like it, which is a plus in some eyes.
As the American frontier moved west, many European plants followed. They included grasses eager
to take up residence in the wide-open spaces. Among them was cheatgrass (
Bromus tectorum
). This old-
world grass fed some of the first domesticated animals thousands of years ago in the Middle East. It
is now a truly global citizen, found from New Zealand to Greenland, and South Africa to Japan. It ar-
rived in America with livestock, making a first appearance in Pennsylvania in the 1860s. “After that it
came west with the stagecoaches, and began taking over from sagebrush,” says DiTomaso, author of the
standard textbook on the weeds of California. It traveled well. Its hairy seeds blew in the wind, attached
to clothes, and got hooked into the hairs of cattle and sheep. It also mixed with the hay that fed the an-
imals pulling coaches and wagons.
Cheatgrass likes things out west. The knee-high yellow grass is today the most common plant in the
region, covering millions of acres, especially in Idaho, Nevada, and Utah.
21
It steals a march on native
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