Reading Comprehension

Pesticides (including insecticides, fungicides, nematicides, and herbicides)
are chemicals used in agriculture to increase production by combating
organisms that damage or destroy plants. However, pesticides by their
very nature can result in serious harm to wildlife both by directly killing
animals and through more subtle effects on reproduction, development
and behaviour. Organophosphates are pesticides that interfere with the
enzyme cholinesterase, which is essential for the proper functioning of
the nervous systems of insects, as well as of humans and other
vertebrates. Toxic exposure to organophosphates results in fatal
respiratory failure. The first indicator of toxic absorption is a reduction in
the enzyme cholinesterase in red blood cells, and contact with
insecticides is the only known cause of a marked depression of this
enzyme.

In a recent study, researchers collected specimens of both adult and
tadpole Pacific treefrogs from sites located both within the Sierra Nevada
(representing northern and southern areas) and also to its west
(representing the foothills and the Pacific coast of California). When
cholinesterase levels were then examined they were significantly lower in
tadpoles taken from the mountains east of the San Joaquin Valley, such
as Yosemite and Sequoia National Parks, than in those taken from similar
sites farther north in the Sierra Nevada, which lie east of the Sacramento
Valley where agricultural activity is less intense.

Moreover, lower cholinesterase activity levels were correlated with
distance away from the coast and toward the higher elevations of the
Sierra Nevada. Similar, although less significant, trends were seen in
adult frogs. Concentrations of particular organophosphate pesticides in
the collected tadpoles and adult frogs were also measured. More than
fifty percent of the adult frogs and tadpoles at Yosemite National Park
had measurable levels of diazinon and chlorpyrifos, compared to only
nine percent at coastal sites. Since both diazinon and chlorpyrifos
degrade very rapidly in organisms, the detection of either compound
indicates recent exposure to the chemicals. The red-legged frog is now
listed as threatened under the U.S. Endangered Species Act, and the
mountain yellow-legged frog and Yosemite toad have been proposed for
listing. Many amphibian population declines have occurred in some of the
state's most seemingly pristine areas, such as the Sierra Nevada
mountain range of eastern California which includes Sequoia, Yosemite,
Kings Canyon, and Lassen Volcanic National Parks as well as Lake Tahoe
and Mt. Whitney.

Because the southern parts of the Sierra Nevada lie east of the
intensely agricultural San Joaquin Valley, environmentalists have
suspected that pesticide use may be responsible. Pesticides could be
transported from the San Joaquin Valley to the Sierra Nevada on the
prevailing eastward summer winds, and then affect populations of
amphibians that breed in mountain ponds and streams.

With respect to pesticides, the author takes time in the passage to assert that
they:

A. are transported for long distances by wind currents.
B. are detrimental to both insects and vertebrates.
C. are not used in the Sierra Nevada.
D. have benefits in agricultural applications.
E. should be absolutely banned