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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