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The converse of this is that organisms may be very

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Unformatted text preview: t reduces the probability of exposure where the endocrine modulating substances are not persistent and not released on a continuous basis. Thus, for endocrine modulating substances that are not persistent and are released into the environment on a sporadic basis only, the probability of exposure coinciding with sensitivity is reduced and the risk is thus smaller (Figure 3-27). Endocrine modulating responses from non -persistent substances have not been as well documented as those from the persistent organic pollutants. Because of their lack of persistence, these substances present a special case with respect to endocrine modulation as consideration of the probability of exposu re and sensitivity becomes more important. Exposure Sensitivity Persistent endocrine modulating substances or continuous release EFFECTS ON WILDLIFE: THE EXAMPLE OF THE GREAT L AKES Time Higher probability of response Sensitivity Exposure Non-persistent endocrine modulating substances, noncontinuous release Time Lower probability of response Figure 32 Illustration of the relationship between cyclical sensitivity and exposure to endocrine modulating substances As recently as last year, wildlife in the great lakes was thought to be affected by pesticides. ―Eagles, cormorants, and other birds residing near the Great Lakes have been born with crossed beaks and other cranio-facial abnormalities. Abnormal nesting behaviour has also been observed.‖(USEPA 1996) The Great Lakes are an interesting case in point. Wildlife in the Great Lakes did suffer declines and near -extinctions as a r esult of exposure to pollutants. Similar observations were made elsewhere. These observations of population -level effects on birds and other organisms exposed to persistent and bioaccumulative substances lead to regulatory action to reduce production a nd release. Many of these substances was restricted or banned, starting in the early 1970s. Much of this activity was initiated in North America, where considerable attention was focused on persistent substances released into the Great Lakes. Regulation of these types of substances has since spread to other jurisdictions. Environmental concentrations have declined since regulations were implemented. Thus, while the Great Lakes were heavily polluted with persistent organic pollutants, such as PCBs, DDT, hexachlorobenzene, mirex, dieldrin, chlordane, nonachlor, toxaphene, and PCDD/PCDFs from the 1950s through the 1960s, concentrations of these in the ecosystem peaked in the late 1960s and early 1970s and have declined in sediments, water, air and biota. The rates of these decreases differ from lake to lake but the downward trend is consistent. Consistent with decreases in concentrations of persistent organic pollutants in the Great Lakes, several bird species also have shown r eductions in effects. A case in point is the Double -crested Cormorant ( Phalacrocorax autitus), a non-native species that first colonized the Great Lakes in the early 1900s. Starti...
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This document was uploaded on 04/07/2014.

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