ECOSYSTEMS NORMAL

The sensitivity of a region to acidic deposition is related primarily to its capacity to reduce or chemically neutralize acidity. If there is an ability to quickly and effectively return conditions to nearly normal, the effects of the deposition will be minimized and the ecosystem is not considered to be sensitive. However, if there is little or no capacity to reduce the acidity, the ecosystem is highly sensitive and severe damage can occur if the acidic deposition is significant. 

Water birds have not been shown to be directly affected by acidification. However, the prey of waterbirds may be of concern as these lower food-chain organisms may have elevated levels of toxic metals related to acidification of their habitat. Moreover, most water birds rely on some component of the aquatic food-chain for their high protein diet. Invertebrates that normally supply caJcium to egg-laying birds or their growing chicks are among the first to disappear as lakes acidify. As these food sources are reduced or eliminated due to acidification, bird habitat is reduced and reproductive rate of the birds is affected. The Common Loon is able to raise fewer chicks, or none at all, on acidic lakes where fish populations are reduced 37 and 5S). However, in some isolated cases, food supplies can be increased when competitive species are eliminated (e.g.. Common Goldeneye ducks can better exploit insects as food when competition from fish is eliminated). The collective influences of acidification are difficult to quantify on a specific area basis but for species that rely on a healthy aquatic ecosystem to breed, acidification remains a continuing threat in thousands of lakes across eastern North America 14). 

Water scarcity is a structural, persistent drought affecting resources and aquatic ecosystems, with implications in water quality and societal needs. Scarcity results in repeated drought episodes. While drought is a temporary (and often normally associated to climatic patterns) decrease in water resources, water scarcity occurs when water demand exceeds the water resources exploitable under sustainable conditions. 

Wind-throw and wind-snap of forest trees is a normal process in natural forests (Spurr Barnes, 1980). Runkle (1982) has estimated that, annually, as much as 1% of the area of a forested ecosystem may be blown (or fall) over and create canopy gaps. Such a figure appears to be appropriate for a range of forested ecosystems from equatorial to boreal. The canopy gaps are critical for the establishment of new species and maintaining species diversity in the forest (Woodward, 1987). 

The first step in a wildlife exposure assessment is to document the occurrence and persistence of a pesticide in the study area throughout the study duration. Several articles in this book describe the experimental designs and best practices to conduct field crop and environmental dissipation (air, soil and water) studies. This article presents methods to quantify spatial and temporal distributions of pesticide presence in ecosystems following normal application and resultant exposure of nontarget wildlife. 

Environmental risk assessment examines the potential adverse effects to ecosystems from exposure of the aquatic, terrestrial and air components. Initial assessment normally focuses on the aquatic compartment, including effects on microorganisms in waste water treatment plants. This first tier risk assessment can be extended to cover the sediment part of the aquatic compartment and the soil compartment. At higher tonnage levels, effects relevant to the food chain are evaluated, i.e., secondary poisoning. Diderich in Chapter 8 of (73) discusses the principles of EU environmental risk assessment. 

The characterisation of LAS degradation in the marine environment requires laboratory experiments, although due to the special characteristics of this compartment (e.g. its high salinity and its normally oligotrophic status) and the numerous variables that affect it, divergent results may be obtained. Marine-specific bacterial communities cannot be cultivated as a whole in standard media due to the difficulty of reproducing original ecosystem conditions where they have been... 

Concentrate can be harmful to the environment due to either its higher than normal salinity, or due to pollutants that otherwise would not be present in the receiving body of water. These include chlorine and other biocides, heavy metals, antisealants, coagulants and cleaning chemicals. Of particular concern is the effect of pollutants on delicate ecosystems and endangered or threatened species. However, with appropriate measures in place, the discharge of concentrate to surface water can remain a viable method for seawater desalination plants. 

Ecosystems may be described developmentally as young (serai, successional) or mature (climatic). In a young ecosystem, developmental stages or communities are rapidly replaced by other communities. This succession leads ultimately to the mature stage. The distinguishing characteristic of the mature, or climax, stage is that the dominant species that form the community can replace themselves thus, the community is in equilibrium with its normal environment. 

Two bioassays are employed to evaluate the effect of samples on terrestrial life forms. For gas samples, the plant stress ethylene test is presently recommended. This test is based on the well-known plant response to environmental stress release of elevated levels of ethylene (under normal conditions plants produce low levels of ethylene). The test is designed to expose plants to various levels of gaseous effluents under controlled conditions. The ethylene released during a set time period is then measured by gas chromatography to determine toxicity of the effluent. For liquid and solid samples, a soil microcosm test is employed. The sample is introduced on the surface of a 5 cm diameter by 5 cm deep plug of soil obtained from a representative ecosystem. Evolution of carbon dioxide, transport of calcium, and dissolved oxygen content of the leachate are the primary quantifying parameters. 

Despite the impression made by occasional widespread pest outbreaks such as those of the gypsy moth, severe defoliation of forested ecosystems is quite unusual. Fewer than 10% of the species listed in the Canadian Forest Survey of Lepidoptera (1, 2) exhibit periodic or occasional outbreaks. Generally, defoliation in forests is less than 7% of primary production per year P, but see. The vast majority of forest Lepidoptera are quite rare almost all of the time, and their numbers do not fluctuate to a noticeable degree. These observations suggest that some factor or factors normally regulate forest insect populations and keep defoliation at low levels. 

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