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<br />3 <br /> <br />tions in species populations of many plants and animals on a wide variety <br />of sites (94), upon which changes due to weather modification will be <br />superimposed. <br /> <br />~ <br /> <br />This conclusion should not be confused with the fallacy that, be- <br />cause the change anticipated from weather modification is smaller :than <br />the normal variabili ty of weather, weather modification will have 11 ttle <br />or no biological effect. It has been adequately shown that, exceplt .for <br />some exceptional circumstances in particularly harsh climates, the long- <br />term structure of plant and animal commurdties is a response to aV4arage <br />climatic conditions, not to isolated events. An increase of 10% ill mean <br />annual precipitation will inevitably lead to an adjustment in comm1LUlity <br />structure over a period of time, even if the normal year-to-year v,ariation <br />in rainfall is much greater, say 30%. <br /> <br />This adjustment will normally take place more slowly in a reg:lon <br />of highly variable weather than -in one of relatively uniform weath,er from <br />year to year (112). Likewise, sensitivity to deliberate precipitation change <br />is likely to be greatest in semiarid climates, least in humid climates, <br />and intermediate in truly arid regions (113). <br /> <br />SPECIFIC ANTICIPATED EFFECTS OF WEATHER MODIFICATION <br /> <br />PopuLations of PLants and AnimaLs <br /> <br />Organisms will in general respond to increased moisture as individual <br />species,.not as communities. In mountainous areas, there will not be a <br />uniform downward migration of biological communities under the influence <br />of artificially increased precipitation. Rather, at least some plants and <br />animals will become regularly associated with species among which they were <br />not commonly found in the past (110). <br /> <br />Changes in vegetation composition and structure will mostly be <br />the result of changes in abundance of species already present in the <br />area. Invasions by species previously unable to survive may occur, <br />but such invaders are unlikely to build up to spectacular numbers. The <br />appearance of a major. invasion could be created, however, by multiplica- <br />tion of undesirable species previously present in such low numbers as to <br />be relatively unnoticeable (111). <br /> <br />. <br /> <br />Major or catastrophic changes in'insect abundance or in incidence <br />of plant diseases are not. likely from the kinds of weather modification <br />now visualized. In part this is because most insect populations and <br />plant pathogens are more directly affected by temperature and humidity <br />than by precipitation. Outbreaks of a sort that are now occasionally <br />triggered by unusual weather sequences may possibly be more frequent <br />after weather modification (49). Weather modification is not likely to <br />be very useful even as an auxiliary measure in pest control programs (51). <br />