Greenhouse gases are likely to change insect distribution patterns both directly, via increases in temperature and rainfall, and indirectly, via changes in the distribution of host plants. Recent research on a sample of 35 nonmigratory European butterflies showed that 63% had ranges that shifted to the north by 35 to 240 km during the 20th century, while only 3% shifted to the south. Thus for many insects, global warming has already changed range boundaries. The data appear to be robust because for most of these species, northward shifts have been shown in more than one country. Furthermore, the data appear to be robust across families, with many members of the Lycaenidae, Nymphalinae, Satyrinae, and Hesperiidae showing such range shifts. The northward shifts of the butterflies are of the same magnitude as the shift in climatic isotherms, which have moved about 120 km north as Europe has warmed by about 0.8°C.
Changes in rainfall, likely to have at least as big an impact as rising temperatures, have not been much studied. Global rainfall patterns clearly will change as a result of changes in global temperature, with many coastal areas becoming wetter and many interior continental areas becoming drier. This set of changes will affect the distribution of host plants and the insects that live on them. In addition, rainfall changes can directly affect the hatching of immatures from eggs laid in the soil, including eggs of many species of locust. Increased soil moisture increases the likelihood of locust outbreaks because it increases hatching and stimulates growth of host plants on which the locusts feed. The threat of locust plagues in new areas of the globe is therefore very real.
Margaret Davis, a paleobotanist from the University of Minnesota, showed that in the event of a CO2 doubling, beech trees, presently distributed throughout the eastern United States and southeastern Canada, would die back in all areas except northern Maine, northern New Brunswick, and southern Quebec. Of course favorable new locations would develop in central Quebec, but the trees would take a long time to colonize such areas. Presumably the animals that feed on beech trees, including insect herbivores, would suffer a severe range contraction too, though this has not yet been studied.
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