Adult Flight

Temperature and weather also affect the physiology and activity of adult butterflies. Mating, feeding, and oviposition require flight, and flight is strongly affected by ambient temperature and solar radiation. Butterflies thermoregulate to achieve body temperatures within a small thermal window for flight (Clench 1966, Watt 1968, Heinrich 1974, 1986, Pivnick and McNeil 1987). Individuals orient themselves toward the sun for maximal absorption of solar heat when their body temperatures are below the thermal window and, either by basking or by using the reflectivity of their wings, elevate their body temperatures (Douwes 1976, Kingsolver 1985). (Several butterfly species also generate heat by shivering; e.g., Kammer 1970.) Above the thermal window, butterflies orient away from the sun to minimize additional thermal load. Thermal stress occurs when ambient temperatures are above suitable body temperatures, and body temperatures too low for flight can occur during periods of cloudiness or cooling by convection (i.e., wind) (Digby 1955). The number of days or hours available for flight during a flight season is inversely related to the number of extreme temperature days, the number of cloudy days, and the frequency and duration of weather systems.

For those species that spend an approximately constant portion of their life laying eggs, total available flight time directly equates to total reproductive output. Females of many species spend a majority of their flight time searching for suitable oviposition sites (Wik-lund and Persson 1983). For example, studies on Colias populations showed that weather can constrain fecundity to only 20 to 50% of potential fecundity, suggesting that poor weather leads to a depressed population size in the following generation (Kingsolver 1983). Failure to lay a full complement of eggs as a function of limited sunny days also drives population fluctuations in the British butterfly, Anthocharis cardamines (Courtney and Duggan 1983). In A. cardamines, the number of eggs deposited per season is directly proportional to the number of sunshine hours. Therefore, climate change could lead to either population increases or declines depending on whether climate change limits or expands flight time (i.e., searching and egg-laying time) and, therefore, reproductive output.

Climate change also could impact butterfly dispersal and migration. First, by changing the total time available for adult flight, changes in weather could depress the frequency with which individuals colonize unoccupied patches and disperse to other established populations (i.e., frequency of gene flow). Alternatively, increased flight time due to decreased frequency of cold, very hot, or cloudy days could lead to more frequent dispersal events. Second, climate and weather changes could impact seasonal migratory species (Johnson 1969, Baker 1978). Research on migration in other insects suggests that migratory species can be blown off course by extreme weather events (Southwood 1981, Johnson 1995, Dingle 1996). Butterflies, however, may avoid flight during strong winds and heavy weather or fly near the ground to maintain control of their flight path (Williams 1930,Taylor 1974, Gilbert and Singer 1975).To the extent that the former is true, increased frequency of extreme events under climate change could lower the number of individuals that successfully arrive at the migration destination. In cases where the latter pertains, more numerous extreme events could slow or delay migration time. Migratory species are also susceptible to climate changes in multiple habitats along their flight path (Malcolm 1987). The conservation of migratory species in the face of climate change will require cooperative efforts in several locations.

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