Climate Surprises

If global climate change causes local warming, some butterfly larvae may be able to develop and survive at higher latitudes or elevations (Brakefield 1987). Climate change could allow northerly expansion into regions where prechange temperatures were too low for larval development. However, climatologists are predicting that climate change will alter the frequency of extreme events or climate "surprises" (e.g., harsh freezes or drought) in addition to changing mean conditions (e.g., average temperature or precipitation) (Overpeck 1996, Schneider and Root 1996).The occurrence of surprises may be more important in determining range limits than mean climate because surprises can kill individuals and increase the probability of population extinction (see also Kingsolver and Watt 1983).There-fore, surprises, rather than site suitability determined by degree days, may control the potential occupancy of habitat on the edges of a butterfly species' range.

For example, extreme cool weather in Michigan led to population declines of northerly expanding species of Papilio in 1992 (Scriber and Gage 1995). Several southern Papilio species had colonized Michigan in a warming period extending from 1986 to 1991, but extreme cold weather and a late frost in 1992 led to large larval mortality and population die-back. The effects of this extreme event overrode the impacts of the warming trend with respect to Papilio persistence in the region.

Topography can play an important role in mediating these regional climate surprises (Wellington et al. 1999). The ends of topographic gradients can experience qualitatively different weather, and some habitats within a region can be protected from climatic extremes. At the ends of a gradient or in protected habitat pockets, individuals may be buffered from the mortality effects of extreme temperatures or catastrophic weather events. Protected populations could then recolonize the entire landscape when favorable conditions return. On a regional or landscape scale, we might observe exaggerated or more frequent population dieback and expansion under climate change, and complex topography may play an important role in protecting species from extinction under altered weather regimes.

For example, the Bay checkerspot butterfly, Euphydryas editha bayensis, inhabits topographically diverse areas of native grassland in California. Regular shifts in butterfly density on various slope types were observed in the mid-1980s; the occurrence of individuals on warmer slopes increased when the total population size grew, and the population retreated to cool slopes when total abundance declined (Murphy and Weiss 1988). Larval survivorship differences by slope type drive this spatial expansion and contraction, suggesting that cool slopes offer an important refuge in years of extreme heat or drought. If the frequency of these extremes increases, the availability of cool slopes will be critical to the persistence of E. e. bayensis.

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