6.11.2. Climatic change and insect distributions
The modeling techniques above lend themselves to back-tracking, allowing reconstruction of past species distributions based on models of previous climate and/ or reconstruction of past climates based on postglacial fossil remains representing past distributional information. Such studies were based originally on pollen remains (palynology) from lake benthic cores, in which rather broad groups of pollens, with occasional indicative species, were used to track vegetational changes through time, across landscapes, and even associated with previous climates. More refined data came from preserved ostracods, beetles (especially their elytra), and the head capsules of larval chironomids. These remnants of previous inhabitants derive from short-lived organisms that appear to respond rapidly to climatic events. Extrapolation from inferred bioclimatic controls governing the present-day distributional range of insect species and their assemblages to those same taxa preserved at time of deposition allows reconstructions of previous climates. For example, major features from the late Quaternary period include a rapid recovery from extreme conditions at the peak of last glaciation (14,500 years ago), with intermittent reversal to colder periods in a general warming trend. Verification for such insect-based reconstructions has come from independent chemical signals and congruence with a Younger Dryas cold period (11,400-10,500 years ago), and documented records in human history such as a medieval 12th century warm event and the 17th century Little Ice Age when “Ice Fairs” were held on the frozen River Thames. Inferred changes in temperatures range from 1 to 6°C, sometimes over just a few decades.
Confirmation of past temperature-associated biotic changes leads to the advocacy of such models to predict future range changes. For example, estimates for disease-transmitting mosquitoes and biting midges under different climate-change scenarios have ranged from naïve estimates of increased range of disease vectors into populated areas currently disease-free (where vectors actually already exist in the absence of the virus) to sophisticated models accounting for altered development rates for vector and arbovirus, and altered environments for larval development. Future levels of predicted climate change remain unclear, allowing certain policy makers to deny its existence or its biotic significance.
However, by the turn of the millennium Europe had warmed 0.8°C in the 20th century and realistic expectations are for a further increase of between 2.1 and 4.6°C mean global change in this century, along with commensurate variation in other climatic factors such as seasonality and rainfall. That predicted changes in distributions of insects are occur- ring is evident from studies of individual species, but the generality of these examples has been unclear. However, a study of species of western European butterflies (limited to non-migrants and excluding monophagous and/or geographically restricted taxa) is quite conclusive. Significant northward extension of ranges is demonstrated for many taxa (65% of 52 species), with some stasis (34%), and retraction south from an earlier northern limit for only one species. Data for the southern boundary, limited to 40 species, revealed retraction northward for 22%, stasis for 72%, and southward extension for only one species. The subset of the data for which sufficient historical detail was known for both northern and southern boundaries comprised 35 species: of these 63% shifted northward, 29% were stable at both boundaries, 6% shifted south- wards, and one species extended both boundaries. For the many species whose boundaries moved, an observed range shift of from 35 to 240 km in the past 30–100 years coincides quite closely with the (north) polewards movement of the isotherms over the period. That such range changes have been induced by a modest temperature increase of <1°C surely is a warning of the dramatic effects of the ongoing “global warming” over the next century.