Radiocarbon Date (ka BP)
FIGURE 8.6 Changes in majo'r vegetation zones in rocky areas of central-south Nevada over the past 22,000 yr, as recorded by macrofossils in the middens of packrats. Dashed lines indicate the approximate elevation of ecotones.The pygmy conifer (pinon-juniper) woodland/desert scrub transition varied from wet to dry sites, as indicated by the broad ecotone (Spaulding, 1990).
Insects are the most abundant class of animals on Earth and representatives of the group can be found in virtually every type of environment, from polar desert to tropical rainforest. Naturally this ubiquitous distribution is only possible because of the great diversity of insect types, each of which has adapted to particular environmental conditions. Of overriding significance to the distribution of an individual species is the climate, and in particular the temperature conditions, of an area. Species that are restricted to specific climatic zones are said to be stenothermic, whereas species with less rigorous climatic requirements are eurythermic; clearly the former group are of most value in paleoclimatic reconstructions and it is these on which paleocli-matic inferences are based. It would be unwise, however, to place too much faith in the presence of any particular individual insect as a climatic indicator, as insects are often extremely mobile and inevitably individuals will be blown far from their optimum habitat. More reliable interpretations can be placed on assemblages of insects that are commonly found in associations characteristic of a particular climatic regime. Such assemblages are observed today and it is reasonable to assume that similar fossil assemblages represent similar climatic conditions in the past. In this respect, the approach resembles that of palynology, but in insect studies abundance is not of major significance. Abundance is considered to be more indicative of local conditions rather than the macroclimate, which is of primary interest. It is the characteristic fossil assemblage that provides the climatic information (Coope, 1967). An excellent, comprehensive guide to the study of Quaternary insects and their use in paleo-environmental reconstruction is provided by Elias (1994).
Most paleoclimatic work utilizing insects has involved the study of fossil beetles (Coleoptera; Coope, 1977 a,b), but other insects such as flies (Diptera), caddis flies (Trichoptera), and wasps and ants (Hymenoptera) have also provided additional information (Morgan and Morgan, 1979). Insect fossils are commonly found in sedimentary deposits such as lake sediments or peat, where their chitinous exo-skeletons may be extremely well preserved. This is of great value because taxonomic differentiation of the class is primarily based on exoskeleton morphology. Fossils can, therefore, often be identified down to species level by an examination of mi-croscale features in the exoskeleton. One result of this work has been the demonstration of morphological constancy for many species throughout the Quaternary. This is considered to be evidence that they have also exhibited physiological constancy; in other words, they have not altered their ecological requirements, at least over the last 2 million years or so. Although no direct evidence of this can be obtained, the fact that fossil assemblages are often so similar to modern assemblages, in what are assumed to be similar environmental conditions, suggests that radical changes in physiological development have not occurred. This is a fundamental assumption in using fossil insects as paleoclimatic indices, as any change in their climatic tolerances would, of course, invalidate any conclusions that might be drawn from their presence. However, this problem is no different from that facing palynol-ogists or marine microfaunal analysts, and indeed entomologists have considerably more evidence for genotypic stability in their fossils than can be provided in many other branches of biology.
From a paleoclimatic viewpoint, one of the most important attributes of insects is their ability to occupy new territory fairly rapidly following a climatic amelioration. They thus provide a more sensitive index of climate variation than plants, which have much slower migration rates. Indeed, Coleoptera may occupy and abandon a new territory in response to a marked but brief warm interval, whereas there may be no evidence for such an event in the pollen record because of the lag in vegetation response time (Coope and Brophy, 1972; Morgan, 1973). In short, "this combination of sensitivity and rapidity of response to climatic changes, coupled with their demonstrated evolutionary stability, makes the Coleoptera one of the most climatically significant components of the whole terrestrial biota" (Coope, 1977a). Evidence for the extreme mobility of insect populations is found by comparing the modern distribution of a species with its fossil occurrence. For example, Tachinus caelatus has been found in glacial-age sediments in Great Britain, but today it appears to be restricted to the mountains of Mongolia where an extreme continental climate prevails (Coope, 1994). Numerous other species from glacial deposits in the United Kingdom are today found only in tundra regions of Siberia.
Thus the insects have responded to climatic change by mass migration, effectively maintaining a more or less constant environment for themselves by shifting geographically as the global climate changes of the Quaternary ebbed and flowed around them.
8.3.1 Paleodimatic Reconstructions Based on Fossil Coleoptera
A great deal of paleodimatic work using insects has been carried out in Europe, especially in Great Britain, where the temperate assemblages of Coleoptera today were replaced in the past by an alternation of boreal or polar assemblages during glacial and stadial events, and by more southern or subtropical assemblages during interglacials and interstadials (Coope, 1975a, 1977b). A large number of sites have been studied, ranging in age from interglacial to postglacial (Flandrian). Figure 8.7 illustrates the estimated average July temperature record of the last 120,000 yr, since the last (Ipswichian [= Sangamon = Eemian]) interglacial. July temperatures are assumed to be a major control on insect distribution as the northern limits of most thermophilous (warmth-loving) species more closely parallel July or summer season isotherms than isotherms of winter months (Morgan, 1973). Nevertheless, some estimate of winter temperatures can be made by considering the occurrence of species that are today characteristic of continental Eurasia. A species may be an arctic stenotherm (having a northern distribution) but it may live in continental areas where July temperatures are relatively high and winter temperatures extremely low. Bearing such factors in mind, and considering modern climate in areas where the (fossil) species are found today, it is possible to assess the annual temperature range at intervals in the past (see Fig. 8.7; Coope, 1977b).
Over the last 125,000 yr, there appear to have been three distinct periods when temperatures in central England were at least as warm, or warmer, than they are at present: the Ipswichian interglacial; the Upton Warren interstadial; and the Lake Windermere interstadial. The last interglacial was, by definition, the warmest of these episodes, with Coleopteran assemblages characteristic of southern Europe today, present in lowland England; July temperatures are estimated to have been -3 °C higher than today (Coope, 1974). Between 50,000 and 25,000 yr B.P. the climate of Great Britain appears to have fluctuated rapidly between temperate and cold continental conditions. This inference is based on the occurrence of climatically contrasting Coleopteran assemblages that follow one another quite abruptly in stratigraphic sequences. The period has been termed the Upton Warren interstadial complex, and includes one brief period (-43,000 yr B.P.) when temperatures seem to have been warmer (by 1-2 °C) than the present day (see Fig. 8.7). The duration of this interval is uncertain (indeed uncertain radiocarbon dates, close to the limit of the method, may account for the apparently rapid temperature fluctuations of this period) but it may have lasted for 1000-2000 yr, followed by a gradual fall in temperature. This cooling was accompanied by more continental conditions, as evidenced by a beetle assemblage typical of parts of Eurasia today; average February temperatures of -20 °C and July temperatures of only +10 °C seem probable. In spite of periods of relative warmth during the "interstadial complex," central England was devoid of trees all the time and there is little palynological evidence for
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