Largescale Ecological Change Effects Of The Pleistocene

The ecological relations of extant (i.e., existing) insects are assumed to have been the same in the past as they are now; if different assumptions are made, they must be explained. The drastic climatic changes since the end of the Tertiary and especially during Pleistocene glaciations profoundly

FIGURE 3 Transberingian and arctoalpine disjunctions in circumpolar Noctuidae of the genus Xestia; the Old World X. speciosa is represented in western North America by the subspecies aklavicensis; the origin of the eastern North American X. mixta is thought to predate the Wisconsin glaciation. [From Mikkola, K., Lafontaine, J. D., and Kononenko, V. S. (1991). Zoogeography of the Holarctic species of the Noctuidae (Lepidoptera): Importance of the Beringian refuge. Entomol. Fenn. 2, 157-173.]

FIGURE 3 Transberingian and arctoalpine disjunctions in circumpolar Noctuidae of the genus Xestia; the Old World X. speciosa is represented in western North America by the subspecies aklavicensis; the origin of the eastern North American X. mixta is thought to predate the Wisconsin glaciation. [From Mikkola, K., Lafontaine, J. D., and Kononenko, V. S. (1991). Zoogeography of the Holarctic species of the Noctuidae (Lepidoptera): Importance of the Beringian refuge. Entomol. Fenn. 2, 157-173.]

impacted insect distributions. The north polar ice expanded south, leading to a southward displacement of zonal biomes in the Holarctic. The tops of major mountains further south also acquired ice caps. The Silvaea and its fauna were driven into southern refugia, whereas cold-adapted oreotundral insects became established in their former place. The process was reversed when the ice began to retreat. The zonal biomes and their associated biota then shifted again northward; they probably continue to do so today.

The essentially north-south orientation of major mountain ranges in North America facilitated the displacements. In Europe, where major mountains run mainly from east to west, there remained an ice-free corridor between the polar ice front and the glaciated Alps, inhabited by a mixed fauna of northern and alpine origin, respectively. Many animals were driven further southwest, toward the Pyrenees, or southeast into the mountains of the Balkan peninsula. About 18,000 years ago, the ice began to retreat and cold-adapted insects followed it. There was a partial faunal exchange, and a number of fracturing of ranges, or disjunctions, resulted. Today, representatives of several insect orders exhibit boreoalpine or boreomontane (or arctoalpine and arctomontane, respectively) disjunctions (Fig. 3), but only rarely has this led to perceptibly divergent evolution, or even speciation.

Because of European topography, Pleistocene refugia of the south-retreating Silvaea and its fauna were mainly on the three large Mediterranean peninsulas. Many of the present central European species can clearly be assigned to one particular refugium because their ranges tend to coincide and occupy all of the former refugium, even though postglacial climate change and human impact strongly fragmented the Mediterranean deciduous forests. In contrast, at the northern range limits the individual species returned, variably far into once devastated areas. Postglacial recolonization by European insects was apparently fast because not all insects were affected by barriers like the English Channel, which formed about 8000 years ago, or the Baltic Sea straits separating Jutland from Scandinavia, which definitely formed some 6400 years before the present. The foregoing scenarios, initially inferred from distribution patterns, were later backed up by fossils, especially well-sclerotized and easily preserved beetles. Today, molecular genetic studies in a new line of research, phylogeography, provide support to these historical reconstructions.

The binding of much water as ice during the glacial periods lowered the sea level by about 100 meters during the last glacial period, which made important land bridges available. Tasmania was connected to Australia, which in turn established contact with New Guinea, which itself had ties with the Oriental region. The latter included a large continuous landmass, the Sunda plate, where numerous separate islands remain today. Japan was connected to the Asian, and England to the European mainland.

The Bering bridge connected East Asia and western Alaska, which were covered by tundra. Thus, there is a fair number of shared species or pairs of sibling species in East Asia and northern North America, mainly among tundral insects. Numbers of terrestrial as well as aquatic insect species are of circumpolar distribution (Fig. 3). Ecological demands seem to mostly prevent a southward spread of these northern species.

0 0

Post a comment