Читать книгу Ecology - Michael Begon - Страница 28

Evidence of changes in vegetation that followed the last retreat of the ice provides clues about the likely consequences of global warming associated with the continuing increases of carbon dioxide and other greenhouse gases in the atmosphere. Warming of 0.7°C in average global mean surface temperature was recorded between 1970 and 2010. Future climate will depend on warming still to occur but as a result of past anthropogenic emissions, future emissions, natural climate variability and whether or not major volcanic eruptions occur. Models that take into account various scenarios indicate that, relative to 1850–1900, global surface temperature increase by the end of the 21st century is likely to exceed 2°C (IPCC, 2014). But note that the scale of current temperature change is dramatically different from that which has occurred since the last ice age. Postglacial warming of 8°C over 20 000 years, or 0.04°C per century, must be compared with the current rate of global warming of about 1.75°C per century. It is disturbing to note that changes in the vegetation failed to keep pace even with a rise of 0.04°C per century. Projections for the 21st century require range shifts for trees at rates of 300–500 km per century compared with typical rates in the past of 20–40 km per century (and exceptional rates of 100–150 km). It is striking that the only precisely dated extinction of a tree species in the Quaternary period, that of Picea critchfeldii, occurred around 15 000 years ago at a time of especially rapid postglacial warming (Jackson & Weng, 1999). Clearly, even more rapid change in the future could result in extinctions of many additional species (Davis & Shaw, 2001).

The Pleistocene ice ages undoubtedly eliminated biota from many mid‐ to high‐latitude areas of the planet. However, in the case of alpine species of the Pyrenees, Himalayas, Andes and Southern Alps, evidence is accumulating that glaciation may sometimes have promoted allopatric speciation by severing continuously distributed populations along the length of mountain ranges (Wallis et al., 2016). In the Southern Alps of New Zealand, for example, comparative phylogeographic analysis, based on mitochondrial and nuclear DNA, has revealed a phylogenetic split 2 mya (the date of the first major glacial epoch), found in both insect and bird biotas, that bisects each into northern and southern assemblages (Figure 1.20).

Figure 1.20 A phylogenetic split bisecting insect and bird biotas into northern and southern assemblages in New Zealand. (a, b) Glaciation of the alpine region of the South Island of New Zealand which is hypothesised to be responsible for fracturing the ancestral biota into north and south lineages that have subsequently diverged. Green shading represents lowlands. (c–f) North–south phylogeographic breaks for alpine birds and insects with dendrograms showing phylogenetic relationships and approximate divergence times. Sampling sites are shown as circles, the shading shows approximate taxon ranges, and the dotted square represents a particularly highly glaciated narrow alpine neck.

Source: After Wallis et al. (2016).