Ecology. Michael Begon

Ecology - Michael  Begon


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species (above) and oak species (below) from 21 500 years ago to the present. Note how the ice sheet contracted during this period.

      Source: (a) After Emiliani (1966) and Davis (1976). (b) After Davis & Shaw (2001).

      During the 20 000 years since the peak of the last glaciation, global temperatures have risen by about 8°C. The analysis of buried pollen – particularly of woody species, which produce most of the pollen – can show how vegetation has changed (Figure 1.17b). As the ice retreated, different forest species advanced in different ways and at different speeds. For some, like the spruce of eastern North America, there was displacement to new latitudes; for others, like the oaks, the picture was more one of expansion.

      We do not have such good records for the postglacial spread of animals associated with the changing forests, but it is certain that many species could not have spread faster than the trees on which they feed. Some of the animals may still be catching up with their plants, and tree species are still returning to areas they occupied before the last ice age. It is quite wrong to imagine that our present vegetation is in some sort of equilibrium with (adapted to) the present climate.

Schematic illustration of contrasting changes between fossil and current distributions of 10 species of woody plant from the mountains of the Sheep Range, Nevada. The red dots represent fossil records, while the blue lines show current elevational ranges.

      Source: After Davis & Shaw (2001).

Schematic illustration of forest species richness is positively related to forest stability in north-east Australia. (a) A predictive study of the distribution of Australian wet forest in Queensland (see inset), based on climatic conditions at 2000 random points in the current forested region. From left to right are the predicted overall distribution of forest across the region in cool-dry, cool-wet, warm-wet and current climatic conditions. (b) Forest stability, which is calculated simply as the sum of the values in the four figures in (a). (c) Current species richness in 21 forest subregions increases with estimated stability.

      Source: After Graham et al. (2006).

      APPLICATION 1.4 Global warming and species distributions and extinctions

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