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FIGURE 4.4 Latitudinal relationship between taxonomic richness (left) and geographical range size (right) for all 9,754 bird species at three different taxonomic levels: species, family, and order. Only breeding distributions were included, and range sizes were measured over dry land and averaged across all species, families, or orders occurring at a given latitudinal band.

FIGURE 4.4 Latitudinal relationship between taxonomic richness (left) and geographical range size (right) for all 9,754 bird species at three different taxonomic levels: species, family, and order. Only breeding distributions were included, and range sizes were measured over dry land and averaged across all species, families, or orders occurring at a given latitudinal band.

nomic levels, but the rates of latitudinal decline are less rapid, because many of the bird orders and families that evolved in the tropics have representatives that radiated into the temperate and arctic zones. In contrast, few evolutionary radiations in the temperate (or arctic) zones have spread back into the tropics. A major future challenge is to examine how the pattern of parasitic helminth diversity maps onto this pattern of host diversity. Our null expectation is that the two patterns should be concordant, but the high levels of host species diversity per order (and per family) in the tropics would suggest we are likely to see more generalist parasites (using closely related host species) in the tropics and more specialist species in the taxonomically poorer temperate and arctic zones.



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