Chapter 16. Chapter 16 Graphic Content

There are two graphs. Each presents data on the number of species present at different latitudes, from the equator toward the Poles. The x-axis shows the number of species: in the top graph, the number of mammalian species, and in the bottom graph, the number of marine copepod species. The y-axis shows the latitude. In the top graph, data are presented for the numbers of species at 40° and 20° south, the equator, and 20°, 40°, and 60° north. In the bottom graph, data are presented for the equator and 20°, 40°, and 60° north.

The independent variable is latitude. The dependent variable is species number. Typically, the x-axis is used for the independent variable and the y-axis for the dependent variable. In these graphs, the variables are placed on the opposite axes. This is because the data are overlaid on a world map, which aids in identifying exactly where in the world 20°, 40°, and 60° latitudes fall. This makes it easy to see, for example, that 20° north latitude includes Central America, while 60° north latitude cuts across southern Alaska and northern Canada.

For mammalian species and for marine copepods, the largest numbers of species occur at 0°, which is the equator.

The value of having data both for mammalian species and for marine copepod species is that it enables us to see that the trend of decreasing species diversity toward the Poles holds true for a terrestrial species and for a marine species. This suggests that the phenomenon is robust and not just a quirk of one group of organisms. It would be helpful if data were presented for all species, but such data do not exist. (And, as we saw in Chapter 10, for prokaryotes and many other organisms it is difficult to even define what a species is.) Only a fraction of the world’s species have been described. In this figure, it is more valuable to include animal groups for which most species have been described. Also important is that the figure uses groups of organisms for which any bias in the proportion of total species described at the equator versus the proportion described at higher latitudes is unlikely.

There are more mammalian species than marine copepod species at 20° north latitude. A person might answer this question incorrectly because the bar representing the number of copepod species is longer than the bar representing mammalian species—but this is because the scales for the axes differ. There are more than 300 mammalian species and about 70 copepod species at 20° north.

The bars here represent exact counts of species rather than some repeated measure of a variable (such as height), which would have a variance associated with it.