Latitudinal Gradient

by Carl Strang

One of the classical challenges of community ecology is the latitudinal gradient of species diversity, species counts diminishing from the tropics to the poles. The observation is easy to make and applies across practically the entire range of species groups. Many theories have been proposed to account for this pattern, and probably some combination of factors is at play. It never had occurred to me to see whether the gradient applies across the relatively narrow latitude range of my Chicago region survey area for singing insects.

One addition to the next edition of my regional species guide will be an expanded introductory section on range extensions, along with new sections on landscape ecology and community ecology. As I drafted that material, it occurred to me that I might check for latitudinal changes in species counts.

It turns out that the pattern appears in the region’s 6 rows of counties. From south to north, median species numbers are 60, 60.5, 57, 53, 44.5 and 43.

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