The Meaning of “Range”

by Carl Strang

This is the third installment of a weekly series on singing insect species that supposedly occur in northeast Illinois or northwest Indiana but which I have yet to find after several years’ field work. Today I will consider two crickets representing different groups. They have in common a certain peculiarity in their range maps. We’ll start with the melodious ground cricket.

As you can see, the shaded area on this map (from the Singing Insects of North America website, or SINA) places nearly all of Illinois within the range of this species. However, that shaded area is computer generated from the only concrete records, which are represented by the black dots. Note that there are no dots anywhere in Illinois. As far as the SINA database goes, the species never has been found in Illinois, and there are only two records from northern Indiana. Illinois is included in the range thanks to a single record from northern Missouri. The species first was described from Ohio in 1957 by Edward S. Thomas and Richard Alexander. It is very similar to the Carolina ground cricket physically, but its song is described as a more melodious trill (lacking the Carolina’s discordant overlay of tones), and its habitat is narrower, limited to bogs and marshes. Even in Ohio there are few locations. The map shows melodius all over Florida, where Thomas Walker (who runs the excellent SINA site) is located.

The next example is the prairie tree cricket.

Northeast Illinois and northwest Indiana are included in the range for this species thanks to two records for Cook County, Illinois, from 1934 and 1935. On the other hand, two records from Iowa probably would have led the computer to shade our region even without those old Cook County records. I mentioned in an earlier post this year that I am looking for prairie tree crickets in sweep samples from meadow and prairie areas, but so far have found only four-spotted, and black-horned/Forbes’s tree crickets in that sampling.

These examples underline the need to be careful in thinking about the geographic range of species. Some singing insects are relatively general in their habitat choice, or tolerant of human alterations in the landscape, or simply have been fortunate to have passed through the sieve of history unscathed. They are the common ones. Shaded areas in range maps like SINA’s represent them best. Other species are much pickier, or their lower densities have made them subject to more frequent local extinctions over time. They are best represented by dots. The best example I have encountered here is the sphagnum ground cricket, which appears indeed to be limited to the narrow confines of sphagnum moss areas. These were more ubiquitous in the broad zone which trailed the last continental glacier north, but then in southern parts became isolated in little bits here and there.

I am not removing melodious ground crickets or prairie tree crickets from my hypothetical list for our region, but until I find them I will not list them as definitely occurring here today.

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