The Other Side of Range Extensions

by Carl Strang

The aspect of singing insect studies that has been of greatest general interest, and about which I have presented talks most frequently, is range extensions. Most of the several examples uncovered so far have been northward extensions of species ranges, and it is easy to jump to the conclusion that this is a product of global climate change.

The first species I found well north of where it previously had been known was the broad-winged tree cricket.

The first species I found well north of where it previously had been known was the broad-winged tree cricket.

If climate is the primary influence in these range extensions, we would expect the southern boundary of a species’ range to shift north as well. However, sticking with the example of the broad-winged tree cricket, its range has expanded south as well as north. I know that only because Tom Walker, the authority responsible for the Singing Insects of North America website, informed me that he has found them spreading down into parts of Florida where they had not been before.

Documenting an expansion northward is easier than demonstrating a disappearance in the south portion of a species’ range. It is easier to prove a positive than a negative. Also, when one has limited time and financial resources, it is not possible to travel south and conduct the necessary regional surveys. So, what to do?

The best idea I have had so far is to identify species whose southern range boundaries occur in my region, and make a point of documenting their presence and estimating their abundance from year to year. There are four best candidates.

The dog day cicada Tibicen canicularis has the northernmost southern range boundary. It is mapped down into the very northern edges of Indiana and Illinois, but I have found it a little farther south, at Braidwood Dunes in Will County, Illinois, and in southern Marshall County, Indiana. My notes suggest that its numbers may vary considerably from year to year in the latter location.

Dog day cicada

Dog day cicada

The broad-winged bush katydid Scudderia pistillata is mapped a little farther south, but I have found it infrequent in DuPage County, Illinois, and its season of activity seems to conclude sooner here than it does farther north. So far my surveys have turned it up in Lake County, Illinois, as well, but not farther south.

Broad-winged bush katydid

Broad-winged bush katydid

The striped ground cricket and sword-bearing conehead are abundant in the region. Their southern range boundaries are mapped about half a state south, but that is the same distance as a number of the northern range shifts I have observed. If entire ranges are shifting, one would expect them to be more scarce.

Sword-bearing conehead

Sword-bearing conehead

This is yet another topic of interest I am keeping in mind while surveying the region’s singing insect species.

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