by Carl Strang
Some of the formalism developed by landscape ecologists clarifies the current status of our singing insects and their prospects for survival. From the standpoint of a given species, the landscape is viewed as patches of habitat imbedded within a matrix of non-habitat. If that matrix is hostile to the species, the habitat edge is a “hard boundary,” and unless the species is good at dispersing long distances, they will be confined to their habitat island. To different degrees that extreme may not be met if there are steppingstones or corridors that can act as acceptable temporary refuges for dispersing individuals. I suspect that such is the case with pine tree crickets, for example. These habitat specialists are common in conifer groves across the region, many of which seem too isolated for such a small, specialized insect to cross the intervening distances. Nancy Collins, a Wisconsin specialist on the subfamily, has noticed that pine tree cricket nymphs can live, at least temporarily, in non-conifer, herbaceous habitats. Edges of conifer groves thus are not hard boundaries for pine tree crickets. In other cases, river corridors and highway rights-of-way can serve as travel lanes. Thus, the little-known dispersal abilities of our various species are key to understanding their status.
Pine tree cricket
In some of the crickets and katydids which normally are short-winged and flightless, long-winged morphs occasionally appear. Roesel’s katydid, mentioned in the previous post, essentially is never long-winged in its native Europe, but commonly is so here, where the species is expanding its range. There is a tradeoff in play for such species. Long-winged variants are good dispersers, but their fecundity is reduced.
Roesel’s katydid, long-winged variant
Dispersal also can be facilitated by humans. I have seen examples, with Japanese burrowing crickets and jumping bush crickets, of individuals and small groups showing up far ahead of the front of their range expansion, in places where landscape materials are stored or sold.
Mate finding motivates some movement by singing insects. The females in general must travel to meet up with singing males. The males themselves also may need to change their position. In DuPage County, where swamp cicadas are relatively few and scattered, I have observed males frequently changing position between songs by tens of meters at a time. An Iowa study (Shaw, Bitzer and North 1982) found that sword-bearing coneheads shifted position an average of 6.2m between nights, but otherwise remained associated with their group of other males.
Swamp cicada
The singing insects whose habitat needs are met by landscape alterations for agriculture and residential areas are the abundant, widespread ones. They are easily picked out by the large number of sites marked on their maps in my singing insects guide. Dispersal is relatively easy for them as there are large habitat blocks well connected by amenable corridors. Even they can suffer local extinction as land is cleared of vegetation for buildings or roadways. As new vegetation grows into such places, the weedy species are quick to re-establish themselves, but this underlines the dynamism of the habitat patch mosaic.
Of greater interest, and greater concern, are those species whose needs are not met by human-created habitats. Here the habitat patches are only a small percentage of the landscape, and elements of patch size, patch isolation, and insect dispersal ability become critical to understanding. Theory suggests that when a habitat falls below 10-20% of the landscape, dispersal ability is expected to drop dramatically in the absence of viable corridors or steppingstones; such is certainly the case for many of our habitat specialists. A few species anecdotally are good dispersers. I have reports, or have seen myself, instances of slender meadow katydids and long-tailed meadow katydids showing up in locations remote from their respective habitats. These individuals were unusual among the small meadow katydids in being long-winged morphs; most cannot fly.
Slender meadow katydid
Patch isolation in some cases is such that successful dispersal is impossible. Distances among the few surviving sphagnum bogs, for example, are too great to be crossed by sphagnum ground crickets. Prairie cicadas can fly, but apparently are disinclined to do so far enough to matter. At one of their sites, the West Chicago Prairie in DuPage County, they remain confined to one area of mixed grasses and forbs. They have not crossed the 350 meters of grasses, small shrubs and wetlands that separate them from a similar area within the same preserve. So far, their populations have persisted in remnant prairies ranging from 1 to a few acres. Can such small populations survive long term?
Prairie cicada
That question raises another concept from landscape ecology: extinction debt. This is the idea that a species’ habitat may have been so reduced that it still may be present but is doomed to fade away to extinction because its numbers are inadequate to maintain reproductive viability. Such may or may not be the case for prairie cicadas. I suspect this is what happened to northern wood crickets (NWC) in the region. NWC were known from two forested areas in northern Indiana at the beginning of the 20th Century (Blatchley 1903). One of these was in Marshall County. Years ago I used the original survey notes and county soil map to reconstruct the presettlement landscape of the township in question.
Presettlement map of Union Township, Marshall County, Indiana. Mesic forest was the large green area east of Lake Maxinkuckee.
Blatchley found NWC in the area of mesic forest which, in 1834, was very large. By the end of the 19th century agricultural clearing would have been well under way, and today only a tiny portion of that forest remains, the rest having been converted to pasture and crops. The same is true of forested areas in Lake County, Indiana, the other area where NWC lived in the late 1800’s. Though fragments of the forests where Blatchley found NWC remain, the crickets are gone, and I have checked all the other relatively large forests in the region without finding them. This raises the disturbing question: how many other species presently in the Chicago region are in a state of extinction debt? Patch size needed to maintain a species is dependent upon the characteristics of the species, its population dynamics, and patch quality. These are unknowns for all the uncommon species.
Our two species are Linnaeus’s 17-year cicada, on the left, and Cassin’s 17-year cicada, on the right
I will close this section with a case study on the periodical cicadas in DuPage County. In pre-settlement times the county was, from the cicadas’ perspective, a matrix of prairies and wetlands with 18 forested areas of various sizes scattered throughout. Historical maps and aerial photos allowed me to trace changes in those forests from the early 19th Century to the present day. I measured the forest sizes, noting their smallest (bottleneck) extents and how those were reflected in the presence of periodical cicada choruses in the 2007 emergence. There was a clear threshold of local extinction: forests which had remained above 61ha (hectares) still had cicadas, those which had fallen below 52ha did not, even when they had grown larger subsequently. Old newspaper accounts placed cicadas in at least some of these. Three forests which fell between those sizes appeared to have been affected by isolation, a remote one lacking cicadas, and two near persisting populations having them. The human history peculiar to the county is important here. DuPage County is immediately west of Chicago. The western half of the county quickly became agricultural, with forests cut back to make room for fields and pastures. Forests lacking cicadas in 2007 were mainly in the west. The eastern half developed residential commuter communities, with forests being protected and expanded as people planted trees around their homes. The largest area with cicada choruses in 2007 was lobular in shape, the various lobes following the routes of commuter-serving railways and the towns that expanded along them, connecting several of the pre-settlement forest locations. When the cicada choruses reached their peak in the first half of June, numbers of the insects suddenly appeared in flight, crossing over highways and other hostile environments. Subsequently, small groups showed up in places remote from the concentration areas. Whether these will result in significant expansion of the species in the county remains to be seen, but this observation supports the notion that competition and population pressure produce responses by the cicadas. They have the advantages of stronger flight capabilities and better vision than other singing insect groups.
Nature Inquiries 