Gauging local rarity. Since 1982, we have amassed more than 3200 element occurrences (EO's) for the conservative species that inhabit the CW region. We used this database to assign objective, ordinal “rarity” ranks to each species. To facilitate comparisons between sites and regions, each conservative species has been assigned an objective “rarity coefficient” reflective of its local, and, in some cases, regional rarity. Species assigned rarity values of 1 are known to occur on many site within the Chicago region and are presumed to be secure (e.g., Papaipema silphii is known from 33 sites). At the other extreme, species with rarity coefficients of 5 are known to inhabit few sites and are considered to be potentially imperiled. Species considered to be globally rare (e.g., Papaipema eryngii) are included in this category, irrespective of their local abundance.
Controlling for sampling bias. Our sampling efforts over the years have varied appreciably. In the beginning, we focused very heavily on butterflies. We soon added Papaipema moths. Next came Schinia moths, leafhoppers, froghoppers and some plant hoppers. We didn’t begin to seriously consider micro moths until around 2001. This has resulted in significant disparities in sampling effort, as seen in Table 1. To control for uneven sampling bias in our data, we use weighted element occurrence criteria to assign rarity values. The EO ranges chosen for each taxonomic class were chosen to maximize the correlation between mean taxonomic class values and search efforts for the four groups of species shown in Rarity criteria ( r2 = 0.99; p = 0.001; n = 4).
Appling the criteria shown in Table 1, a butterfly species recorded on 15 sites is assigned a rarity coefficient of 4 (very uncommon), whereas a micro moth known from the same number of sites is assigned a lower rarity coefficient of 2 (common). This disparity in rankings reflects the fact that the “relative frequency of occurrence” (percentage of searched sites that were inhabited by each species) for the butterfly was roughly half that of the micro moth. |
Problematic species. We have chosen to reserve judgement as to the rarity of 44 species. These species can be placed in one or more of five categories.
1). We continue to encounter new species. We have recorded at least 150 within the past few years, a couple dozen of which appear to be remnant-dependent. Many seem to be scarce in our region but we are reluctant to draw this conclussion because we have searched so few sites since we became aware of their existance. Examples would include the planthoppers Delphacodes parvula and Prokelisia crocea, and the micro moth Haimbachia squamulella.
2). Our heavy focus on butterflies, moths and leafhoppers has resulted in the undersampling of a few species from other groups. Examples would include the kaydids Orchelimum campestre, Orchelimum concinnum, Orchelimum delicatum and the cone head Neoconocephalus lyristes.
3). We question the taxonomic status of a few species (e.g., Satyrodis eurydice fumosa, Chlorotettix borealis, Schinia oleagina). If these are in fact real species, our ability to separate them from closely related species is questionable.
4). Some species closely resemble one or more sibling species and are easily overlooked in large samples. Thes species have quite possibly been overlooked on one or more sites.
5). Finally, our experience, as well as that of others, leads us to suspect that prairies and sand savannas may represent marginal or "fringe habitats" for several of our putative “rare species”. These species may be much more prevalent (common) in minimally surveyed habitats such as fine-soiled savannas and woodlands (e.g., Chlorotettix dentatus and Chlorotettix luscorius) and wooded floodplains (e.g., Elymana acrita and Elymana inornata).
To guard against inflated rarity estimates for these problematic species, we have assigned each a default values of “3”, pending additional information. Note that default values are shown in italics.
Nearly all of the prairie and savanna ecosystems that once florished east of the Mississippi River have been destroyed. Not surprising , many of those species that require these habitats are thought to be doing poorly throughout much of their ranges. Nintey-nine of the prairie and savanna-requiring species on our list have been determined to be imperiled in one or more states, as indicated in Table 1. The following symbols are used to denote levels of protection throughout USFWS Region 3 (Illinois, Indiana, Iowa, Ohio, Michigan, Missouri, Minnesota, Wisconsin): E = endangered, T = threatened, SC = special concern, SI = special interest. W = watch species, V = vulnerable, R = rare, X = extirpated.
Distribution of rarity among taxa. A review of E & T insect listings for the Midwest suggests that butterflies may be inordinately vulnerable to the high extinction rates associated with extreme fragmentation (see "regional status" assessment for butterflies). In disagreement with this apparent trend, we found species rarity to be evenly distributed among the (1) butterflies, (2) Homoptera, (3) moths and (4) “other species” (W=0.93), as shown in Table 2. This pattern holds for sand-inhabiting and sand-only species as well. |
CW status. Nearly 50% of the conservative species we list are known to occur on fewer than 9 sites within the CW region; many of these rare species are, in our opinion, locally (CW) imperiled. Taking into account our sampling thoroughness, our ability to capture each species (some are much more difficult to capture than others), and their status in other regions, we have taken the liberty to designate 189 of 390 extant species as Endangered (96), Threatened (55), or of Special Concern (38) in the CW region. Recognizing that some of these species could yet be found to be less scarce than we currently believe to be the case, we encourage comments from those with information to share (email button).
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