Gauging local rarity. The protection of vulnerable species will hinge on our ability to identify those species most in need of protection. To accomplish this, the relative vulnerability of each species must be established (Diamond 1976, Kirchhofer 1997, Beissinger et al. 2000, Noss 2000). We have amassed more than 3200 records (element occurrences) for conservative species since 1982. We used this database to assign each species an objective rarity coefficient (RC) reflective of its local rarity. To accomplish this, it was necessary to control for sampling bias.
Controlling for sampling bias.Our total search efforts, defined here as the number of sites on which surveys were conducted, varied among the taxa examined. Whereas butterflies and Papaipema moths were surveyed on most sites, all other taxa were studied less frequently. Survey efforts for four groups, (1) butterflies and Papaipema moths, (2) Homoptera, (3) other macro moths and (4) other species (dragonflies, damselflies, micro moths grasshoppers, katydids, walking sticks and beetles) were limited to 72, 62, 48 and 37 sites respectively. We adjust for this sampling disparity by calculating the proportion of searched sites that were found to support each species (species occurances/sites surveyed). Based on this calculation, each species was assigned a discrete ordinal rank or “rarity coefficient”, ranging from 1 to 5. Species assigned RCs of 1 (RC1) were found to inhabit at least 41% of the sites searched (for that species) and are presumed to be currently secure. At the other extreme, species with RCs of 5 (RC5) were found to inhabit 10% or fewer of the sites surveyed and are considered to be imperiled. Species found to occur on 11 - 20%, 21 – 30% and 31 – 40% of those sites searched were assigned intermediate rarity coefficients of 4, 3 and 2, respectively.
Using this approach , 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 46 species. These species can be placed in one or more of 6 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 conclusion because we have searched so few sites since we became aware of their existence. 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 katydids Orchelimum campestre, Orchelimum concinnum, Orchelimum delicatum and the conehead 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. These species have quite possibly been overlooked on one or more sites.
5). We included 15 thrips species based solely on published host plant data, and have no knowledge as to their current status.
6). 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.
Midwestern status. Nearly all of the prairie and savanna ecosystems that once flourished 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. Ninety-nine of the prairie and savanna-requiring species on our list have been determined to be imperiled in one or more Midwestern states. The following symbols are used to denote levels of protection throughout US Fish & Wildlife Service 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” in our region (W=0.93), as shown in Table 1. 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 cwinsect).
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