Editor’s Note. This essay was subsequently published as:
Dillon, R.T., Jr. (2019d) Unlocking the Keystone State. Pp 219 - 222 in The Freshwater
Gastropods of North America Volume 4, Essays on Ecology and Biogeography. FWGNA Press, Charleston.
The Commonwealth of Pennsylvania spans every aquatic habitat that one might characterize as "northeastern," across the Delaware, Chesapeake, Ohio, and Great Lakes drainages, both the glaciated and the not. The Keystone State also includes two large and important cities, Philadelphia and Pittsburgh, each with a fine natural history museum. The diverse waters of Pennsylvania have been sporadically but professionally surveyed for almost 200 years.
In 2008 our colleagues Ryan Evans and Sally Ray published a thorough review of museum holdings in Pennsylvania freshwater gastropods, not just at the Academy of Natural Sciences in Philadelphia and the Carnegie Museum in Pittsburgh, but through the electronic databases of 9 other institutions as well (1). Perhaps not surprisingly, they found records of an impressive 63 species.
Now in the most recent American Malacological Bulletin, Evans and Ray have published the results of the first modern survey of The Keystone State, "Distribution and environmental influences of freshwater gastropods from lotic systems and springs in Pennsylvania, with conservation recommendations (2)." The authors sampled 398 sites selected to cover the range of USGS "hydrologic units" encompassed by the state (3), measuring water chemistry variables and extracting a variety of landscape variables using GIS techniques. And the number of species they have confirmed by field collection was ... 37.
Has there been some catastrophic extinction? Almost as alarming as the complete absence of 26 specific nomina from Evans and Ray's field survey were the details of their Table 1, which reported 7 of the 37 species actually recovered at but single sites, of the 398. Has a meteor smashed into the Keystone State in the last 200 years, leaving no trace but the bleached shells of 52% of the freshwater gastropod fauna?
Of course not. We must not overlook the fact that Evans and Ray focused their fieldwork almost entirely upon wadeable streams and springs, excluding marshes, ponds and lakes, and gave very little coverage to large rivers. And natural lakes and ponds are not common in Pennsylvania in any case; the Erie/Ontario drift and lake plains ecoregion just barely nips the northwest corner of the state.
So downloadable from Note (4) below is a spreadsheet listing the 63 freshwater gastropod species that Evans and Ray documented from Pennsylvania in 2008, ranked by the number of sites at which they were recovered by the field survey of 2010. The 26 missing species are listed at the bottom, with number of sites = 0.
Subtracted in Column D are 13 specific nomina with taxonomic problems, leaving 50 species I wouldn't question. Then in Column E I have listed 17 species as "Northern Lentic" - primarily characteristic of lakes, ponds, and marshes, becoming much more common north of Pennsylvania. This subset includes 11 of the 26 species missing from Evans and Ray's 2010 field survey, and 3 of the species collected at but single sites.
Column F subtracts five species for "other sampling problems" as noted by Evans and Ray themselves, and Column G subtracts six introduced species. The bottom line seems to suggest that just two Pennsylvania freshwater gastropod species may warrant conservation concern if viewed from a larger perspective - Lioplax subcarinata (5) and Gillia altilis.
Think Continentally, Act Regionally. It is clear that the conservation implications of the data collected by Evans and Ray can only be interpreted in the context of the larger freshwater gastropod faunas north, south, and west. But it is equally clear that the field survey that brought us these marvelous data was funded by the Pennsylvania Department of Conservation and Natural Resources, an organization with no mandate outside the state lines. Evans and Ray and the PaDCNR are to be highly commended for this effort. If the FWGNA project can only be built one stone at a time, they have contributed a key.
Notes
(1) Evans, R. R. & S. J. Ray (2008) Checklist of the freshwater snails (Mollusca: Gastropoda) of Pennsylvania, USA. Journal of the Pennsylvania Academy of Science 82: 92-97. [PDF]
(2) Evans, R. R. & S. J. Ray (2010) Distribution and environmental influences of freshwater gastropods from lotic systems and springs in Pennsylvania, USA, with conservation recommendations. Am. Malac. Bull. 28: 135-150. [PDF]
(3) The EPA "Surf your Watershed" website lists 58 eight-digit HUCs for Pennsylvania: Surf Pennsylvania
(4) Download an excel spreadsheet analyzing Evans and Ray's (2008, 2010) freshwater gastropods of Pennsylvania. [FW-gastropods-PA.xls]
(5) Evans and Ray "did not feel that adequate survey data were available to give a conservation status recommendation for Lioplax subcarinata."
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