We have confirmed that 33 species of gastropods inhabit the rivers, ponds, lakes, and streams of our Great Plains study area: Kansas, Nebraska, South Dakota, and North Dakota combined. The complete list is reported in Table 1 [pdf] [excel], subdividing by state, each species ranked by “incidence,” the number of sites at which it was collected.

> Continent-Scale Biogeography

Our data suggest that the five most common species of freshwater gastropods in the Great Plains are Physa acuta, Helisoma trivolvis, Physa gyrina, Lymnaea elodes, and Gyraulus parvus.  This result is strikingly different from our 17-state Eastern synthesis: Physa acuta, Campeloma decisum, Menetus dilatatus, Ferrissia fragilis, and Ferrissia rivularis.  Species two, three, and four on our Eastern list seem nearly to disappear west of the Mississippi River, just 5 records of Campeloma, 3 records of Menetus and 2 records of F. fragilis occurring in our entire four-state Great Plains database.  Meanwhile Lymnaea elodes pops up from number 41 (of 107) on the Eastern list to fourth place in the Great Plains.

Five species new to the FWGNA project were encountered in the Great Plains: Lymnaea cockerelli, Physa jennessi, Promenetus umbilicatellus, Gyraulus crista, and Pleurocera potosiensis.  None of these species is common, but two are widespread; P. jennessi and L. cockerelli were observed in three states.  Promenetus umbilicatellus and G. crista were only observed in North Dakota, and Pleurocera potosiensis (an element of the Ozark/Ouachita fauna) only in extreme SE Kansas.

By areal extent, the four-state Great Plains region under study here is the largest of the eight regions thus far surveyed by the FWGNA Project.  Yet by freshwater gastropod species richness, it is the smallest.  Figure 2 compares the relative size of the FWGGP study area (308,000 square miles) to the 144,000 square mile area of the Ohio Drainage (FWGO) and the 58,000 square mile area of the Tennessee/Cumberland drainage (FWGTN).  Combining subspecies under their parents, the total species richness of the FWGO = 66, FWGTN = 54 and FWGGP = 33.

Figure 2.  Venn diagram showing the freshwater gastropod species shared among the Great Plains (FWGGP), the Ohio drainage (FWGO), and the Tennessee/Cumberland drainage (FWGTN). The size of the rectangles is proportional to the geographic area under study.

Similar trends have long been remarked in almost all elements of the biota of North America, both terrestrial and aquatic.  For plants and terrestrial animals, hypotheses to explain the lower biodiversity one observes as one travels westward from the Mississippi River toward the Rockies have generally focused on the reduction of precipitation.  Such effects could only be indirect for the freshwater fauna but cannot be ruled out entirely.   Extreme temperature fluctuations are also occasionally mentioned by biogeographers studying the terrestrial diversity of the Great Plains, but again, the freshwater fauna should be relatively protected.  

We suggest that the primary reasons for the reduced biodiversity of the freshwater gastropod fauna of the Great Plains are not environmental, but rather historical.  Two tightly linked, coequal factors seem to be involved.

First notice in Figure 2 that the Ohio drainage hosts 17 unique species, the Tennessee/Cumberland 12, and the Great Plains only 6.  None of those 6 species is endemic to the Great Plains.  The four pulmonates (Lymnaea cockerelli, Physa jennessi, Promenetus umbilicatellus, and Gyraulus crista) are all widespread further north and west.  Pleurocera potosiensis is very common in the Ozark/Ouachita highlands of Missouri and Arkansas.  Melanoides tuberculata is an invasive species, native to the Old World, introduced into the American southeast (FWGSC, FWGGA) but not spread, as yet, into the Ohio or Tennessee/Cumberland drainages.

The Late Cretaceous map of North America shows a “Western Interior Seaway” covering most or all of the modern states of Kansas, Nebraska, and the Dakotas, separating the island of “Appalachia” (Missouri and eastward) from “Laramidia” (Rockies and westward).  Sea levels fluctuated across the Great Plains over the next 30 million years, not ultimately receding until the end of the Eocene.

The freshwater biota evolves slowly.  Paddlefish strain plankton from the waters of the American interior even today, as sturgeon root bottom sediments for the larvae of dragonflies. Dillon and Robinson (2009) have reviewed evidence suggesting that the pleurocerid snails of the Southern Appalachians have not speciated since their origin in the Paleozoic.  So one of our co-equal hypotheses for the surprisingly low freshwater gastropod species richness of the Great Plains is that not enough time has passed since the seas have receded for endemic freshwater gastropod diversity to have evolved.

Now notice in Figure 2 that the Great Plains share just 16 + 2 = 18 species with the Tennessee/Cumberland, and 16 + 9 = 25 species with The Ohio, while The Ohio and The Tennessee/Cumberland share 16 + 24 = 40.  Those 24 species, demonstrating dispersal capabilities sufficient to spread across the Ohio, Cumberland, and Tennessee regions in their entirety, but not apparently able to extend their ranges into the Great Plains, are almost entirely prosobranchs.

We suggest that the absence of landform diversity is coequal with the absence of time as an hypothesis to explain the reduced diversity of the freshwater gastropod fauna of the Great Plains.  The seas invaded the middle of North America during the Mesozoic Era because that region was flat, and that region has remained flat since the seas have receded.  Landform diversity is among the chief hypotheses offered by continental biogeographers to explain the high species richness of the aquatic biota of the American Southeast, as compared to the homogeneous Great Plains, for example (Lydeard and Mayden 1995; Neves et al. 1997; Jenkins et al. 2015).

And the element of landform diversity missing from the Great Plains is, of course, rock.  With local exceptions (e.g., Black Hills of SD) the rivers that have developed since the Western Interior Seaway receded are not entrained by rocky hills and mountains; they shift on plains of clay and sand.  They run broad and slow, carrying large volumes of sediment, especially during flood events.

Among the 24 species shared by FWGO and FWGTN but absent from FWGGP is a single pulmonate (the limpet, Laevapex), and 23 prosobranchs (4 viviparids, 7 hydrobioids, and 12 pleurocerids).  Prosobranchs, as a broad generality, require more dissolved oxygen than pulmonates, and are not as well-adapted to warmer temperatures or lentic environments.  Populations of pleurocerid snails in particular are typically associated with riffles, rapids, and rocks.  They are best adapted for grazing on solid substrate. They require solid substrate for egg laying.

We suggest that the absence of landform diversity – rocky riffles and falls in particular – together with the absence of time for a regionally adapted fauna to evolve, accounts for the reduced freshwater gastropod diversity of The Great Plains.

Protection from Pleistocene glaciation is also high on the list of hypotheses for the aquatic biodiversity of the Southeast, but if anything, patterns in The Great Plains are directly counter.  Much of the (relatively meager) landform diversity detectable in the Prairie today can be credited to the glaciers that covered most of North Dakota, half of South Dakota, and eastern slivers of Nebraska and Kansas.  And our modern surveys find the richest freshwater gastropod fauna in North Dakota (23 spp), the poorest in Kansas (16 spp).

> Regional Biogeography

The distributions of the 10 species of planorbid snails we have confirmed from the Great Plains are mapped together in Figure 3.  Click for larger.  Their joint biogeographic patterns up 12 degrees of latitude are striking.  Most species become more common in northern latitudes: Helisoma anceps, Gyraulus deflectus, G. crista, G. circumstriatus, Promenetus umbilicatellus, and P. exacuous.  Menetus dilatatus is most common in the south.  The net result is that Kansas hosts only three species of planorbids, Nebraska and South Dakota five, and North Dakota seven.

Figure 3.  The distributions of the ten planorbid species confirmed from our Great Plains study area.  Click for larger.

These trends continue onward north into Canada.  Prescott & Curteanu (2004) reported that Gyraulus deflectus is the most widely distributed species of freshwater gastropod in the Central Parklands of Alberta, being found at 54.8% of 197 sample sites, with G. crista (30.0%), G. circumstriatus (23.4%) and Promenetus exacuous (22.8%) all quite common as well.  And identical trends are also readily apparent in the Great Plains lymnaeids (7 spp), the physids (5 spp) and the hydrobioids (3 spp).  See the maps available for download from the individual species pages.

> Historic Trends

State subtotals were generally lower than expected from the literature.  We have confirmed just 18 gastropod species from the waters of Nebraska, against 31 expected from the review of Stephen (2015), just 19 in South Dakota against 25 expected (Stephen 2018), and just 16 in Kansas, against 20 expected (Leonard (1959).  North Dakota was the exception, with 23 species of freshwater gastropods observed against 22 expected (Cvancara 1983).

At least three factors may be involved in these deficits: taxonomic correction, sampling bias, and local extinction.  Taking Nebraska as an example, we think it likely that previous records of Physa pomilia are misidentifications of Physa acuta, and that previous records of Lymnaea (Stagnicola) catascopium may have been misidentified Lymnaea (Pseudosuccinea) columella.  The former correction reduces the expected number of freshwater gastropods in Nebraska, since Physa acuta was already in Stephen’s (2015) list of 31, but the latter correction does not, since Lymnaea columella was not on Stephen’s list.

We also note that we have reidentified previous collections of Lymnaea (Galba) bulimoides as a mixture of L. cubensis and L. cockerelli, raising the expected species richness by one, and that we have newly discovered Menetus dilatatus in Nebraska, raising the expectation by two.  So, the revised expected species richness of freshwater gastropods in Nebraska is 31 – 1 + 2 = 32.

Have 32 – 18 = 14 species of freshwater gastropods gone extinct in Nebraska?  Certainly not.  All 14 of the missing species are (of course) rare.  Seven are known from the state only by a single historic record.  The sampling bias against rare species has been well-understood by community ecologists since the veil line of Preston (1948) and has been extensively reviewed for freshwater mollusks by Dillon (2000: 421 - 428).  In fact, we think it likely that Menetus dilatatus is missing from the expected list (but present in the observed list) as a result of (historic) sampling error.

All that stipulated.  We consider it likely that some gastropod species may have indeed disappeared from the waters of Nebraska in historic times.  Most of the missing species are at the southern limits of their range, appearing much more common in the Dakotas and further north: Amnicola, Probythinella, Valvata sincera,  Lymnaea stagnalis, and five of the ten planorbid species mapped above.  It is certainly possible that some of these have become locally extinct in Nebraska very recently, and that climate change has been a factor. 

> References

Angelo, R.T., Cringan, M.S., and Fry J.E. 2002. Distributional re-vision and new and amended occurrence records for prosobranch snails in Kansas. Transactions of the Kansas Academy of Science 105: 246–57.
Cvancara, A.M. 1983. Aquatic Mollusks of North Dakota. Report of Investigation, no. 78. North Dakota Geological Survey. 141.
Dillon, R.T., Jr. (2000) The Ecology of Freshwater Molluscs.  Cambridge University Press, 509 pp.
Dillon, R T. and J. D. Robinson (2009)  The snails the dinosaurs saw: Are the pleurocerid populations of the Older Appalachians a relict of the Paleozoic Era?  Journal of the North American Benthological Society 28: 1 - 11.  [pdf]
Jenkins, C.N., Van Houtan, K.S., Pimm, S.L., and Sexton, J.O. 2015. US protected lands mismatch biodiversity priorities. Proceedings of the National Academy of Sciences 112(6): 5081–5086.  [html] 
Leonard, B. 1959. Handbook of Gastropods in Kansas. Misc. Publ., no. 20.
Lawrence: Univ. Kansas Mus. Natl. Hist.
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Neves, R.J., Bogan, A.E., Williams, J.D., Ahlstedt, S.A., and Hartfield, P.W. 1997. Status of aquatic mollusks in the southeastern United States: A downward spiral of diversity. Pp. 43-86 in: Aquatic Fauna in Peril: The Southeastern Perspective (Benz GW, Collins DE, editors). Special Publication 1, Southeast Aquatic Research Institute, Lenz Design and Communications, Decatur, GA. Proceedings of a UMRCC symposium, 12-14 October 1992, St. Louis, Missouri. Upper Mississippi River Conservation Committee, Rock Island, Illinois.
Prescott, DRC., and Curteanu, MM. 2004. Survey of Aquatic Gastropods in the Central Parkland Subregion of Alberta. Alberta Sustainable Resource Development, Fish and Wildlife Division, Alberta Species at Risk Report No. 92, Edmonton, AB. 50 pp.
Preston, F. (1948) The commonness, and rarity, of species.  Ecology 29: 254 – 283.
Stephen, B.J. 2015. Species composition of Nebraska’s freshwater gastropod fauna: A review of historical records. American Malacological Bulletin 33(1): 1-11.
Stephen, B.J. 2018. Distribution and conservation status of the freshwater gastropods of South Dakota: A review of historical records and recent sampling. BioRxiv (Pre-print). [html]
Van der Valk, A. 1989. Northern Prairie Wetlands. Iowa State University Press, Ames, Iowa. Pp. 228–267.