FWGNA > Synthesis
Synthesis V3.0
As of 4June19, the complete 15-state FWGNA database contains 19,643 records.  The 102 species (with their 9 additional subspecies) that comprise the freshwater gastropod fauna of this vast and ecologically diverse region are not all equally common.

In recent years a widespread practice has developed of prioritizing species for conservation purposes by a system of “status ranks.”  The nonprofit environmental organization “NatureServe,” for example, prioritizes the worldwide biota globally (G-ranks), nationally (N-ranks) or regionally (S-ranks) into five categories:  1 = critically imperiled, 2 = imperiled, 3 = vulnerable, 4 = apparently secure, and 5 = secure.  The International Union for the Conservation of Nature also uses five ranks: CR = critically endangered, EN = endangered, VU = vulnerable, NT = near threatened and LC = least concern.  A system of four ranks was advocated in the spurious review of Johnson et al. (2013), E = endangered, T = threatened, V = vulnerable, and CS = currently stable.  A review of the many methods by which species have been categorized according to perceived conservation concern has been offered by Munton (1987).


Although the appeal of such systems to the natural resource agencies charged with protection of potentially endangered species is undeniable, such concepts as "threat" or "peril" or "endangerment" are by their nature entirely subjective. And although some connection is almost certainly made between conservation status rank and rarity in the minds of natural resource managers, the relationship has never been formally explored, at least in the case of freshwater snails.  So, in 2011 we first suggested a ranking system based on incidence, offered as a compliment to (if not necessarily as an objective substitute for) the subjective systems of conservation status ranking currently in vogue.

> Version history

FWGNA synthesis 1.0 was initially proposed and forcefully advocated in essays posted on the FWGNA blogs of 12Dec11, 9Jan12, and 19Mar12.  It was published in March 2012 as "FWGNA Circular No. 1" [pdf].  The database at that juncture contained 8,864 records of 57 species and subspecies, covering southern Atlantic drainages only.  The distribution of commonness and rarity for those 57 species seemed to fit a lognormal model.  Thus, a parametric system of incidence rankings was suggested, the rarest 5% being ranked “F1,” the remainder being divided by standard deviations around the mean: F2, F3, F4, and F5.

FWGNA synthesis 2.0 was proposed in the FWGNA website expansion of October 2013 and developed in the FWGNA blog post of 9Dec13.  The analysis at that juncture was of 11,471 records collected from the Atlantic drainages of Georgia to the New York line.  Number-theoretical analysis (EstimateS, Colwell 2013) returned no evidence that any rare species might have been missed in the list of 67 species (combining subspecies) returned.

The distribution of commonness and rarity demonstrated by the 67-species fauna did not appear lognormally distributed, but rather bimodal, with a secondary peak of high-incidence species.  Thus, we shifted to a nonparametric model inspired by the work of Gaston (1994), suggesting a quartile system of I-ranks (I for “incidence”), again setting aside the rarest 5% as I-1.  We also introduced the concept of peripheral species at that time, with the related phenomena of pseudo-rarity and non-apparent rarity.

FWGNA synthesis 2.1 was announced in my blog post of 19Nov15 and went up on the FWGNA website that same day.  The analytical methods remained identical to that of version 2.0, although the database was expanded to 12,211 records and 69 species.  This was the version published in Volume 1 of the FWGNA series by Dillon et al (2019).

> Version 3.0

All previous versions of the FWGNA synthesis have focused exclusively on the fauna of the Atlantic drainages.  Here we expand our analysis to include both the fauna of the Tennessee drainage above the Alabama line and the fauna of The Ohio above the mouth of the Tennessee/Cumberland at ORM 920.  

We have also updated our database from the Atlantic drainages.  Table 1 shows that the 12,211 Atlantic drainage records we analyzed in 7July15 sorted into 9 states, plus the District of Columbia, with double-counting at state lines artificially inflating the column total to 12,346.  Since 2015 we have accumulated 384 records from Atlantic drainages, over 8 states, with a column total of 399.

Table 1 also shows that the most recent (2013) version of our FWGTN database contains 1,674 records, sortable into 4 states.  Since that date we have accumulated an additional 124 records from the drainage of the Tennessee.

The database we have just released on our FWGO website contains 5,250 records, inflated to 5,734 by double counting.  Thus the combined, 15-state database analyzed in Synthesis 3.0 = 12,211 + 384 + 1,674 + 124 + 5,250 = 19,643 records.

The 102 species recorded from that vast region are listed in Table 2, ordered by the number of records in our database.  We will not re-analyze these data for lognormality nor evidence that rare species may have been missed, referring interested workers back to Dillon et al (2019).

Unsurprisingly, the most common species in our 15-state region would appear to be Physa acuta, with 3,219 records.  The 1,575 records of the next-most common species, Campeloma decisum, amount to less than half of the Physa records.  In striking contrast, ten freshwater gastropod species are represented in our database by but a single record.

The ecological requirements of this 102-element fauna are exceptionally diverse.  Four of the species are stygobionts – obligately adapted to the cave environment – Antroselates spiralis, Holsingeria unthanksensis, Fontigens tartarea, and F. turritellaFontigens cryptica seems to be an obligate inhabitant of subterranean interstitial spaces, although so rarely collected alive that its biology is unknown.  It seems unlikely to us that the data presented in Table 2 accurately reflect the abundance of these five species.

Ecologically, Pomatiopsis lapidaria is a land snail.  Essentially all 31 P. lapidaria records in our database have come from museums, primarily in collections from forest litter.  As was the case with the five subterranean species, it seems likely to us that our sampling methodology significantly underestimates the true abundance of P. lapidaria.

> Rarity

Rabinowitz (1981) has famously pointed out that there are seven forms of rarity.  We have no data on population sizes, however, nor any rigorous measure of habitat specificity for our 102 freshwater gastropod species.  Our analysis here has focused entirely upon geographical distributions.  The species found in the most spots are common by our definition, and those in the fewest spots are rare.  This may be termed “incidence rarity.”

Convention would dictate that some special consideration might be extended to the extreme 5% of any distribution, normally distributed or otherwise.  Thus, we suggest that the 5% of the species demonstrating the lowest number of incidences in a biota under consideration be assigned the rank “I-1,” by analogy to the “G1” rank of NatureServe.  The “I” prefix here designates “incidence,” to emphasize that the present ranking system is based on incidence data, rather than subjective impressions of “global imperilment.”

Gaston (1994) has offered an admirable review of the term “rare” in all its various origins and biological usages.  On the basis of clarity, versatility, consistency and ease of use, he has suggested that the term "rare" be defined as “the first quartile of the frequency distribution of species abundances.”  So because we have just set aside the 5% of species with the lowest incidence frequency as I-1, perhaps the 20% of the species remaining in the lowest quartile ought to be designated I-2.  Then a straightforward application of Gaston’s system would suggest that the second quartile (between the rarest set and the median) be designated I-3, and the third quartile I-4, and the fourth (most common) quartile designated I-5.

We here exclude the five subterranean species and the terrestrial P. lapidaria from further analysis.  Then the vector of I-ranks resulting from application of the system suggested above to the 102 – 6 = 96 freshwater gastropod species inhabiting our study area is shown in the rightmost ("FWGNA") column of Table 2.  Note that we have rounded to accommodate some natural breaks in our incidence data, such that all nine singleton species are included in rank I-1.

> Peripheral Species

Murray et al. (1999) reported that a remarkable 91% of the species in the “tail” of a rank-abundance curve generated from the canopy-forming vegetation of the Australian dry sclerophyll woodland were significantly more abundant elsewhere.  Murray and colleagues called these 91% the “somewhere-abundant” species, to distinguish that group from the 9% that were “everywhere-sparse.”

Similar phenomena have been noted in many animal communities.  Gaston (1994) has reported that the vast majority of the British bird species demonstrating incidence rarity are “vagrants,” which he defines as “not permanent members of the assemblage, do not breed, or do not have self-sustaining populations.”  Gaston notes that other terms that have been applied to describe such species include accidentals, casuals, immigrants, incidentals, strays, tourists and tramps.  Magurran & Henderson (2003) have added the term “occasionals” for rare species in estuarine fish communities.  The literature of plant community ecology includes the terms "peripheral" and "waif."

Similar to the situation described by Murray and colleagues, the data in Table 2 suggest to us two categories of rare species, the “somewhere-abundant” and the “everywhere-sparse.”  But we are not aware of any term in malacology analogous to “vagrant” or “occasional,” probably because such terms imply greater dispersal capability than is ordinarily assumed for freshwater mollusks.

We therefore suggest adopting the botanical term “peripheral” for use in mollusk community ecology to describe the situation where a rare species is “somewhere-else-abundant.”   We formally define a peripheral species as demonstrating less than median incidence in a region under study, but greater than median incidence elsewhere.  And we suggest that all non-peripheral species in a study region be called “core” species.

Although there are few rigorous estimates of the relative incidence of freshwater gastropod species outside the present analysis, our reading of the malacological literature suggests to us that 19 of the 48 species listed below the median in Table 2 probably demonstrate above-median incidence elsewhere.  Five of these 48 are exotic invasives – Bithynia tentaculata, Potamopyrgus antipodarium, Pyrgophorus parvulus, Melanoides tuberculata and Pomacea maculata.  The excellent New York survey of Jokinen (1992) and the Canadian survey of Clarke (1981) suggest to us that the following 9 species are more common to the north of our study area: Valvata tricarinata, Marstonia lustrica, Helisoma campanulata, Lymnaea catascopium, L. stagnalis, L. caperata,  Gyraulus deflectus, G. circumstriatus and Aplexa hypnorum.  And the Florida survey of Thompson (1999) suggests that the following 5 species are more common further south: Hebetancylus excentricus, Pleurocera floridensis, Biomphalaria havanensis, Pomacea paludosa and Floridobia floridana.   A lower-case “p” has been appended to the incidence ranks of all 19 of these species in Table 2, to indicate their (hypothesized) peripheral status in our 15-state study region.

> Pseudo-rarity and Non-apparent rarity

The 6 core species marked I-1 and the 8 core species marked I-2 are clearly rare, demonstrating incidences in the bottom quartile.  Gaston (1994) coined two terms relevant to the situation regarding the other 24 – 14 = 10 species in the bottom quartile of Table 2, however, "pseudo-rarity" and "non-apparent rarity."  The former term would describe the 3 species we have listed as I-1p and the 7 species we have listed as I-2p, because although their incidence ranks them in the bottom quartile of the 96 species in our study area, there is reason to think that they are not rare elsewhere.  Indeed, those 10 pseudo-rare species have displaced 10 species that should have occupied their spots in the bottom quartile.  Thus the 10 core species marked I-3* in Table 1 (without the modifier "p") all demonstrate non-apparent rarity.  They are genuinely rare, and deserved to have appeared in the bottom quartile, but their rarity was obscured by the 10 pseudo-rare, peripheral species. 

> Future Prospects

Note that (at least) 9 of the species listed in Table 1 have been considered “invasive,” demonstrating a potential for rapid range expansion in historic times: Cipangopaludina japonica, C. chinensis, Viviparus georgianus, and V. subpurpureus, and the five species listed two paragraphs above.  At least two other species may have obtained representation in Table 1 by artificial introduction (Pomacea paludosa and Biomphalaria havanensis), although demonstrating little potential to spread.  We have elected not to treat these species differently.  The four viviparids listed above have now spread to above-median incidence in our 15-state study area and are hence considered “core” species by our definition.  Some of the other species may ultimately transfer from peripheral to core status, as well.  We do not expect the designations in the far rightmost column of Table 2 to remain static.

Indeed, we expect the opposite.  Our long term plans call for expansion of the FWGNA survey southward and westward, with concomitant augmentation of our database and (doubtless) addition of new species to our faunal list.  The I-ranks currently shown in Table 2 should be considered subject to revision for quite a few years to come.

> Essays

  • Our initial effort to develop a (parametric) theory of commonness and rarity for freshwater gastropods was based on the incidence of 57 species in four states only.  This analysis was introduced and justified in my blog posts of 12Dec11 and 9Jan12: "Toward the Scientific Ranking of Conservation Status."
  • I introduced the first nine-state, nonparametric version of this analysis in my blog post of 9Dec13, "What is Rarity?"  That particular essay focused on Gaston's quartile definition.  The 2013 FWGNA database at that juncture contained 11,471 records from the Atlantic drainages, representing 67 species.
  • I focused on the subjects of "peripheral" species, pseudo-rarity and non-apparent rarity in my follow-up essay of 6Jan14, "Why is Rarity?"
  • Version 2.1 of the FWGNA synthesis (12,211 records, 69 species) was an incremental (although not negligible) expansion of our 2013 analysis.  It was announced in my blog post of 19Nov15.

> References

Clarke, A. H. 1981.  The Freshwater Molluscs of Canada.  National Museum of Natural Sciences, National Museums of Canada, Ottawa.
Colewell, R. K. 2013. EstimateS: Statistical estimation of species richness and shared species from samples. Version 9. User's Guide and application published at: http://purl.oclc.org/estimates.
Dillon, R.T., Jr., M.J. Ashton, W.K. Reeves, T.P. Smith, T.W. Stewart, & B.T. Watson (2019) Atlantic drainages, Georgia through Pennsylvania.  Freshwater Gastropods of North America, Volume 1.  FWGNA Press.  199 pp.
Gaston, K. J. 1994.  Rarity.  Chapman & Hall, London.  205 pp.
Johnson, P.D. Bogan, Brown, Burkhead, Cordeiro, Garner, Hartfield, Lepitzki, Mackie, Pip, Tarpley, Tiemann, Whelan & Strong 2013.  Conservation status of freshwater gastropods of Canada and the United States.  Fisheries 38: 247- 282.
Jokinen, E. H. 1992.  The Freshwater Snails (Mollusca: Gastropoda) of New York State.  Albany: New York State Museum.  112 pp.
Munton, P.  1987.  Concepts of threat to the survival of species used in Red Data books and similar compilations.  Pp 72- 95 In The Road to Extinction (R. Fitter & M. Fitter, eds.)  IUCN/UNEP.  Gland, Switzerland
Magurran, A. E. & P. A. Henderson 2003.  Explaining the excess of rare species in natural species abundance distributions.  Nature 422: 714-716.
Murray,  B. R., B. L. Rice, D. A. Keith, P. J. Myerscough, J. Howell, A. G. Floyd, K. Mills & M. Westoby 1999.  Species in the tail of rank-abundance curves.  Ecology 80: 1806-1816.
Rabinowitz, D. 1981.  Seven forms of rarity.  Pp 205 – 217 in The Biological Aspects of Rare Plant Conservation (H. Synge, ed.)  Wiley, NY.
Thompson, F. G. 1999.  An identification manual for the freshwater snails of Florida.  Walkerana 10 (23): 1 – 96.