If you had to pick one state to represent the entire freshwater mollusk fauna of North America, how about Iowa? Bounded by the Mississippi River on the east, with its western third in the Missouri drainage and its northern half pocked by prairie potholes of glacial origin, I can't think of a better place to sample the American heartland. And so the article by our colleague, Tim Stewart, in the most recent American Malacological Bulletin (21:59-75), "The freshwater gastropods of Iowa (1821 - 1998): species composition, geographic distributions, and conservation concerns" arrives as a most welcome contribution.
Tim did an extraordinarily thorough job of surveying both the published literature and the electronically-available museum collections in compiling the data for this paper. Lumping several nominal Campeloma species and several nominal species of the lymnaeid subgenus Fossaria into single categories, as well as combing the various synonyms of Physa acuta, Tim documented 49 freshwater gastropod species in Iowa. He also eliminated 6 species that seem to have been reported falsely.
It is interesting to compare Tim's list to the only other previously-existing database of which I am aware, that of NatureServe. My query to the NatureServe Explorer database this morning returned a list of 44 freshwater gastropod species from in Iowa which, paring down the Campeloma, Fossaria, and Physa and eliminating dubious entries, reduced to just 38. The 11 species missed by NatureServe appear to be a random subsample of the fauna: 2 Valvata, 3 lymnaeids, 2 physids, 2 planorbids and 2 ancylids.
Tim briefly reviewed the natural history of Iowa, as it has been developed from a tallgrass prairie to a breadbasket 95% under cultivation. Broadly examining collection records for trend, he found evidence that as many as 25 of the 49 Iowa freshwater snail species may warrant some conservation concern.
The problem with a literature-based approach is, admittedly, the difficulty of controlling for trends such as a decline in the publication or curation of malacological collections. What is needed, of course, are fresh data, and not just in Iowa. Tim concluded his discussion with a call for a "comprehensive field survey to determine which species are truly endangered in this state." And in fact, he assures me personally that such a survey is already underway. Amen, brother!
Keep us posted,
Subject: Reasonable expectations for NatureServe
Date: Thu, 27 Apr 2006
From: "Dillon, Robert T. Jr."
To: FWGNA group
I might need to correct a misunderstanding from my message of 4/20/06, "Surveying the Heartland." I did not mean to imply anything negative regarding the on-line database maintained by NatureServe. Science is the construction of testable models about the natural world. The NatureServe database constitutes such a model, where essentially no other models exist, and thus makes a valuable contribution.
Any biologist with a minimum of field experience will understand, I hope, that continent-scale distribution maps such as those provided by NatureServe must be based on the broad ranges of the organisms involved, which come from general reviews, large monographs, and regional surveys. Such maps cannot possibly duplicate the precision of a more finely-detailed inventory, such as that for an individual state. So I should hope that nobody would be surprised to read that the results of Tim Stewart's intensive survey of Iowa didn't precisely match expectation from NatureServe's broadly-drawn national ranges.
Nor in fact is it reasonable to expect that the NatureServe database will be kept meticulously current. In addition to his freshwater snail duties, our good friend Jay Cordeiro of NatureServe is also in charge of freshwater mussels, terrestrial gastropods, crayfish, fairy shrimp, clam shrimp, tadpole shrimp, mayflies, stoneflies, caddisflies, and odonates. I myself can't update the freshwater gastropods of South Carolina any more than about annually. Jay is doing a great job. And hey, we're snail people - we can wait.
Finally, it is not reasonable to expect that the NatureServe list (or the list Tim Stewart developed from his more detailed study, for that matter) will accurately reflect the true freshwater snail fauna of Iowa. Neither estimate has been confirmed by any recent field work. And there are some systematic biases in literature review as a method of biotic inventory - removing a dubious record is more difficult than adding one, for example.
Philosophers of science tell us that the true number of freshwater gastropod species is everywhere either trivial or unknowable. But certainly, all the models we've got today can be refined.
So let's get busy!