Dr. Rob Dillon, Coordinator

Monday, November 6, 2017

Loved to Death?

Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2023c)  Loved to Death?  Pp 29 – 35 in The Freshwater Gastropods of North America Volume 7, Collected in Turn One, and Other EssaysFWGNA Project, Charleston, SC.

Back in 1976, when I first joined the American Malacological Union, the community of shell collectors played an important role in the day-to-day operations of the society.  It seemed to me that the amateurs actually outnumbered the professionals at our annual meetings.  Shell clubs ran the registration table, operated the A/V equipment, and sponsored the receptions.  Receptions, heck – the Houston Shell Club threw a rip-snorting party, with music, and dancing, and pretty girls.  And (then-Treasurer) Connie Boone came through kissing everybody who held still.  Man, those were the days!

My 1974 edition of a 1966 classic
So the climax of the annual meeting of the American Malacological Union in those golden days was always the shell auction, for the benefit of the student fund.  Dick Petit would get half-tuned and wisecrack from lot to lot of colorful specimen shells, offering drop-dead gorgeous little jewels of nature to the highest bidder, and some of the prices paid were eye-popping.

Although I myself never offered any bids for any of the lovely seashells on auction, I felt a strong connection to those who did.  I was quite the hobbyist myself in my youth.  I did collect a lot of marine gastropods and bivalves for their shells, and traded shells with friends all over the world, and (yes) did indeed purchase specimen shells from shell dealers and shell shops, till the day I left home for college.  Shell collecting gave me my start in malacology.

Alas, in 1996 the American Malacological Union threw it all away.  Citing vague conservation concerns, that summer the AMU Council [1] banned the sale or trade of shells at annual meetings, implying strongly as it did that overcollecting by hobbyists posed a threat to natural mollusk populations.  The hobbyists felt as though they had been insulted, which they had, and they left the AMU and never came back.  It was the stupidest thing ever done by a roomful of mollusk people.

So last month we took a peek into the worldwide trade in living freshwater gastropods for the aquarium hobby [2].  Most of the 47 species at least occasionally available from Singapore, the primary wholesale exporter of aquarium stock to the global marketplace, are widespread and trashy, as one might expect.  But our colleague Ting Hui Ng and her coauthors [3] also identified several freshwater gastropod species as “narrowly endemic,” mostly Tylomelania and other Southeast Asian pachychilids, with a viviparid or two thrown in for diverse measure.  Quoting Ting Hui directly, “The rarity of the species may drive increased demand, which may ultimately lead to a decline of the species.”

So what evidence might there be to suggest that hobbyists or collectors might drive populations of gastropods such as these to extinction by overharvest?

The discipline of fisheries management is generally considered to have been born in the early 1930s, with the development of the concept of maximum sustained yield [4].  Stated simply, MSY = Kr/4, where K is the carrying capacity of the environment, and r the intrinsic rate of natural increase.  These are difficult and near-impossible parameters to estimate, respectively, and so the concept of MSY has rarely seen application, even in the management of the most commercially valuable fisheries.  Much less snails.

But, if you’ll allow Captain Obvious to take the helm here, note that the concept of MSY depends on the assumption of density-dependent population regulation.  And, to be quite frank, as your Captain always is, it is my strong impression that essentially all our colleagues with research interests in the ecology of mollusk populations, or indeed with interests in any aspect of the biology of any invertebrate population whatsoever worldwide, carry with us a (near-universally unstated) assumption of density-independence.  In other words, we assume that the size of our study populations is not a function of r and K, but of extrinsic factors such as weather, or floods, or harvest by Indonesian locals who might want to gather up a few small but exotic-looking snails from the lakeshore to sell to feed their families. 

Well, shame on us all.  The tiny little scraps of evidence available today suggest that the regulation of freshwater gastropod populations is as density-dependent as any population of living things on this earth.  The figure below, reproduced from Chapter 5 of my (2000) book, shows the densities of five freshwater gastropod populations over records of 5 – 10 years.  My PBLR test on the longest record available (the Ancylus data set of Russell-Hunter) returned a value of t significant at the 0.01 level, showing strong evidence of density-dependent regulation [5].

Dillon (2000) Figure 5.11
The grand mean density of the Ancylus population in the stream studied by Russell-Hunter, 260 per square meter, can therefore be taken as a rough estimate of carrying capacity.  The grand means of the other populations (per meter squared) were Physa = 77, Lymnaea = 93, Hydrobioides = 41, and Pomacea = 0.11.  The Pomacea estimate did not include juveniles, however.  Shall we take, as a rough estimate of carrying capacity K for freshwater gastropod populations, about 10 per square meter?

Table 5.1 of my (2000) book offers a big compilation of demographic data for 36 populations of freshwater gastropods [5].  The median value for intrinsic rate of natural increase tabulated was around r = 1.5, but the two prosobranch values were systematically lower, just r = 0.09 for Pomacea and r = 0.24 for Melanoides.  Then if we roughly estimate the value of r for freshwater prosobranch populations as r = 0.1 per generation, and carrying capacity K = 10 per square meter, we find MSY = Kr/4 = 0.25 snails per square meter per generation.

My biological intuition suggests that Tylomelania probably mature at around age one year and reproduce continuously thereafter. And the surface area of Sulawasi’s Lake Pozo, where all the Tylomelania live, is approximately 300 square km.  I realize all the lake bottom is not equally inhabitable by snails, but you get the picture.  Back-of-the-envelope estimates suggest that any subtraction of snails by collectors from the Tylomelania populations of Lake Pozo below roughly 10 million snails per year will not ultimately lower the population size, but will be replaced.  And there is zero evidence suggesting otherwise. 

OK, I understand the justification.  My colleagues want grant funding to gather hard data on the size and demographics of Tylomelania populations, so that an informed decision can be made.  In the meantime, since we don’t know, we must err on the side of caution, yes?  Well, I agree with that first sentiment, but not the second one.  Science does not make recommendations based on the premise, “Since we don’t know.”  I understand the fear, but fear is not reason.  Fear is the opposite of reason.

The chairman of my graduate committee 1977 – 1982 was a prominent ecologist named Dr. Robert E. (Bob) Ricklefs.  At some point in my graduate career, he made a point that has stuck with me for 40 years.  Bob observed that if we assume population size is a function of density-independent effects, populations are like drunks on a subway platform, veering left and right randomly until they either go extinct or cover the earth ass deep.  I realize that sometimes it seems extinction is frighteningly common.  But if the size of freshwater gastropod populations were indeed a function of density-independent factors [6], the two phenomena (extinction and ass-deep-coverage) would be equally common, with the frequency of extinction = frequency of coverage = 0.50.  Since that is not the case, density-dependence must prevail.

Well, we’ll return to that ass-deep-earth-coverage thing next month.  But the bottom line for today is that there is no evidence that natural populations of mollusks can be driven to extinction by the love of hobbyists [7].  And a lot of evidence that the love of hobbyists can be beneficial to the work of science.  The birdwatchers are a tremendous asset to ornithology, and the stargazers a tremendous asset to astronomy.  Indeed, the vibrant amateur communities of birders and gazers are where the professional ornithologists and astronomers are born. 

Meanwhile we malacologists are so wrapped up in the remote likelihood that our study organisms might disappear into extinction that we cannot see the imminent extinction of our own profession.  And I strongly suspect the attrition rate in the halls of malacology has been well above Kr/4 for quite a few generations now.


[1]  I myself was not called to the AMS Council until 1999.  I did campaign heavily for repeal of the shell ban during my 2001-02 presidency, but could not find the votes.  The best I could do was neglect the issue entirely when I redrafted the AMS constitution & bylaws 2002-03.

[2] What’s Out There? [9Oct17]

[3] Ng Ting Hui, Tan SK, Wong WH, Meier R, Chan S-Y, Tan HH, Yeo DCJ (2016) Molluscs for Sale: Assessment of Freshwater Gastropods and Bivalves in the Ornamental Pet Trade. PLoS ONE 11(8): e0161130. https://doi.org/10.1371/journal.pone.0161130

[4] Russell, E. S. (1931). Some theoretical Considerations on the "Overfishing" Problem. ICES Journal of Marine Science. 6 (1): 3–20.
Graham, M. (1935). "Modern Theory of Exploiting a Fishery, and Application to North Sea Trawling". ICES Journal of Marine Science. 10 (3): 264–274.

[5]  Dillon, R. T., Jr. (2000) The Ecology of Freshwater Molluscs.  Cambridge University Press.  All the details for the demographic analyses referenced above are found in Chapter 5, with the material on carrying capacity pp 202 – 207, and intrinsic rate of natural increase on pp 172 – 182.

[6] Or if such populations could be driven to extinction by harvest rates very much above Kr/4.

[7] In fact, it seems unlikely to me that even unregulated commercial harvest could drive marine shellfish populations to extinction.  And I don’t think there’s any evidence that the pearl-button industry was responsible for any of the freshwater mussel extinctions.  The only cases of human overharvest I’ve ever heard of, for any mollusk population in any environment whatsoever, are the occasionally-successful handpicking efforts to control land snail pests.  See:

Simberloff, D. 1997. Eradication. Pages 221–228 in D. Simberloff, D. C. Schmitz, and T. C. Brown, editors. Strangers in paradise. Island, Washington, D.C., USA.