The fall line as it arcs through the midlands of South
Carolina is a rather indistinct region of broad shoals and rocky flats, featuring
no actual falls, constituting no real line.
The Catawba River enters this region at Great Falls, SC, and, finding nothing
especially remarkable, much less great, traces a lazy path about 25 miles south
to the vicinity of Camden, changing its name to the Wateree River somewhere
along the journey, possibly out of boredom.
Prior to the 20th century, I feel certain that this section of
the river presented at least occasional rocky shoals and rapids. But it was on the last shoal of the Catawba/Wateree,
perhaps 6-8 miles north of Camden, that Duke Power built a hydroelectric dam in
1920.
Undistinguished by its 225 foot height, but stretching a
longish 3,380 ft. across the wide river channel and associated flood plain, the
Wateree Dam and Hydro Station has a 56 megawatt capacity, typically generating
a daily 3-5 foot cycle at the gauge in its tailwaters during the warm months. Its 13,864 acre reservoir also sees
substantial recreational use, Duke Power having thoughtfully provided boat
ramps and fisherman’s accesses throughout.
Invasive populations of Bellamya (or Cipangopaludina) japonica
have been established in South Carolina since at least 1995 [1]. And indeed, my good friend Bill Poly of the
SCDNR alerted me to the introduction of a Bellamya population in the Wateree
Dam tailwaters back in June of 2011. So
I was not surprised by the email I received last month from another of my good
friends among the ranks of state aquatic biologists, David Eargle of
SCDHEC. But the way he described the gastropod
situation downstream from the Wateree Dam piqued my curiosity. David reported “Unbelievable numbers of
Bellamya. One area I thought at first
was a gravel bar was all snails. Amazing.”
So I picked out a nice Saturday late last month, loaded my
kayak into the back of my pickup, and headed up the interstate northwest about
2.5 hours from Charleston to the tailwaters of the Wateree Dam. And indeed, the adjective “amazing” describes
the situation quite well.
The fisherman’s access is on the right (descending) bank of
the river. I launched my kayak and
paddled maybe 20 - 30 yards across the main channel about mid-day, to the broad
and (in spots) marshy region, dissected by rocky pools and shoals, that extends
across the left 90% of the riverbed. The
photo below was snapped in that left-side rocky/marshy region looking
downstream. The photo at the top of this
essay was snapped from the same vantage point, looking upstream.
So the third photo in this series is a typical vista looking
across one of those rocky pools downstream from the Wateree Dam. The water levels had been dropping steadily
all afternoon on this particular Saturday, reaching an extreme low around 4:30
PM, at which time the horn sounded and the hydro station began to
generate. The photo below was taken
around 4:00 PM.
And the next photo was snapped looking directly down through
about a foot of clear water, showing that the bottom of a typical channel is
essentially a “bed” of gastropods, piled on top of each other, perhaps three or
four snails deep. If you click on the
image below you can download a high-resolution (4.5 mb) jpeg, zoom in, and see that the snails are not grazing. They are lying on their backs on the bottom
of the Wateree River, apparently filter feeding like a bed of mussels.
The ability to filter-feed is fairly well-documented in
viviparids [2]. In fact, if you shop in
one of those nurseries that specialize in backyard water-gardens, the salesmen
will often advise you to purchase “Japanese Mystery Snails” for $1.00 each (or
$10 per dozen) to help “clarify” the water in your ornamental lily pond. Which I think they probably do.
Notice in this next photo that the snails in my net seem to
be strikingly uniform in size, all approximately 30 – 40 mm in standard shell
length. It was my impression that the
population was comprised almost entirely of the one-year-old age class,
probably born in the spring and summer of 2013.
I noticed a few young-of-the-year juveniles the afternoon of my visit, all
in the 10 mm size range, and just a couple 60 mm “lunkers,” which must have
been 2+ years old. But overall, the size
distribution of the snail population struck me as unusually homogeneous.
So the tide hit dead low at about 4:30 PM, and I started
back through the rocky marsh, more dragging my kayak than paddling it. And I happened to wade through one warm pool
with a slightly muddy bottom, apparently not receiving as much current as some
of the others. And when I turned back to
look in my wake, this is what I saw:
These are empty shells, of course. The snail has died and rotted, and the empty
shell filled with gas. I must have kicked
up several hundred such “floaters” as I walked back to the channel.
It seems likely to me that the Bellamya population
downstream from the Wateree Dam is on the verge of a bust. Dramatic die-offs of invasive viviparid
populations are not uncommon in the southeastern United States [3],
occasionally even reaching the attention of the popular press. I got telephone calls about a stinking
Bellamya die-off in Lake Murray on the other side of Columbia a couple years
ago, and there was a huge mess on the Neabsco River up in Virginia in 2010,
necessitating the deployment of heavy equipment. But in all cases of my personal experience, the
phenomenon is discovered after the crash.
Last month’s observations were the first I have ever personally made
during the population flush phase.
The population age distribution was especially interesting
to me. So I loaded my kayak back in my
pickup and drove around the fisherman’s access roads to Lake Wateree just above
the dam, maybe 300 yards upstream from where I had spent most of the
afternoon. The size distribution of the
Bellamya population in the shallows of the reservoir seemed dominated by big
lunkers 50-60 mm in shell length, which is normal year-round in the Carolinas,
in my experience. Typically very few
animals aged one year and younger will be apparent upon a causal census of a
Bellamya population around here. I took
this as evidence that the Bellamya population downstream from the Wateree Dam
had reproduced explosively in the last year or so.
“Boom-and-bust” or “flush-crash” population dynamics are a
familiar aspect of invasive species biology.
But it is my impression, after a couple hours of poking around the
published literature, that about 95% of everything we know is anecdotal [4]. Danielle Haak and her colleagues [5]
suggested that a 17-39% die-off of the adult Bellamya population in a Nebraska
reservoir was due to “an extreme drought event, which was coincident with
abnormally hot weather.” Moore and
colleagues [6] documented the community effects of a Potamopyrgus boom-and-bust
cycle in California, reviewing the three most obvious hypotheses to account for
the bust (weather, intraspecific competition and predators/pathogens), but not
ultimately selecting a favorite [7].
Simberloff and Gibbons [8] conducted an “exhaustive” review
of the worldwide literature on population crashes of established introduced
species, together with systematic “queries to experts on invasives by
particular taxa.” They observed:
“even quantitative data documenting perceived declines were exceedingly scarce, while the great majority of proposed explanations were simply more-or-less reasonable ad hoc suggestions with no supporting evidence.”
Across the N=17
case studies Simberloff considered the best-documented, four of the putative
causes were “competition with other introduced species,” with one case each for
“parasitism by subsequently introduced species,” “adaptation by native
herbivore,” and “exhaustion of resource.”
And for ten of the 17 best-documented invasive species population busts
[9], “there is no strong evidence suggesting a cause.”
One would think, with all the dump trucks full of money
being spent to study the causes and consequences of biological invasions
worldwide – all the weeds, all the bugs, and all the slugs combined – at least
a little funding would be available to study why this problem, at least
occasionally, solves itself. Or am I
wrong, again?
Notes
[1] This is the fifth post I have authored in the last ten
years on Bellamya invasion. If you’re
interested in digging into the phenomenon, perhaps the best approach would be
to first go to the FWGNA pages on Bellamya japonica and the closely-related
Bellamya chinensis, read the general biology, and then follow the links from
the “Essays” sections at the bottom of those two species pages back to this
blog. So start here:
[2] See pp 99 – 100 in my book:
Dillon, R. T. (2000) The Ecology of Freshwater
Molluscs. Cambridge University Press.
[4] The irony that I myself am adding yet another anecdotal
report here does not escape me.
[5] Haak, D. M., N. M. Chaine, B. J. Stephen, A. Wong, &
C. R. Allen (2013) Mortality estimate of
Chinese mystery snail, Bellamya chinensis in a Nebraska reservoir. BioInvasions Records 2: 137-139.
[6] Moore, J. W., D. B. Herbst, W. N. Heady and S. M.
Carlson (2012) Stream community and
ecosystem responses to the boom and bust of an invading snail. Biological Invasions 14: 2435-2446.
[7] Have there been similar (local) crashes of Potamopyrgus
populations throughout the American West?
This was the impression that I took from the Snake River below Minidoka
Dam in 2010, mentioned in footnote #4 here:
- The Mystery of the SRALP: A Twofold Quest! [1Mar13]
[8] Simberloff, D. & L. Gibbons (2004) Now you see them, now you don’t – Population
crashes of established introduced species.
Biological Invasions 6: 161-172.
[9] Including the giant African land snail Achatina on
Pacific islands.
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