Banff Springs snail (Physella johnsoni) COSEWIC assessment and status report: chapter 9

Population Sizes and Trends

Search Effort

No historic population estimates exist as the few individuals who collected snails up to 1996 (see Distribution) did not count snails.

The Banff Springs Snail research and recovery program began in January 1996 with a number of objectives, including determining the distribution and abundance of the species. Protocols were designed whereby thermal springs where the snail was historically found (Clench 1926; Clarke 1973, 1977, 1981) were visited systematically and periodically and examined visually for snails. Maps of the thermal springs and outflow streams, drawn with the aid of compass and tape measure, were used to delineate sections or microsites. The microsite divisions were based on local geomorphology at the scale of less than one metre. Visual surveys consist of intensively searching the microsites, using a headlamp where necessary, without disturbing the substrate. A hand tally counter aided the counting. Some microsites have never been examined due to their inaccessibility e.g., areas adjacent to cliff faces, steep water falls, pipes directing water at the C&NHS, and under boardwalks. Since January 1997, the origin pools and outflow streams of springs where the species was not observed in 1996 have not been intensively examined for snails except for the areas where water physicochemistry is measured. From January 1996 through July 2000, the frequency of regular surveys was tri-weekly (once every three weeks); since then the surveys have been quad-weekly (Lepitzki 2000c).

As the same person (D.A.W. Lepitzki) has counted the snails using the same protocol initiated in January 1996, results from surveys should be consistent and comparable. No doubt, some snails are missed during the surveys so population estimates should be considered minima. Repeat counts were done in the Basin Spring pool in September 1996 over five days (Lepitzki 1997b) in order to determine snail count precision (or repeatability). The number of snails counted varied from day-to-day (117, 131, 156, 98, and 123) but averaged 125 ± 9.5 (S.E.M.) (95% CI ±26.4). Unfortunately, illegal swimming may have confounded results. If an error of± 10% is added to the snail counts, 86% and 96% of additional, weekly snail counts at the two re-introduced sites (after the initial decline, see next section, Kidney n=28 and Upper Middle n=24, respectively), fell between those made during the regular quad-weekly counts, attesting to the precision of the visual counts.


Abundance, Fluctuations, and Trends

Results from regular tri-weekly and then quad-weekly snail counts at each thermal spring and at all thermal springs combined from January 1996 through May 2007 are shown in Figure 11. While these numbers are total numbers of snails observed during the visual counts, they can be considered estimates for mature individuals as larger snails are easier to see and reproduction has been observed in snails as small as 3 mm shell length (see Life Cycle and Reproduction).


Figure 11: Number of Physella johnsoni in each thermal spring and all seven springs combined, January 1996 through May 2007

Figure 11. Number of Physella johnsoni in each thermal spring and all seven springs combined, January 1996 through May 2007.

Re-introduced subpopulations at Upper Middle and Kidney Springs and augmented population at the Basin Spring outflow stream are included. Up until August 2000, population surveys occurred once every three weeks; thereafter they occurred once every four weeks.

Based on direction in the Parks Canada approved Resource Management Plan (Lepitzki et al. 2002b) and Environmental Assessment (Lepitzki and Pacas 2001) and subsequent re-evaluations (Lepitzki and Pacas 2002, 2003), snail subpopulations were re-introduced at the Upper Middle and Kidney Springs. Fifty snails were translocated from Lower Middle to the Upper Middle in November 2002; 50 snails (25 from the Upper Middle and 25 from the Lower Middle) were subsequently translocated to Kidney Spring in November 2003. Following initial drops to 16 and eight snails within three and two weeks of the re-introductions, respectively, both subpopulations appear to be sustainable and self-maintaining (Figure 11).

The Basin Spring subpopulation also has been augmented, based on the Parks Canada approved Environmental Assessment (Lepitzki 2005) and SAR (Species at Risk) permit (BA-2005-859) issued to D.A.W. Lepitzki. When the six captive-breeding tanks were decommissioned from 8 December 2005 through 13 February 2006, 7345 snails were added to the upper reaches of the Basin outflow stream. These translocated snails were responsible for the unprecedented increase in the last snail count of 2005 (Figure 11). As previously suggested, the success of this habitat enhancement and population augmentation (see Adaptability) continues to be monitored.

Populations of the Banff Springs Snail typically fluctuate by over two orders of magnitude annually with lows occurring during the summer and highs during the late-winter (Figure 11). These seasonal population fluctuations are exactly opposite those of other North American physids. For example, densities of P. integra peaked in August-September in Michigan (Clampitt 1974), and in Manitoba densities of P. gyrina peaked in June-July (Pip and Stewart 1976). A general limitation of these studies is that year-round sampling is not possible because of winter ice. While Sankurathri and Holmes (1976) also did not sample P. gyrina under the ice in their control area, a thermally influenced experimental area of a lake near Edmonton, Alberta was sampled year-round. Even though P. gyrina was found to reproduce year-round in the experimental area, peak snail densities still occurred during the summer.

The causes of the annual population fluctuations of P. johsnoni are uncertain but may be related to food supply and/or the seasonal dynamics of the thermal spring ecosystems (Lepitzki 2002b).

Including the two re-introduced subpopulations, minimum and maximum population sizes for each spring over the past 10+ years (January 1996 through May 2007) are shown in Table 1. Linear regressions of yearly minima, maxima, and mean for each separate subpopulation (excluding the re-introduced subpopulations) and all subpopulations combined (including those re-introduced) for the years 1996 through 2005 are shown in Figure 12. Due to the magnitude of yearly changes, none of these 10-year regressions were significant (P<0.05) except for the significant increase in yearly maxima at Lower Middle, and significant increases in yearly minima and mean at the Basin (Figure 12). When all five original subpopulations are combined, a significant increase in yearly maxima also was apparent. Highly significant (P≤0.005) increases in yearly minima, maxima, and mean were found only after the re-introduced subpopulations were added to the original five.


Figure 12: Yearly population minima, maxima, and mean for each subpopulation (except Upper Middle and Kidney), for all five original subpopulations combined, and for all subpopulations including the re-introduced subpopulations at Upper Middle and Kidney, 1996 through 2005

Figure 12. Yearly population minima, maxima, and mean for each subpopulation (except Upper Middle and Kidney), for all five original subpopulations combined, and for all subpopulations including the re-introduced subpopulations at Upper Middle and Kidney, 1996 through 2005.

Linear regressions with 95% confidence intervals are plotted as are r-square and the P for the ANOVA F-test.

Tischendorf (2003) modelled snail populations in the five original springs using snail counts from 1996 through 2002 (seven years). Parameters for RAMAS GIS population models were extracted and estimated from the time series analyses. One thousand replicate simulation runs were used to estimate probability of population extirpation and species extinction over a 40 year time horizon. While extirpation probabilities increased over time, most likely due to stochastic events (Tischendorf 2003), there was a low (≤~7.5%) extirpation risk for a time frame of 10 years for each population. Extirpation probabilities still remained around 4% for the Basin and 3% for the Upper C&B subpopulations after 40 years; however, the probability of extirpation was around 21% for the Cave subpopulation, around 29% for the Lower C&B subpopulation, and around 27% for the Lower Middle subpopulation after 40 years (Tischendorf 2003). If all five subpopulations were combined, the probability of extinction was 0% even after 40 years. While these are the “best possible educated ‘guess’ based on the current knowledge of the biology, life history and habitat requirements for this species”, Tischendorf (2003) further and clearly notes there is “substantial uncertainty in the knowledge of demographic data, such as fecundity, survival and dispersal distances” and that these “absolute numbers should be interpreted with caution”.


Rescue Effect

There is no possibility of a rescue effect from populations elsewhere as the species is endemic to selected thermal springs in Banff National Park.

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