Banff Springs snail (Physella johnsoni) COSEWIC assessment and status report: chapter 10

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Limiting Factors and Threats

• Thermal Water Flow Stoppages, Reductions, and Redirections
• Limited or Low Quality Habitat
• Soaking and Swimming
• Population Lows and Genetic Inbreeding
• Trampling and Other Local Disturbance
• Other Threats

Physella johnsoni is naturally restricted to the upper reaches of a limited number of thermal springs within Banff National Park and consequently its total population is severely fragmented. Individual subpopulations typically undergo natural, annual fluctuations of over two orders of magnitude and few opportunities exist for natural migration among springs except by birds (see Dispersal/Migration).

Natural and human caused threats to the species and its habitat, whether incidental to facility operations at the Upper Hot and C&BNHS or through other actions by people, have been identified in the Recovery Strategy and Action Plan (Lepitzki and Pacas 2007), are shown in Table 2, and are summarized below. The severity of each threat for each subpopulation has been ranked as H (high), M (medium), or L (low). Criteria for ranking included:

• whether the threat is confirmed and real (there is evidence that the threat has occurred in the past, present, or is anticipated, and results in mortality or decreases in probability of survival), and
• the magnitude of the impact to the snail population.

The severity of the identified threats varies among springs but in general, the higher certainty and severity threats appear in the upper sections of the table.

Whether the threat is N (natural) or caused by humans through FO (facility operations) or other actions (Hu) is indicated. Threats, ranked as H (high), M (medium), L (low) (see text for criteria), unk (unknown), and – (not applicable), are generally arranged vertically from most (top) to least (bottom) severe and certain. The threats indicated for Upper Hot and Gord’s Springs are anticipated if the snails are re-introduced at these sites.

Thermal Water Flow Stoppages, Reductions, and Redirections

Thermal water flow stoppages are a localized threat that may affect one or more springs simultaneously and can have severe consequences. Evidence has been presented elsewhere that Sulphur Mountain thermal springs have dried, with an accelerating frequency over the past 10 years. The frequency is expected to increase due to global climate change. While it is uncertain if thermal spring drying led to extirpations since the species was first discovered, flow stoppages were most likely involved. Consequently, the severity of this threat is high for all springs.

Natural seasonal reductions or fluctuations occur in the amount of water flowing in the springs. Mortality of snails has been observed in lower reaches of outflow streams that have dried as a consequence of flow reduction. Because the magnitude of natural flow reduction and fluctuation varies among springs, the severity of the threat also varies with recent history and yearly magnitude of flow variation contributing to the rank.

Flow reductions and fluctuations as a consequence of facility operations occur at the Upper Hot and springs at the C&BNHS. At the Upper Hot, thermal spring water is shunted to the bathing facilities resulting in little suitable habitat for the snail. Van Everdingen (1991) also states that ecological damage to thermal springs occurs through water diversions as well as through returning chlorinated water to outflow streams after use for swimming and bathing. The severity of flow reduction and fluctuation varies at the four springs of the C&BNHS with the highest severity occurring at the springs where all water flow from the thermal spring origin is controlled by pipes and valves. Pipes and valves clogging with microbial growth, debris, and garbage exacerbate natural changes and create unnatural fluctuations in water levels. Water fluctuations have killed snails and eggs at both the Basin and Cave springs.

Water flow redirections occur naturally and as a consequence of facility operations. Because natural flow redirections typically have been observed in lower reaches of outflow streams (e.g., passage of a herd of wapiti Cervus elaphus redirected flow of the east Cave outflow stream in April 1998) where snail numbers are lower, the severity of this threat is low. Flow redirections can also occur naturally through tufa mound growth, tree fall, debris deposition, or erosion. Flow redirections due to facility operation occur at the same springs where facility operation flow reductions and fluctuations have been identified and have been ranked identically.

Limited or Low Quality Habitat

Suitable, preferred habitat is naturally limited for this species. Facility operations have further manipulated and changed the habitat. For example, the rapid discharge of water through piping into steep terrain decreases both the quality and quantity of snail habitat in outflow streams.

Soaking and Swimming

Soaking and swimming have been documented at most of the thermal springs inhabited by the snail but the extent of this threat varies among springs, primarily as a consequence of what the spring’s origin pool looks like. Entering and exiting the pool can crush snails and eggs. Observed disruptions to the floating microbial mat include sinking, stranding, and fragmentation. Significant (P<0.05) changes in water clarity, physicochemistry, and snail microdistribution have been observed and measured (Lepitzki 1998, 1999). Dislodged mat can clog pipes and result in facility operation water level fluctuations. Several thousand snails were stranded, froze, and died in February 2005 following illegal swimming in the Basin Spring pool when pipes became clogged with microbial mat and the pool flooded (Lepitzki pers. comm. 2005.). This incident provided snails for isotopic analyses (Londry 2005a, b). While not having yet been examined experimentally, chemicals such as suntan oil, deodorants, and insect repellents could impact snails and their habitat. Others (Kroeger 1988; Lee and Ackerman 1998; Heron 2007) have speculated that the addition of toxic substances (e.g. soap, shampoo, bath and suntan oil, repellents) by bathers may threaten thermal spring flora and fauna. While the level of these illegal activities (prohibited by Park Superintendent closures, except at the Upper Hot) has declined as public knowledge of the snail has increased and habitat protection actions have been implemented, continued vigilance and testing of surveillance devices is required.

Population Lows and Genetic Inbreeding

The natural, annual population low, characteristic of this species, is a natural threat that increases the risk of population extirpation, with those subpopulations that have experienced the most extreme lows during the past 10+ years (Figure 11, Table 1) being the most susceptible (Table 2). A consequence of the seasonal low is genetic inbreeding and a continuous, repeating genetic bottleneck.

Trampling and Other Local Disturbance

Trampling and other local disturbance such as littering, substrate movement or removal, and dam construction have been observed within the past 10 years at all thermal springs historically inhabited by the snail. The frequency and magnitude of this impact varies but is highest at those springs in high visitor-use areas (Table 2). Trampling of fragile riparian habitat and movement or removal of substrates such as the microbial mat, rocks, and floating or emergent woody debris can crush adhering snails or cause them to freeze or desiccated. Littering and the tossing of coins, snow balls, ice chunks, rocks, and logs have been detected (Lepitzki et al. 2002b). The addition of copper containing coins may be particularly damaging as copper sulphate was used as a molluscicide (Swales 1935). Even the removal of garbage from snail habitat by well-meaning visitors could kill snails and eggs if the inappropriate substrate is not carefully examined for snails. Boardwalks and barrier fencing at the C&BNHS reduce much of this damage but it still occurs, especially on weekends and during the busy summer tourist season (Lepitzki et al. 2002b).

Other Threats

The dipping of feet or hands is widespread and occurs regularly, especially at the C&BNHS (Lepitzki 2000d; Thomlinson 2005). Visitor behaviour was observed in 1999 and 2000 (Lepitzki 2000d): 73% of visitors dipped their hands into the Cave Spring pool while significantly (P<0.05) fewer did so at the other springs (12% Basin; 6% Upper C&B; and 8% Lower C&B), possibly because kneeling was required. Thomlinson’s (2005) social science study found that limb-dipping continued to occur at the Basin and Cave Springs and suggested that many of the visitors who did so were unaware that the activity was prohibited. Like swimming and soaking, limb-dipping potentially crushes snails and adds toxic substances and is a continuing threat given current and anticipated visitation levels at the C&BNHS. Further actions are proposed to curb this activity and provide opportunities to touch the thermal water without affecting the snail and its habitat (Lepitzki and Pacas 2007).

By definition, stochastic events are random and somewhat unpredictable; however, because of its fragmented habitat and annual population fluctuations this species may be very susceptible to catastrophic population loss though a single, unpredictable chance event. The main reason for increased probability of extirpation was propagation of stochastic events (Tischendorf 2003).

While predation and competition are natural threats, they could result in population extirpation when combined with other threats and especially if they occurred during population lows. Similarly, while unquantified, poaching and shell collecting also are potential threats. Twitch-ups occur along outflow streams when snow-laden branches bend into the water, are colonized by the microbial community and snails, and suddenly twitch-up into the air under warming conditions. Over 40 and 60 of these “quick-frozen” snails have been found in separate incidents along the Lower Middle and Basin outflow streams (Lepitzki 1998).