Hotwater physa (Physella wrighti) COSEWIC assessment and upate status report: chapter 7

Population Sizes and Trends

The population of P. wrighti in Liard River Hotsprings Provincial Park (LRHPP) occurs in three definable areas: Alpha Stream, lower Alpha Pool and Beta Pool. These areas probably remain connected by wildlife and human movements, and by flood events. Beta Pool is uphill from and has an exit stream in the direction of Alpha Pool and Stream. It is likely that snails from Beta Pool could be washed downhill during heavy rains, flooding or snow melt and mix with Alpha snails. The snails at LRHPPprobably constitute one population although confirmation of this would be obtained by microsatellite studies (see Genetic description).

Several documents are available regarding the presence of P. wrighti in LRHPP (see Table 1). These are Te and Clarke (1985), Lee and Ackerman (1999), Salter (2001), Salter (2003) and the 2006 survey by Lee reported here. Abundance data were collected by Lee and Ackerman (1999), Salter (2001) and during the 2006 survey.

Search effort

In August 1973, Clarke spent 0.4 hours collecting “Physa in main stream” at LRHPP (Clarke 1973). Te and Clarke (1985) reported that in 1979, the whole population of P. wrighti was in Alpha Stream although other springs and pools within the park were searched. Based on Clarke’s information, Lee and Ackerman (1999) restricted their survey to Alpha Stream spending 6.0 hours collecting population data. Salter (2001) was the first to report P. wrighti from lower Alpha Pool, and Salter (2003) reported P. wrightifrom Beta Pool. During the 2006 survey, P. wrighti was found in all these sites; search effort was 12.0 hours in Alpha Stream (2 people for 6 hours), 0.4 hours in lower Alpha Pool and 1.0 hour in Beta Pool collecting population data (see Table 1).

Abundance

Lee and Ackerman (1999)

This survey was conducted in September 1997 and found P. wrighti to inhabit a reach of Alpha Stream extending from 5–31 m below the dam at the outflow from Alpha Pool. Most P. wrighti were within partially submerged Chara mats that occurred in 11 areas along this reach. To estimate the P. wrighti population, the size of each Chara mat was measured and an 18 cm diameter stainless steel mesh net on a pole was pushed under the mat and was then shaken to dislodge snails. Live and empty shells were separated; live snails were counted and immediately returned to the Charaand empty shells were retained for measurement. The number of live snails collected in the net area (254 cm²) was used to estimate the total number of snails in each Chara mat. As this method was disruptive to the habitat, no replicate samples were collected. When shaken Chara was subsequently examined, this method appeared to remove only 1/3 of the live snails; thus, the abundance data were adjusted accordingly. When the 1997 data from Lee and Ackerman (1999) were examined for this report, an error in calculation was found in that the net area used was 364 cm² and not 254 cm². The abundance estimates were therefore recalculated. Within the Chara, 1,526 snails were found in a habitat area of 3.2 on the west and east banks. Snails were also found on open sediment on the west bank and were counted within several quadrats for an estimated 601 snails in 0.3 m² of habitat. Water temperature was recorded at each collection site (see Figure 4). Therefore, the reach of Alpha stream extending from 5-31 m below the dam contained 2,127 snails in suitable habitat of approximately 3.5 m² (see Table 1 and Appendix B). Most snails occurred in Chara or on open sediment on the west bank (1,388; 65%) where most of the habitat was found (2.7 m²; 73%; see Appendix B). To facilitate plotting and comparison with 2006 data, the distance below the dam was reported as per the sections identified in 2006 and the habitat area was calculated as number of 100 cm² quadrats. The abundance of snails was found to be correlated with the amount of habitat available in 100 cm² areas on both the west and east banks (Figure 5a, figureb); Spearman rank correlation coefficients were R=0.76 and R=0.70 for the west and east banks, respectively, and R=0.72 overall.

Salter (2001)

This survey was conducted in August 2000 and found P. wrighti to inhabit a reach of Alpha Stream that extended 200 m below the dam. This reach was divided into two sections; within the 0–140 m section, P. wrighti was found in Charamats and on small and large woody debris, and within the 140–200 m section was found within a continuous border of Charaand on the adjacent sediment substrate.

To estimate snail abundance within the Chara mats, a 10 cm diameter stainless steel mesh kitchen sieve was inserted under the Chara and this section of the plant was removed. Snails were counted and returned to the same area. Visual estimates were made on woody debris, Chara border and substrate. Within the 0–140 m section, 82 linear metres of Charamat contained an average of 23 snails/m for a total of 1,886 snails, and 14 m of woody debris supported an average of 30 snails/m for a total of 420 snails. Within 140–200 m section, the 120 m of Chara border contained an average of 16 snails/m for a total of 1,920 snails, and the 120 m of adjacent substrate held an average of 8 snails/m for total of 960 snails. Thus, the upper 140 m of Alpha Stream contained 2,306 snails and the lower 60 m contained 2,880 snails for a total of 5,186 snails in this 200 m reach (Table 3).

Figure 5. Correlation (R) of abundance of Physella wrighti to amount of available habitat in sections downstream of dam in Alpha Stream

Figure 5a. west bank 1997

Figure 5a. west bank 1997

Figure 5b. east bank 1997 [1997 R=0.72]

Figure 5b) east bank 1997 [1997 R=0.72]

Figure 5c. west bank 2006

Figure 5c. west bank 2006

Figure 5d. east bank 2006 [2006 R=0.86]

Figure 5d. east bank 2006 [2006 R=0.86]
Table 3. Survey data for Physella wrighti: August 2000 (Salter 2001)
Distance below dam (m) Habitat Type Habitat Length (m) No. Snails per m Snail Abundance
0–140 Chara mats
82
23
1,886
0–140 Woody debris
14
30
420
140–200 Continuous Chara
120
16
1,920
140–200 Substrate
120
8
960
Total abundance of Physella wrighti  
 
 
5,186

In January 2001, Salter surveyed lower Alpha Pool. The pool diameter was about 28 m, and approximately 75 snails/m were found in a 5 cm band both above and below the water surface around the sediment perimeter and on wooden structures for a total estimate of 2,100 snails in a habitat area of 1.4 m² (see Table 1).

2006 Survey

This survey for P. wrighti was conducted in August 2006 and included the reach of Alpha Stream extending 95 m below the dam. While all previous surveys noted the seemingly preferential use of Chara by snails, the 2006 survey found no submerged vegetation within Alpha Stream (see Habitat trends) and all snails were found on sediment or woody debris. The area immediately below the dam was wide and shallow with large woody debris and was designated “Section 1”. Below Section 1, each side of the stream was marked at 1 m intervals and data were collected within each 1 m long, consecutively numbered section except when stream curve necessitated inclusion of an extra metre to keep the start of each section relatively even on both banks. The 1 m measurements continued to 45 m below the dam on the east bank and thereafter data were collected within 10 m long sections of the stream. The water temperature (see Figure 4) and distance across the stream (Appendix B) were measured for each section. At about 85 m downstream the stream made an abrupt turn to the west, and about 10 m beyond this turn the sediment became too soft to continue the survey although suitable temperatures and habitat for P. wrighti appeared to continue for some distance down the stream. Geoposition data within this reach were recorded with a hand-held GPS (Garmin eTrex Legend®) at the dam and approximately every 10 m downstream (see Appendix B).

Snails visible on the substrate in each section were counted and a 10 cm × 10 cm (100 cm²) quadrat was used to estimate the amount of apparently suitable habitat available. P. wrighti occurred most abundantly in the area extending from the dam to Section 33, or to about 37 m along the east bank, and 1,041 snails were counted within this reach. From Section 34 to 43 (37–65 m along the east bank), only 11 snails were counted on both sides of the stream; the reach had relatively high flow and provided little habitat. The stream widened and slowed in Section 43 (65 m) and suitable snail habitat was again available. Within the remaining 30 m that could be surveyed (65-95 m), 374 snails were counted. There were undoubtedly many P. wrightibeyond this point that could not be surveyed. Therefore, the reach of Alpha Stream that extended 95 m below the dam contained 1,426 snails in an area of suitable habitat of about 3.0 m² (see Table 1 and Appendix B). Most snails occurred on the west bank (836; 59%) where the most habitat was found (2.0 m²; 67%; see Appendix B). The abundance of snails was found to be correlated with the amount of habitat available in 100 cm² areas on both the west and east banks (Figure 5c, figured); Spearman rank correlation coefficients were R=0.93 and R=0.86 for the west and east banks, respectively, and R=0.86 overall.

The perimeters of upper and lower Alpha Pool were searched. The temperature in the upper pool was 43–48°C at the stairs and no snails were seen. The temperature in the lower pool was 36°C at the stairs and 23 snails were counted around the perimeter, occurring on the wooden stairs and just above the water line on the sediment walls of the pool.

The perimeter of Beta Pool was also searched. The water temperature at the stairs was 41.5ºC. Twenty snails were counted on the support structures of the pool deck in an area of 0.02 m². On the north bank near the small stream that exits from the pool, the water temperature was 36–38°C and many snails were found just above the water/air interface. Counts were made within several 100 cm² quadrats for an estimate of 890 snails in 0.2 m² of habitat. Therefore, 910 snails were found in a 0.2 m² area of Beta Pool.

The abundance estimate for P. wrighti in LRHPP in August 2006 was 2,359 snails (see Table 1).

Fluctuations and trends

Alpha Stream

Three abundance estimates of P. wrighti in Alpha Steam have been made with Salter’s (2001) estimate of 5,186 snails the highest and the 2006 estimate of 1,426 snails the lowest (see Table 1 and Figure 6). The reduced snail abundance noted in 2006 in the upper reach of Alpha Stream appears to be due to a loss of submerged macrophytes, which may be the result of an increase in water temperature. In 1997, all 2,127 snails were in the first 31 m of Alpha Stream in 3.5 m² of habitat, and in 2006 this same reach had only 769 snails or 36% of the previous population estimate (see Figure 6), in 1.8 m² of habitat or 55% of the previous habitat observed. However, while the water temperature increase measured in 2006 might be responsible for loss of Chara and the consequent reduction in P. wrighti abundance in this upper reach, this temperature increase might also extend the reach of the stream optimal for P. wrighti, so the population might be redistributed rather than reduced. The stronger correlation of snail abundance to habitat available in 2006 vs. 1997 (R=0.93 and R=0.71, respectively) suggests that under the circumstances of loss of preferred habitat, P. wrighti is more strongly exploiting the remaining suitable habitat.

Additional surveys with standardized protocols are required to assess the population trend within Alpha Stream but the current evidence does not indicate a decline in the population of P. wrighti.

Figure 6. Abundance estimates of Physella wrighti in Alpha Stream, lower Alpha Pool and Beta Pool.

Figure 6.Abundance estimates of Physella wrighti in Alpha Stream, lower Alpha Pool and Beta Pool.

Alpha Pool

In lower Alpha Pool, 2,100 snails were estimated in January 2001 and 23 were counted in August 2006 (see Table 1 and Figure 6). This might be a seasonal population trend. Observations on P.johnsoni in Banff National Park from 1996 to 2006 have shown that the population typically fluctuates by two orders of magnitude annually with lows occurring during the summer and highs during the winter (COSEWIC 2008).

Beta Pool

While many P. wrighti were observed in Beta Pool in November 2001 (Salter 2003), the 2006 abundance estimate of 910 snails are the only data available (see Table 1 and Figure 6).

Rescue effect

As there is only one population of P. wrighti, there is no possibility of rescue effect. If a catastrophic event were confined to Alpha Pool/Stream or Beta Pool, snails from the other pool could be used to repopulate restored habitat. Rescue effect from outside populations of P. gyrina is unknown. It is uncertain if P. wrighti and P. gyrina are conspecific, and it is uncertain how P. gyrina would adapt to the hotsprings environment.

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