Oregon forestsnail (Allogona townsendiana) COSEWIC assessment and status report 2013: chapter 10
The most studied population of Oregon Forestsnail in BC is within TWU-ESA, where snail movement and life history patterns were initially studied periodically over six years beginning in 2000 (Steensma et al. 2009); these studies are continuing (Steensma pers. comm. 2012). Data were collected during spring months, when climate conditions are optimal and snails are most active (e.g., during breeding season). Four 24 m2 study sites were assessed for snail population densities using the Jolly-Seber mark and recapture technique (Krebs 1989). Sites were searched for individuals for 30 minutes each on five consecutive days (June 8 - 12, 2004), at exactly the same time each day. Population estimates were made using the computer program JOLLY (USGS Patuxent Wildlife Research Center, Laurel, MD, USA) (Pollock et al. 1990) with the assumptions: all snails have equal survivability and equal catchability, processing time is brief, marked snails are immediately released and marks are easily seen.
The population of Oregon Forestsnail at one of the training areas managed by the Department of National Defence (DND) at Area Support Unit (ASU) Chilliwack also was studied (Hawkes and Gatten 2011) to determine the area of occupancy, distribution and estimate population size throughout the federal property. Data were gathered in the fall, which is not considered ideal timing (e.g., not breeding season); however, snails are known to be active and visible during the wet fall months. A total of 32 plots, each 25 m2 (5 m x 5 m plots = total 800 m2), was randomly selected within habitat deemed likely to have Oregon Forestsnail. Habitat was chosen based on aerial photograph interpretation of sites for habitat elements (see Habitat Requirements). Plots were searched for 20-minute intervals; searches under leaf litter and coarse woody debris allowed discovery of aestivating snails. Snail populations were estimated by summing gastropods recorded per plot and calculating density (snails/m2) (Hawkes and Gatten 2011).
There are insufficient data to provide an accurate estimate of Oregon Forestsnail abundance across the entire range in BC. Data on populations at each site has minimal information on population size or trend. Oregon Forestsnail sites mapped by the BC Conservation Data Centre (2013) and data gathered during the preparation of this status report provide some information on snail abundance. Oregon Forestsnail site abundance ranges from one individual (at least 17 sites) to counts greater than 20 snails (9 sites). The largest number of observations at one time is 670 individuals at Colony Farm Regional Park (Figure 10) (Parkinson and Heron 2010).
Two studies are available on population estimates. Steensma et al.’s (2009) population estimates among the four study areas within the TWU-ESA ranged from seven to 47 snails in their four 24-m2 sampling sites with an overall mean population density of 1.0 snail/m2. At the other population estimation site (Chilliwack), the estimated density of Oregon Forestsnail was highest in riparian habitats (0.14 snail/m2) and second-growth mixed deciduous forests (0.13 snail/m2) (Hawkes and Gatten 2011). These data from Chilliwack were not gathered in the breeding season (ideal time) but were collected in the wet fall when snails are known to be active and visible. Until a survey is repeated in spring mating season, the Chilliwack results should be treated with uncertainty.
These minimum and maximum density estimates were multiplied by the total biological area (m2) of mapped occurrences (~ 3,278,300 m2) (see Table 1; BC Conservation Data Centre 2013) to derive a crude population estimation range. The Canadian population for Oregon Forestsnail is therefore estimated to contain from 426,000 to 3.3 million individuals.
There is minimal information on fluctuations and trends for Oregon Forestsnail populations. While the 1903 historical Vancouver Island site was confirmed in 2003 and 2009, it is suspected that populations have been or will be lost from the 17 urban housing developments that have already occurred in the Lower Fraser Valley (see Habitat Trends) and the many others that are planned within the current urban growth boundaries.
The persistence of Oregon Forestsnail populations in the landscape is dependent on interconnected and suitable habitat patches and dispersal. Urban and agricultural development, combined with natural succession, fire suppression and infilling/draining of lowland wetland riparian habitats (see Threats and Limiting Factors) has likely led to the isolation of populations and subsequent inability of snails to disperse and recolonize habitat patches. Eventually, cumulative threats combined with limiting factors likely led to extirpation within some sites. What is certain is that since the initial status report (COSEWIC 2002) there has been substantial loss of Oregon Forestsnail habitat and individuals from urban development (see Threats and Limiting Factors).
It is difficult to assess the effectiveness of Oregon Forestsnail dispersal abilities from one patch of suitable habitat to the next. Distances between known sites can be large (> 10 km) (Figure 4), although if there is sufficient habitat it is likely the snail could disperse, over time, through these areas as must have happened in the past to give the historical Canadian distribution of the species. Snails typically have small home ranges, although Oregon Forestsnail may colonize new habitats including artificially created ones (e.g., ditches, fallow flooded areas) over time if the habitat is not continually disturbed or barriers to movement are not prolonged or permanent.
Using expert opinion, each of the 66 known sites was scored (yes/no) for fragmentation and isolation depending on its size and siting i.e., is it surrounded by city or highway or pending development or within a protected area (Table 1). Fifty-two of the 66 known sites (78.8%) were assessed as fragmented and isolated. At least 50 sites are less than 5 ha and/or are subject to urban development within the next 10 years (see Threats and Limiting Factors). These sites are isolated by distances of more than 1 km of unsuitable habitat as a result of past land development (since 2002) and most likely will not be able to maintain a viable snail population in the future. Examined another way, 50 of the 75 occupied 2 km x 2 km grid squares (66.7%) also are not viable. While the IUCN (2011) definition for severely fragmented does not contain a timeline in which to assess the viability of a subpopulation, if the average generation time is 5 years, three generations will occur by 2028. Some of the IUCN (2011) quantitative criteria for assigning status, which COSEWIC uses as guidelines, use 10 years or 3 generations, whichever is longer (up to a maximum of 100 years) as a time line; however, 100 years is also used in the quantitative analysis criterion E.
The proportion of the known area of occupancy that is fragmented and isolated equates to 108 ha of the 328 ha (33%) of the biological area of occupancy of mapped sites. Calculated this way, the situation does not strictly satisfy the IUCN definition for severe fragmentation when data are available: most (> 50%) of the total area of occupancy is in habitat patches that a) are smaller than required to support a viable population and b) the habitat patches are separated by large distance. But, examined other ways, the nearly 80% of the total number of known sites and 67% of the occupied 2 km x 2 km grid squares being considered too small and isolated to maintain viable populations does meet the spirit and intent of severe fragmentation. One must also not forget that these current, remnant fragments are indicative of larger pieces of habitat that once existed.
There is similar Oregon Forestsnail habitat south of the international border and thus populations likely occur within these areas. The separation distances or habitat connectivity between US and Canadian sites is unknown. Therefore the possibility of rescue is difficult to assess but likely is minimal even given suitable, connected cross-border habitat – snails have limited dispersal capability. Washington State has not been tracking the conservation status of Oregon Forestsnail nor is there recent survey information on the species (Potter pers. comm. 2011; Stellini pers. comm. 2011; Thomas pers. comm. 2011).