Status, distribution, habitat, and stressors of the Sonoran Talussnail

The Sonoran talussnail (Soronella magdalenensis) is known from eight mountain ranges in Pima and Santa Cruz counties in southern Arizona, and also occurs in adjacent Sonora, Mexico. Despite recent conservation concern and various threats to this species, little is known about current distribution and abundance or the ecology and habitat of populations. We assessed the distribution and relative abundance of talussnails across the potential range of the Sonoran talussnail in Arizona, modeled factors that explain occurrence and relative abundance, and estimated persistence of talussnail populations that had been documented historically. We surveyed talussnails in the field by sampling a broad set of randomly-selected plots stratified among four landforms (talus, mountain slope, upper bajada, drainage) and various historical sites, and quantified a range of environmental attributes and land-use intensity at all survey sites. Complications in species identification precluded unequivocal determinations that all observed talussnails were in fact Sonoran talussnails, but morphological measurements and tissue samples we gathered can help foster future efforts to identify individuals to the species level.

We surveyed 130 plots between mid-September 2018 and late February 2021 in 22 mountain ranges and adjacent landscapes and between 627 and 2,350 meters (m) elevation. Most surveyed plots were selected randomly (81%) with others mainly at old historical sites first documented before the 1970’s (n = 15) versus more recently (n = 10). Although we targeted wet periods for surveys, only 37% of plots were surveyed within 2 days of local rainfall and time since the last rain event at the time of surveys averaged 12 ± 1 days (± SE; range 0-60 days).  We found evidence of Sonorella presence within 37.7% of surveyed plots (which included 24.5% occupancy at random plots), and detected an estimated 526 live or dead individuals across the full range of sampled elevations in or around 17 mountain ranges. Relative abundance scaled by linear survey effort averaged 1.18 ± 0.31 individuals per 100 m (range = 0-31.1) across 131.9 km of linear survey effort and 128 person hours. Auspiciously, we documented evidence of recent occupancy of talussnails at 93% of old and 90% of new historical sites, suggesting high levels of population persistence. Plots at old historical localities were in the Santa Rita, Baboquivari, Cerro Colorado, Tumacacori, and Tucson mountains, on Tumamoc Hill, and on Black Mountain, and included most known historical localities from the early and mid-1900s within the range of the Sonoran talussnail. Some old historical localities in the Santa Rita, San Cayetano, Roskruge, and Comobabi mountains were not surveyed, some of which were on private or reservation lands.

Probability of occurrence at random sites varied widely among landforms (P = 0.012) and ranged from 0.58 ± 0.16 on talus, 0.27 ± 0.10 on mountain slopes, to much lower on bajadas and drainages (0.04-0.10 ± 0.04-0.06), with similar patterns for relative abundances that also increased across the same landscape gradient. There was limited evidence (P = 0.12-0.17) that occurrence probabilities and relative abundances varied among seven vegetation types we considered, but no evidence (P ≥ 0.41) of variation among slope aspects or rock types. Across all plots, probability of talussnail occurrence increased with increasing volume of oak vegetation and decreased with increasing volume of succulents and basal cover of woody debris, after controlling for the effects of landform and site classification (e.g. random vs. historical). Across all 49 plots occupied by Sonorella, relative abundances increased with elevation and basal cover of rocks, and decreased with increasing volume of conifers and succulents. Relative abundances also decreased with increasing land-use intensity, which was linked mainly to off-road vehicle and foot traffic, invasive plants, and livestock grazing. Within 39 plots where we obtained measurements, relative abundances also increased strikingly with increasing mean size of rocks.

We submitted 23 specimens from seven mountain ranges to an expert for morphological measurements, and obtained data on external traits for all specimens and internal traits from 18 specimens with developed genitalia. Shell height ranged from 8.8 to 18 mm (mean ± SE = 12.6 ± 0.4 mm), shell width ranged from 14.0 to 28.5 mm (21.3 ± 0.7), and number of whorls ranged from 3.9 to 4.9 (4.4 ± 0.1). Penis length ranged from 3.4 to 22.2 mm (mean ± SE = 8.3 ± 1.3), verge length ranged from 2.3 to 12.4 mm (5.3 ± 0.8), and vagina length ranged from 3.3 to 12.9 mm (6.9 ± 0.6 mm); 50% of specimens had longer penis than vagina lengths, 39% had longer vagina length, and 11% had relatively equal genital lengths. Observed trait values were within known ranges for Sonorella magdalenensis for a specimen from Cat Mountain, Tucson Mountains, whereas specimens from Madera Canyon in the Santa Rita Mountains and one specimen from the Atascosa Mountains were more consistent with Sonorella walkeri based on diagnostic characters from Miller (1967). However, sample sizes were small and there was uncertainty as to whether fully mature specimens were examined, which are required for reliable identification. Therefore, morphological traits did not provide unequivocal identifications to species level.

Our findings provide some of the first recent and some novel inferences on talussnail distribution, abundance, habitat, threats, and population persistence across broad spatial gradients that spanned the entire known geographic and elevation ranges of the Sonoran talussnail. Auspiciously, we found that talussnails were broadly distributed across much of the study region and had relatively high occurrence probabilities in randomly-selected plots on talus and mountain slopes. Although more work remains, our findings suggest talussnails are generalists that use a broad range of vegetation and substrate types across a diversity of landforms, slopes, and aspects. Key habitat factors that explained distribution and abundance were linked to:  1) rock cover, rock size, and presence of talus and other rock-dominated landforms that can provide needed protection from high temperatures and desiccation, 2) factors such as ground cover of woody debris that influence access to sheltered microhabitats between and under rocks, and 3) cover of arid-adapted plants such as succulents and elevation that are linked to local and regional moisture availability. Importantly, we also found evidence that talussnail populations are sensitive to increasing land-use intensity, which poses a range of potential challenges and opportunities for conservation and management. Regardless, high levels of persistence of populations that were documented historically suggest broad-scale patterns of distribution are stable despite threats. Future efforts should assess local demographic attributes and how they are influenced by climate, microhabitat resources and conditions, and land use, and the phylogenetic and taxonomic status of populations.