title: Distributions and Habitat Associations of Blue Point Pyrgulopsis and Blue Point Tryonia
Creators: name: Wilcox, Chenoa
Publisher: name: University of Nevada, Las Vegas
Year: 2021

description: Springsnails and other crenophilic species are often of conservation concern due to endemism, narrow habitat requirements, and susceptibility to habitat degradation. Because there is a distinct lack of natural history information for many of these organisms, management of species that are of concern is often based on data from non-target species related to the species of interest. In this thesis, I provide background information regarding springsnails in western North America (Chapter 1), describe a study investigating the distribution and habitat associations of two springsnails endemic to a single spring (Chapter 2), and provide supplemental information on habitat characteristics associated with these springsnails (Chapter 3). Blue Point Spring is located in southern Nevada within Lake Mead Natural Recreation Area. This spring is occupied by several endemic species, including two springsnails: Pyrgulopsis coloradensis and Tryonia infernalis. These springsnails were known to occupy the upper segment of the spring, with high abundances in the source pool, but further information on their natural history was limited. The goal of my research was to assess the distribution, abundance, and habitat associations of P. coloradensis and T. infernalis at Blue Point Spring, in order to better inform resource management planning at the site. I monitored the distributional abundances of these species across the upper 20 meters of Blue Point Spring, sampling every 8 weeks over one year. I collected springsnails using direct substrate sampling, artificial substrate sampling, and vegetation sampling. Both springsnail species were present in high abundances in the source pool (upper 10 meters of the spring) where fish do not occur. Seasonally high abundances on artificial substrate samplers reached 78844/m2 for P. coloradensis and 26311/m2 for T. infernalis. I identified some level of habitat partitioning between these two springsnail species, as generalized linear mixed modeling indicated that species identity was a significant factor in springsnail distribution and abundance. Pyrgulopsis coloradensis reached its highest densities at meters 2.5–3.5 downstream, and T. infernalis reached its highest densities at meter 5.5 downstream. Modeling further indicated that T. infernalis may be selecting shadier habitats than P. coloradensis. Other factors identified as significant included pH, conductivity, meters downstream, percent vegetation cover, and location on the north or south edge of the spring. I supplemented the modeling with observational studies to assess springsnail movement and predation by nonnative fishes. The observational studies indicated that springsnails move at high rates of speed, averaging ~ 0.7 cm/min for P. coloradensis and ~ 0.9 cm/min for T. infernalis, and could colonize tile samplers in about 2.12–27.06 hours. The rate of movements likely impacted the experiments with fish exclusions. Feeding trials and a fish exclusion experiment indicated that nonnative fishes pose a significant threat to springsnails at Blue Point Spring, with convict cichlids, in particular, appearing to be a major limiting factor in the distribution of both P. coloradensis and T. infernalis. The differences in distributional abundance between springsnail species, as well as the impact of nonnative fishes are critical information to consider when making management decisions at Blue Point Spring, including potential habitat modification to improve conditions for other species of interest at the site or eradication of nonnative fishes from portions of the spring.