Spatial variation in lizard communities in the Tucson Basin 2010-2020 and implications for riparian conservation and management

This work is part of a broad effort by Pima County Regional Flood Control District (PCRFCD) to assess and mitigate the impacts of flood-control management projects on wildlife communities and their habitats, preserve wildlife by translocating individuals away from sites before construction activities, and augment the value of habitats and riparian areas for wildlife across the Tucson Basin. Efforts by PCRFCD have targeted herpetofauna and focused on establishing methods to mitigate ecological impacts, salvage and translocate vulnerable individuals away from construction sites, and generate inferences to help guide restoration and conservation in support of the Sonoran Desert Conservation Plan. This report summarizes efforts to extend lizard surveys and monitoring throughout the Tucson Basin into the 2020 field season and to apply data from 2010-2020 to understand spatial variation in populations and communities across multiple regions and sites to help guide efforts by PCRFCD. Additionally, we also evaluated and refined field methods and study designs to increase precision and best address broader project objectives. Data resources we provide will help establish baselines for future monitoring of flood-control projects and enable rigorous evaluations of impacts and population recovery in ways that guide conservation and management.

To sample lizard communities, we surveyed transects that had been established across the Tucson Basin in prior years and at seven new sites along Pantano Wash. This effort expanded the spatial scope of monitoring into a third major watercourse or riverine region of the Tucson Basin to foster comparisons across the Santa Cruz River, Rillito Creek, Pantano Wash, and two regions in the urban core in Tucson that include smaller riparian areas such as Arroyo Chico. Efforts in 2020 were also significant for understanding lizard populations and broader environmental contexts because it was an unusually dry and hot year. In 2020, we completed 177 surveys along 84 transects that spanned 171.4 km of effort, and observed a total of 1,555 lizards of nine species along transects. We used modeling techniques to assess spatial variation in relative abundances (no./km) of the six most common species of lizards, all lizard individuals combined, and observed species richness across the Tucson Basin at two spatial scales; region and site. We used multivariate ordination techniques to assess spatiotemporal variation in community structure of all 12 species observed across time among the three major watercourse regions across the Tucson Basin and two urban core regions in Tucson based on data gathered over 11 years (2010-2020).

Patterns of spatial variation in communities among the three major watercourses of Tucson offered important insights for management, conservation, and future efforts. We found that lizard abundances and observed species richness generally peaked in the Rillito Creek region due, in part, to high abundances of zebra-tailed lizard and desert spiny lizard and to occurrences of several species that are rare or not present elsewhere in the basin. Communities along Rillito Creek were also tightly clustered in a distinctive portion of community space and showed low temporal variation, suggesting a unique and stable species assemblage with high conservation value. Such patterns are likely linked to high levels of connectivity with adjacent wild areas, an abundance of preferred habitats, and high coverages of large riparian trees and woodlands supported by closer depths to groundwater. In contrast to Rillito, lizard abundances along the Santa Cruz River seemed depressed despite moderate overall richness, and communities were fairly tightly clustered in the center of community space between Rillito Creek and Tucson urban core sites indicating low uniqueness. Such patterns along the Santa Cruz River could be due to overall drier floodplain conditions, degree of urban encroachment, higher abundances of predators, and more limited vegetation cover, which can reduce habitat area and quality and promote vulnerability to predation. Future restoration and local habitat management and enhancement along the Santa Cruz River, however, could augment habitat area and quality, and bolster population sizes and diversity along the Santa Cruz River. Finally, community structure along Pantano Wash in 2020 was most similar to that found along the Santa Cruz River overall indicating lower uniqueness and perhaps lower regional significance.

Importantly, our results also indicate the significance of the Tucson urban core for supporting high abundances and richness of lizards. This included species such as tiger whiptail, desert-spiny lizard, ornate tree lizard, and sometimes, regal horned lizard, which have high local abundances at some key sites that are important for conservation and management. Community structure in both urban core regions of Tucson spanned greater ranges and very different areas of community space than those along the three major watercourse regions we considered, indicating high uniqueness and conservation value of the Tucson urban core for lizards. High abundances and richness of lizard communities in such urban settings are likely driven by ecological subsidies and other resources linked to human environments, but sufficient habitat connectivity and quality will be needed to conserve them over the long term. Because the main source of inter-population connectivity for lizards in these regions are likely small watercourses and arroyos that traverse the valley floor in portions of the Tucson Basin, land managers have an important role in managing and conserving these populations.

Information on spatiotemporal patterns in populations and communities will help efforts to manage and conserve urban lizards and the habitats they depend on. Understanding factors and processes responsible for these patterns, however, can best guide management, restoration, and conservation actions and should be a focus of future efforts. Future efforts in the Tucson Basin should also consider annual monitoring to enhance the value of datasets for evaluating impacts to communities, the precision of estimates, desired effect sizes for monitoring evaluations, and statistical power to detect trends in populations and community parameters to assure efforts are consistent with PCRFCD goals and priorities.