Museum and Citizen-Science Data Indicate Contraction in the Range of Texas Horned Lizards (Phrynosoma cornutum)

Abstract: Contraction in the geographic range of Texas Horned Lizards (Phrynosoma cornutum) has long been suspected but not definitively examined. Contraction of a species' geographic range occurs through extirpation of local populations and thus might often be an early warning sign of further endangerment of the species. As such, there is a need to identify species distributional databases and develop statistical procedures for testing for range contraction. We developed a method based on regression and data randomization and then applied the method to museum (VertNet) and citizen-science (iNaturalist) data compiled for Texas Horned Lizards spanning the time period 1960–2019. The regression analysis examined whether distances between observations and the range center had decreased or increased over time. Decreases in distances could indirectly indicate range contraction, whereas increases could indicate range expansion. We conducted separate regressions for the four directional quadrants to assess the directionality of range change. Distance to VertNet observations in the eastern and western quadrants declined (P < 0.05) between 1960 and 2019, at rates of 4.4 and 1.5 km per year, respectively. The iNaturalist data (2000–2019) did not reveal significant declines in any of the quadrants. Further analyses indicated that these results were generally robust to the exact estimation (placement) of the historic range center. Our analysis provides the first quantitative assessment and test of contraction in the range of P. cornutum. Our method also demonstrates the utility of using museum and citizen-science data to monitor spatial distribution in other species that may be undergoing range contraction, although analyses must recognize inherent limitations of the data.