Flat-tailed horned lizards (Phrynosoma mcallii) exist in one of the hottest, driest deserts of North America. Much of this lizard's habitat has been reduced and fragmented because of conversion from aeolian sand flats to agriculture, golf courses, housing, industrial-scale solar, and off-road vehicle recreation, as well as the construction of multiple highways, railroads, and aqueducts. To inform management actions aimed at sustaining populations in these habitat fragments it is critical to monitor the lizards' population fluctuations within a context of changes to the quality and quantity of their habitat and available resources. Here we describe fluctuations and relationships to habitat variables for a population of flat-tailed horned lizards at the northwesternmost edge of their range. Over 20 years of surveys, the lizards' population did not correlate with rainfall amounts as would be expected in such an arid environment. Rather, the lizards showed a negative correlation with the density of an invasive nonnative plant, Sahara mustard (Brassica tournefortii). In years with high rainfall the mustard formed dense stands, inhibiting native annual plants, and creating a negative trophic cascade that included the lizards' primary food, harvester ants. However, in moderate- and low-rainfall years, when the mustard is absent or at low densities, flat-tailed horned lizard populations rebound. The result is that the last remaining Coachella Valley population of flat-tailed horned lizards continues to persist.
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