POPULATION DYNAMICS OF A THREATENED SAND DUNE LIZARD

Abstract

Understanding how and why the abundance of a species changes in space and time is an essential component to effective endangered species conservation. Key to this understanding is being able to distinguish natural population dynamics from a downward trajectory of a species at risk of extinction. For many species in arid environments, rainfall drives population changes. This is the case for Coachella Valley fringe-toed lizards (Uma inornata), a species listed as threatened under the U.S. Endangered Species Act. At low rainfall levels, the lizards exhibit negative population growth until annual precipitation exceeds 40 to 50 mm. Fluctuation in the population growth of the lizards is also correlated with changes in their diet. A regression model using rainfall and diet to explain lizard population dynamics resulted in a significant R2 value of 0.956. Because drought is common in their arid environment, it is not unusual for this lizard species to endure consecutive years of population declines. Fringe-toed lizard population counts during extended droughts often approach zero, yet the populations quickly rebound during periods of near average rainfall. If counts approaching zero are not reliable thresholds for when remedial management actions are warranted, then monitoring based management decisions need to use criteria that are more heuristic. Departures from the rainfall-diet-population growth model might provide the signal needed for management actions.