Competition and the century-long decline of a once common lizard, Sceloporus consobrinus

Habitat modification and climate change are primary mechanisms responsible for historical and ongoing population declines. Species interactions however, though difficult to study, may be of similar importance. Here we use a combination of historical species records, standardized transect surveys, and staged competition trials to assess the role of competition in recent population trends and distributions of two closely related lizard species: the prairie lizard (Sceloporus consobrinus) and the Texas Spiny lizard (S. olivaceus). Occurrence data reveals divergent population trends. S. consobrinus has decreased while S. olivaceus has increased in relative frequency over the last 100 years. We analyzed spatially-aggregated records of all lizards within the range of S. consobrinus to determine the role of climate suitability, climate change, landcover, and species interactions in shaping the occurrence patterns of S. consobrinus. In contrast to other lizard species, presence of S. olivaceus was associated with substantial reductions of S. consobrinus occurrence, and explains occurrence patterns better than either climate suitability or landcover. To test whether patterns of broad scale co-occurrence are indicative of local competitive exclusion we conducted 200m transects surveys to assess lizard communities and paired this with staged behavioral trials in the lab. Despite occurring in similar habitats, and across similar regions, transect surveys revealed lower-than-expected abundance of S. consobrinus on transects containing S. olivaceus, with both species co-occurring on only 2 of 176 transects. Shifts in habitat use implicate competitive displacement, with S. consobrinus occupying areas with three times less canopy cover at sites with S. olivaceus compared to those without. Finally, behavioral trials revealed competitive dominance of S. olivaceus, which controlled the prime basking position, and initiated more interactions that led to retreat or hiding by S. consobrinus. Our study implicates competitive interactions as in important force in structuring species distributions and population trends.