A typically specialist predator, the greater short-horned lizard (Phrynosoma hernandesi), reveals variable diet across its geographic range, obscuring constraints that most limit distribution

Background/Question/Methods

For nearly two centuries, biologists have sought to understand the mechanisms that shape species’ distributions and we are still searching for those mechanisms today. Ecologists tend to classify organisms into discrete bins such as ‘specialized’ or ‘generalized’ in their ecological traits. While these labels provide a convenient way to compare species’ ecologies and potential responses to environmental change, they can also mask the complexities that define a species’ environmental needs and limitations. In particular, resource specialization can change across spatial scales, altering the factors that most limit a species. Ignoring this spatial variability could impact our ability to identify those limiting factors.

Horned lizards (Phrynosoma spp.) are classically ‘specialized’ species, with narrow diets consisting primarily of ants. Some short-horned species, however, have more varied diets, though it is unclear how diet composition changes among populations. Untangling how diet varies across a short-horned species’ distribution can provide insight into factors that limit its population sizes and influence its responses to environmental change. We asked, how does diet specificity vary geographically among greater short-horned lizards (Phrynosoma hernandesi)?

Results/Conclusions

To address our question, we searched systematically for lizards and collected lizard scats in 100 half hectare plots across Wyoming. In each plot we also sampled relative invertebrate abundance and composition by setting out unbaited pitfall traps and mapping all large ant mounds. We compared prey selection with prey availability to identify differences in specialization among populations. To corroborate the data from our scat samples, we used stable isotope techniques on lizard claws to identify δ¹³C and δ¹⁵N levels in diet and compare those levels with signature values from potential prey sources.

Our results suggest that this species is more general in its dietary habits than other horned lizard species, and that its food use shows strong spatial variation. Harvester ants, the dominant prey for many horned lizard species, weakly predicted the presence of horned lizards (P=0.033) but did not predict lizard abundance. Additionally, there was a thriving population (43% gravid females) where no harvester ants were present. Our research highlights that it can be a mistake to identify a single factor, diet in this case, as the most important limitation to a species across a broad range, and that it is necessary to consider how limiting factors may vary intraspecifically.