Fire and the isotopic niche: Quantifying resource exploitation by tree lizard populations among burned Arizona grassland habitats

Background/Question/Methods   Prescribed burning is a common restoration and management practice used by Land Managers to mimic past episodes of disturbance and enhance faunal diversity. However, the ability to predict the responses of wildlife to prescribed burning has been mixed. Most studies have focused on numerical responses to fire and ignored functional (e.g., diet selection) responses. Determining these responses are critical, because they are a direct measure of whether burned habitats allow species to maintain stable or growing populations. Here, I evaluate the resource use of Ornate Tree Lizard (Urosaurus ornatus) populations among two areas in southeast Arizona differing in burn frequency (1x and 2x, respectively). I use stable isotope analysis of carbon and nitrogen isotopes to track resources across trophic links (vegetation, arthropod prey, lizards) and map resource availablity within my study sites. Within each site, available resources (vegetation and arthropod) were quantified by measuring structural and cover variables along a stratified grid. I quantified used resources (structural and cover) within radius plots around the initial capture sites of each focal lizard. In addition, I used the program IsoSource to determine the specific contribution of C3 and C4 resources to lizard diets.

Results/Conclusions   Both sites were dominated by primarily C4 grasses (>84% cover in all areas), but tree lizards relied on food webs that were 44% C3 based across both sites. Sites were largely similar in composition and cover, with differences being driven by the 2x burn site having greater bare ground cover. Nitrogen isotopes indicate that tree lizards are relying more on herbivorous arthropods than other arthropod functional groups. Preliminary results that include data from other lizard species in the community illustrate species collected in more frequently burned areas rely more on C4 sources than C3 sources. As C3 resources are generally more nutritious (lower C:N ratio), the interaction between fire frequency and variation in shrub availability might have critical effects on the persistence of lizard populations in the long-run. These results will be discussed, and future research directions proposed. Ultimately, these results will build upon and expand past studies by including isotopic analysis to estimate detailed nutrient dynamics and energy flow which facilitates understanding how prescribed burns affect the functional use of affected habitat relative to non-burned controls.