How roads affect population demography in two lizard species in the Northern Chihuahuan Desert

Background/Question/Methods

Roads can potentially impact wildlife populations in several negative ways, such as decreasing population sizes and individual survival rates or by acting as barriers to dispersal.  Here we report initial findings from a 2-year study on how roads affect populations of side-blotched lizards (Uta stansburiana) and western whiptails (Aspidoscelis marmorata) in southern New Mexico.  Three study sites each are located on Bureau of Land Management property along both Interstate 10 (a large road with >16,000 vehicles per day) and New Mexico State Highway 9 (a small road with about 500 vehicles per day).  Arrays of pitfall traps are located near the road, with additional traps at 50 m and 125 m from the road.  Control sites are located 1000 m from each road.  Each site had 32 traps, and two control sites each had 24 traps.  All lizards captured were marked and released.  Trapping occurred from July through October 2009 and May 2010 through September 2010, and we are presenting data from all four months of 2009 and May 2010.  Each site was trapped for around three days per month for a total of 3,240 trap days.  

Results/Conclusions

A total of 1533 individual lizards from 11 species were captured.  Side-blotched lizards and western whiptails were the most commonly captured animals, at 59% and 34% of the total individuals, respectively.  There was no difference in the number of individual lizards captured at the different distances from the road for either species and for either road (whiptails: average of 16 – 20 individuals at each distance, side-blotched lizards: average of 23 – 35 individuals at each distance).  There were no differences in the number of recaptures for either species at the different distances from the road, with about 70% of the individual lizards released never getting recaptured.  Although we did not detect any population-level effects of either road on these lizard species, it is possible that additional data might allow detection of an impact.  Alternatively, the roads themselves are large inhospitable areas without vegetative cover, so perhaps the lizards are simply not venturing onto the roads and thus are not experiencing increased mortality rates.   If that is the case, the roads are acting as barriers to dispersal and the populations on either side of the road might become genetically isolated from each other, potentially decreasing long-term persistence of these populations.