Population demography in two lizard species living near roads in the Northern Chihuahuan Desert

Background/Question/Methods

Although increased mortality rates in wildlife populations living near roads has long been observed, it has only been relatively recently that studies have looked at how increased mortality might affect the demography of these populations.  In addition, species with different behaviors are expected to be differentially impacted by roads.  If individual animals venture into roads, their risk of mortality increases and population sizes might decline.  If individuals avoid roads, they do not risk mortality, but the populations will be isolated, potentially decreasing long-term population viability.  We investigated the impacts of two roads of differing sizes on two sympatric species of lizards: relatively sedentary side-blotched lizards (Uta stansburiana) and wide-ranging western whiptail lizards (Aspidoscelis marmorata).   Study sites were located in southern New Mexico, with three near Interstate 10 (large road with >16,000 vehicles/day) and three near New Mexico State Highway 9 (small road with >500 vehicles/day).  Pitfall trap arrays were located near the road (NR), with additional arrays at 50 and 125 m (FR) from the road.  All captured lizards were marked and released.  Trapping occurred during summer and early fall from 2009 to 2011.  Each site was trapped for around 3 days/month, for a total of 9,200 trap-days.

Results/Conclusions

Side-blotched lizards were the most abundant species, with 1971 individuals trapped 3057 times, followed by western whiptails with 1465 individuals captured 3215 times.  Slightly more whiptails were captured at the sites along I-10 than those along NM-9 (I-10: 0.38±0.06 inds/trap-day; NM-9: 0.33±0.04 inds/trap-day).  However, there was no trend with increasing distance from the road for either road (I-10 NR: 0.40±0.18 inds/trap-day, I-10 FR: 0.31±0.07 inds/trap-day; NM-9 NR: 0.31±0.14 inds/trap-day, NM-9 FR: 0.30±0.13 inds/trap-day).  Similar patterns were seen with side-blotched lizards, with slightly more captures along I-10 than NM-9 (I-10: 0.39±0.04 inds/trap-day; NM-9: 0.30±0.04 inds/trap-day).  Again there was no difference in the number of captures near the roads compared with traps 125 m from the roads (I-10 NR: 0.40±0.09 inds/trap-day, I-10 FR: 0.36±0.09 ind/trap-day; NM-9 NR: 0.33±0.07 inds/trap-day, NM-9 FR: 0.26±0.03 inds/trap-day).  These results do not support our hypothesis that roads are affecting lizard populations.  Because I-10 presents a large expanse with no cover, it is likely that neither species attempts to cross.  We have observed both species crossing NM-9, but the traffic volume is likely too low to significantly increase mortality rates.  This study highlights the need to consider road size and traffic volume when assessing risk to wildlife.