The effects of living near roads on abundance and survival in two lizard species in southern New Mexico
Although many studies have documented roadkills among different taxa, less work has been done to determine how this road-related mortality affects abundance and survival in near-road populations. Studies have also recently begun investigating how species-specific behavior influences road-mortality rates. Animals that behaviorally avoid roads should not experience increased mortality, so near-road populations should have similar abundance and survival as away-from-road populations. We investigated how two roads of different sizes and traffic volumes impact populations of two syntopic lizard species: wide-ranging western whiptails (Aspidoscelis marmorata) and sedentary side-blotched lizards (Uta stansburiana). Study sites were located in southern New Mexico: three near NM Highway 9 (two-lane road with <600 vehicles/day) and three near Interstate 10 (four-lane highway with >16,000 vehicles/day), with one control site per road (1000 m from the road). Pitfall trap arrays were located 0 m (close), 50 m (middle) and 125 m (far) from the road. We trapped each site for 3 days/month during summers from 2009-2011. All trapped lizards were marked and released. Abundance was estimated as minimum number of animals known alive and annual survival was calculated as the number of recaptures in one year divided by the number of individuals marked the previous year.
We captured 1945 side-blotched lizards an average of 1.56 times each. Proximity to I-10 did not affect side-blotched lizard abundance. There were significantly more lizards living close to than away from NM-9 (ANOVA: F=4.42, p=0.028, df=6), although actual abundance was not much greater (close: 0.22 ind/trap-day; middle and far: 0.18 ind/trap-day). Annual survival rates were not affected by road proximity. We captured 1385 whiptails an average of 2.34 times each. Road proximity did not affect abundance for either road (average abundance around 0.15 ind/trap-day at all locations). Only annual survival from 2010-2011 was affected by road proximity: individuals at both the close and control distances had lower survival than those at the other distances (ANOVA, F=7.416, p=0.001, df=3). These results do not show lower abundance and/or survival in near-road populations. The higher traffic volume of I-10 likely causes both species to behaviorally avoid road-crossings, while the lighter traffic on NM-9 allows individuals to cross with minimal mortality. As the local human population increases, NM-9 will experience greater traffic, which is likely to increase lizard mortality rates until there is sufficient traffic to cause behavioral avoidance of road-crossings. This transition period is expected to have the greatest impact on near-road populations.