Factors affecting the use of fauna underpasses by bandicoots and bobtail lizards

The construction of fauna underpasses is becoming increasingly common in road construction projects. However, factors that affect their use by native fauna in Australia are still largely unknown. We monitored 10 fauna underpasses for 13 months to determine factors affecting their use by southern brown bandicoots Isoodon obesulus fusciventer and western bobtail lizards Tiliqua rugosa rugosa. We trapped around each underpass five times to determine the size of the populations of these two species. Bandicoots and bobtail lizards were marked with passive implant transponders (PIT) and PIT readers and motion‐activated infrared cameras recorded the number of crossings and the number of individuals that used each underpass. For bandicoots the mean weekly frequency of use corrected for population size and the proportion of the marked population that used each underpass were modelled against factors hypothesized to impact underpass use using generalized linear models. These were underpass length, cross‐sectional area, vegetation cover at the entrances, presence or absence of logs and branches in the underpass (furniture), use by introduced predators and time since construction. Underpass length was negatively correlated with the frequency of use by bandicoots and time since construction was negatively correlated with the proportion of the population that used the underpasses. For western bobtail lizards none of the factors tested were strongly correlated with the frequency of use, or the proportion of the population that used the underpass. Based on these results, we recommend that the length of underpasses be kept to a minimum, possibly through the use of vegetated median strips allowing shorter underpasses to connect habitat on either side of the road. This would likely result in greater use than one long underpass spanning the entire width of the road.