Use of road underpasses by mammals and a monitor lizard in eastern Australia and consideration of the prey-trap hypothesis

Road networks continue to expand globally with predictable effects on
ecological systems. Research into the effectiveness of road underpasses
and overpasses for wildlife has been concentrated in North America and
Europe. In Australia, most studies of underpasses have been of relatively
short duration and without reference sites to give context to the measured
rates of use. We studied 5–7 road underpasses at two locations in eastern
Australia over 2–3 years, comparing camera trap detections of animals in
underpasses with those at nearby forest sites. Three species of large
macropod (wallabies and kangaroos) were frequently detected in the
underpasses, with some underpasses traversed 1–4 times per week, and in
many cases exceeded detections in the forest. The lace monitor (Varanus
various) was detected in all underpasses, often once per week during
spring and summer, and infrequently in the forest. At each location a
different small macropod species, including one regionally threatened,
showed a higher probability of detection in one underpass compared to
several of the forest sites. The vulnerable koala (Phascolartos cinereus)
was detected infrequently in underpasses and in the adjoining forest. The
short-beaked echidna (Tachyglossus aculeatus) had a high probability of
detection in a single underpass. The ‘prey-trap hypothesis’ postulates
that predators will exhibit increased activity at underpasses as a
consequence of prey being funnelled. We found the red fox (Vulpes vulpes)
had high activity in some underpasses. However, its activity coincided
less than expected with the activity of the mammals most at risk to it.
Our results provide no consistent support for the ‘prey-trap hypothesis’.
Instead, our study confirms the generic value of underpasses for a range
of medium-large mammals as well as one large reptile. Habitat adjoining
underpasses exerts a strong influence on their use and requires greater
consideration to maximise underpass use.