Background information and definitions
Waterways can separate populations of a species or provide barriers to movements. Artificial waterways (such as canals and aqueducts) can disrupt movements between previously connected habitat. This may result in genetic isolation of populations (e.g. Corlatti et al. 2009) or drownings, if animals attempt to cross waterways that have steep sides. Crossing points may be installed for use of animals in an attempt to maintain connectivity and free movement between sites or habitats.
Corlatti L., Hackländer K. & Frey-Roos F. (2009) Ability of wildlife overpasses to provide connectivity and prevent genetic isolation. Conservation Biology, 23, 548–556.
Supporting evidence from individual studies
A site comparison study in 1996–1997 along an aqueduct in Arizona, USA (Popowski & Krausman 2002) found that overpasses over a waterway within a created wildlife corridor were used by desert mule deer Odocoileus hemionus eremicus, collared peccaries Pecari tajacu and coyotes Canis latrans. Mule deer and peccaries used all six wildlife overpasses inside the corridor. Bridges outside the corridor, not designed for wildlife, were also used. However, there were more mule deer tracks on wildlife overpasses inside the corridor (average 0.06–0.11 tracks/reading) than on bridges outside the corridor (0–0.01 tracks/reading). The same held for peccaries (wildlife overpasses: 0.15–0.21 tracks/reading; bridges: 0.06–0.17). There was no difference for coyotes (wildlife overpasses: (0.28–0.45 tracks/reading; bridges: 0.31–0.59). Aqueduct crossings were provided at five points within and one immediately adjacent to the corridor. Crossings were 9–173 m wide. Four crossings to the north were also monitored along 11 km of aqueduct. Crossings within the corridor contained natural soil and vegetation. Those outside were concrete overchutes or overpasses of water. Animal tracks were recorded on sand plots (2–22/crossing) on ≥7 consecutive days/month from August 1996 to July 1997 (total 117 checks/plot).Study and other actions tested
A replicated study in 1993–1998 along a canal in Guardo, northern Spain (Peris & Morales 2004) found that all nine small bridges and six of 14 wider bridges designed for humans and livestock were used as crossing points by mammals. Crossings were made by roe deer Capreolus capreolus (four crossings), red deer Cervus elaphus (four), wild boar Sus scrofa (nine), wolf Canis lupus (three), fox (52) and by mustelids, mainly badgers Meles meles and stone martens Martes foina (14). Iberian hares Lepus granatensis and hedgehogs Erinaceus europaeus were also recorded. Small wildlife bridges were used more than were larger bridges by all mammals as a whole (see paper for details) and bridges near scrubland were used more (12 out of 13 used) than were those near cropland (one out of nine used). Despite crossings being available, 123 roe deer and 34 wild boars were found drowned over the five years. Fourteen concrete bridges (for humans and livestock; 5.0–7.5 m wide) and nine small wildlife bridges (2.5–3.6 m wide) along 24 km of a 5-m-wide concrete water canal were monitored. Tracks in sand and other animal signs were recorded on each bridge every three days from April to September 1998. Drowned mammals were monitored daily from April 1993 to October 1998.Study and other actions tested