Abundance and reproduction of toads (Bufo) along a regulated river in the southwestern United States: Importance of flooding in riparian ecosystems

Abundance and size of toads (Bufo woodhousii and B. cognatus) were related to precipitation, river flow, and groundwater over 7 years along the Middle Rio Grande, a regulated river in the semi-arid southwestern United States. Toads were monitored in riparian areas at 12 sites spanning 140 km of river during summers 2000–2006. Regional precipitation varied between years, with occurrence of both drought and record high precipitation. Abundance of toads was low during most of the study (mean=11 captures/month/site). However, two sites flooded in spring 2005, resulting in a dramatic increase in captures at those sites (mean=214 captures/month/site). Most individuals captured in 2005 were small (median body mass 0.6 g), suggesting that toads used the floodplain for breeding during the flood. Only river flows that exceeded 100 m3/s brought groundwater close enough to the surface to create pools used by toads for reproduction and development. Such flows were once common along the Middle Rio Grande but are rare following regulation. Our results demonstrate that small, managed floods can positively affect abundance of toads by providing off-channel, aquatic habitats along regulated rivers.