Using prey availability to evaluate Lower Colorado River riparian restoration

Abstract

The Lower Colorado River Multi-species Conservation Program (MSCP) is charged with restoring habitat for 26 species such as the southwestern willow flycatcher (Empidonax traillii extimus) impacted by water development projects on the river. As of 2015, the MSCP had spent $200 million to create 1,200 ha of habitat at nine sites, but the benefits to these insectivorous birds and other target species have not been quantified. Many MSCP projects emphasized riparian plantings of willow (Salix exigua, Salix gooddingii) and cottonwood (Populus fremontii) on high terraces disconnected from the river. We documented prey availability for insectivores in constructed habitats as an indicator of restoration effectiveness. Using sticky traps as a proxy to estimate aerial insect flux, we found the number of aquatic insects, proportion of aquatic insects, total number of insects, and number of insect orders were all significantly lower in MSCP plantation sites than at the river's edge. Riparian restoration sites over 100 m from the river had only 4% of the aquatic insects, 20% of the total insects, and only half as many insect orders as sites adjacent to the river. Thus, food availability and overall habitat quality for insectivores are likely low in restoration sites that are distant from the river.