Effects of a biological control agent on the use of saltcedar habitat by passerine birds

Authors: Longland, William - Bill; DUDLEY, TOM - UNIV. CALIF., SANTA BARB

Submitted to: Great Basin Birds
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/2007
Publication Date: 4/18/2008
Citation: Longland, W.S., Dudley, T. 2008. Effects of a biological control agent on the use of saltcedar habitat by passerine birds. Great Basin Birds. 10:21-26.

Interpretive Summary: A major environmental problem along river systems in the southwestern U.S. involves replacement of native trees, such as willows and cottonwoods, by exotic saltcedar trees. Among other problems associated with saltcedar, it appears to offer very poor habitat for birds. An Asian beetle species (the saltcedar leaf beetle) that feeds exclusively on saltcedar has been imported and released in an attempt to control this weedy tree. Although saltcedar control should benefit bird populations, some concern has been expressed among wildlife managers and the interested public about potentially negative effects of this biological control effort on birds. We tested whether the presence of the saltcedar leaf beetle enhances the use of saltcedar habitats by birds by comparing how frequently bird droppings occurred at saltcedar infested sites having high densities of beetles versus nearby sites lacking beetles. Our results suggest that bird use of saltcedar habitat was approximately 5 times greater where beetles occurred than where they were absent. The environmental benefits of introducing this beetle species extend beyond its intended purpose of controlling saltcedar; it also provides an abundant food source that benefits bird populations.

Technical Abstract: Invasion of native riparian habitats by saltcedar (Tamarix spp.) in the southwestern United States, has caused declines in population density of birds. The saltcedar leaf beetle (Diorhabda elongata) has been released at several sites as a biological control agent. At two northern Nevada locations, populations of this beetle have established, increased in explosive fashion, and dispersed over a large area. We used the frequency of bird fecal droppings to index bird habitat use in saltcedar stands where beetles were established in high densities and compared this to paired sites lacking beetles that were outside the expanding distribution. Bird habitat use was approximately five times greater where D. elongata had become established than in paired sites lacking these beetles. We also found a significant rank correlation between beetle abundance and bird fecal frequency in comparing sites with no beetles, high densities of beetles, and intermediate densities. Finally, we found that bird fecal frequency was significantly greater at D. elongata sites where adult beetles predominated versus paired sites where larval beetles occurred in high densities. The establishment of an often abundant biological control agent in northern Nevada has increased bird use of saltcedar stands, which are typically poor habitats for passerine birds.