Ecological interactions in the biological control of saltcedar (Tamarix spp.) in the United States: Toward a new understanding

Document Type

Conference Proceeding

Publication Date


Publication Title

Proceedings of the X International Symposium on Biological Control of Weeds


Montana State University

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Saltcedars (Tamarix spp. Tamaricaceae) (SC) are exotic, invasive shrubs to mediumsized trees native to the Old World. In riparian ecosystems of the western United States, SC replaces native plant communities, degrades wildlife habitat, reduces biodiversity, alters stream channel morphology, uses large quantities of groundwater, increases wildfire frequency, reduces recreational and agricultural usage, and has contributed to the decline of many wildlife and fish species. In recent years, the southwestern willow flycatcher (Empidonax trailii extimus) (sw WIFL) has begun nesting extensively in SC in some of its major breeding areas in Arizona, but not in other areas, since SC has replaced its native willow nest trees. This has caused great concern among flycatcher biologists that the proposed biological control of SC will further reduce sw WIFL populations. Current understanding of the causes of the SC invasion and of its effects on wildlife appear to be guided mostly by two paradigms. First, that only abiotic (mostly anthropogenic) changes such as altered hydrologic regimes, lowered water tables, high soil salinity, wildfires, and livestock grazing are determinants of SC abundance. These factors make SC appear “better adapted” and “more aggressive” than the native vegetation. SC is said to be only a passive invader, after the ecosystem already had been damaged by human disturbances. Second, that in areas now too dry or too saline, SC communities can act as functional analogs of native plant communities. This concept has been extended recently, but without supporting evidence, to include equivalency in providing habitat for wildlife, including the sw WIFL. Neither concept explains the dramatic invasion and dominance by SC along unregulated streams, often far from human disturbances. The extended equivalency concept is counter to consistent field observations of lower bird species diversity and density in SC, lower sw WIFL reproductive success, lack of fruits, seeds, suitable insects and nesting cavities for birds and other wildlife, and higher frequency of wildfires that destroy active sw WIFL nests in SC. Circumstantial evidence also indicates that SC may provide less protection from nest parasitism and predation and from climatic extremes. Nevertheless, the sw WIFL nests extensively in SC in some major nesting areas, even though suitable willows grow adjacently. The birds appear to be attracted to the site by dense foliage in moderate to broad floodplains, near or over free water, and with adequate populations of flower-feeding, flying insects. We propose that they nest in SC instead of willow trees because of the superior branching structure of SC for nest placement, even though other traits of SC are inferior and lead to lowered reproductive success - a case of a “super stimulus”. We project that biological control, by the introduction of host specific insects of SC from Eurasia, would provide the missing insect herbivory needed to negate the appearance that SC is “more aggressive” and “better adapted” than the native plants, and to reduce it to a non-damaging member of western riparian plant communities. This reduction in competition is expected to allow the native vegetation to recover in areas where soil salinity and depth to water table are suitable, which includes nearly all of the major sw WIFL breeding areas. Also, because of the present synergetic interaction with SC, some degree of improvement in these abiotic factors and wildfire frequency will result from biological control. Biological control is expected to gradually reduce SC density, with a concurrent increase in cottonwood/willow and other native habitat. The sw WIFL will continue to nest in the remaining SC trees and will begin again to nest in native vegetation.


Desert floodplain ecosystems; Groundwater level; Invasive species; Nesting; Salt cedar (Tamarix spp. Tamaricaceae); Southwestern willow flycatcher (Empidonax trailii extimus); Southwestern United States


Desert Ecology | Systems Biology | Terrestrial and Aquatic Ecology | Weed Science