One of most significant threats to global biodiversity is the invasion of exotic species into natural areas due to human activities and commerce (Clout, 1995). Insidious effects of invasive exotic species often include the inexorable displacement, or replacement, of native plant and animal species, disruptions in nutrient and fire cycles, and changes in the pattern of plant succession. The objective of this paper is to provide a brief summary of some of the impacts of tamarisk or salt cedar (genus Tamarix) invasion on native plants and animals, with an emphasis on the southwestern United States. This paper summarizes information previously presented by Lovich et al. (1994).
Tamarisk invasion has serious consequences on the structure and stability of native plant communities. The decline of riparian stands of cottonwood (Populus fremontii) along the Rio Grande in New Mexico is partially attributable to the invasion of tamarisk. The thick stands of exotic plants along the floodplain have severely limited the number of germination sites that are suitable to cottonwood (Howe and Knopf, 1991). Similarly, in the desert region of Australia tamarisk is capable of displacing native plant species, resulting in the dominance of native vegetation by a relatively few species of introduced and salt-tolerant plants. Tamarisk dominance also results in a reduction in the numbers of native birds and reptiles (Griffin et al., 1989) relative to native ecosystems.
A secondary effect of tamarisk invasion is related to increased frequency of fire in impacted areas. The drought-deciduous nature of tamarisk contributes to a heavy fuel load in infested areas, promoting a fire rotation of about 10 to 20 years (Kerpez and Smith, 1987; Rosenberg et al., 1991). The fire tolerance of tamarisk coupled with the fire intolerance of many native shrubs in the southwestern deserts effectively leads to tamarisk dominance in native plant communities in a relatively short time period.
Some authors have suggested that invasion of tamarisk is a sign, not a cause, of habitat modification and degradation (Horton, 1977). Replacement of tamarisk by native plant species will likely require correcting environmental factors that favored the invasion of tamarisk in the first place (Anderson and Miller, 1990). Such factors will need to be identified (see Anderson and Miller, 1992) prior to active revegetation efforts. However, an area dominated by tamarisk is likely to remain so unless altered by natural cataclysms or man (Kerpez and Smith, 1987).
The suitability of tamarisk as wildlife habitat has been a subject of considerable debate. Naturally, it is utilized by wildlife in its native range in the old world. For example, elephants (Loxodonta africana) in Namibia, Africa exhibit a definite preference for Tamarix usneoides irrespective of plant availability or size (Viljoen, 1989). However, outside of its natural range in the southwestern United States it generally provides unsuitable habitat for most wildlife because neither its foliage nor its flowers (including seeds) have any significant forage value in contrast to native species such as mesquite (a notable exception being the fact that the exotic honeybee, Apis mellifera, utilizes the pollen). However, from a structural standpoint it does provide cover for some species, particularly birds. For example doves (Zenaida macroura), Mississippi Kites (Ictinia mississippiensis), and various passerine birds are known to nest in tamarisk dominated habitats (Glinski and Ohmart, 1983; Brown and Trosset, 1989; Rosenberg et al., 1991). Rice et al. (1983) determined that tamarisk foliage height diversity was an important determinant of avian community organization, although native plant species were more important determinants.
The value of tamarisk to wildlife appears to vary geographically. Utilization of tamarisk by birds was high on the middle Pecos River, intermediate on the lower Rio Grande, and very low on the lower Colorado River. Avian use of tamarisk along the Pecos River may be enhanced due to the occurrence of seed producing shrubs and annuals within or adjacent to the exotic habitat (Hunter et al., 1988). It is important to note that all published studies of the value of tamarisk as wildlife habitat have focused on birds. Purported benefits to selected birds do not necessarily extend to other animals. Additional research is needed on the relationship between tamarisk and other groups of species including invertebrates as compared to native vegetation types.
In spite of the value that tamarisk may have for wildlife cover, most authors have concluded that the invader has little value to native wildlife (Kerpez and Smith, 1987; Anderson and Miller, 1990; Rosenberg et al., 1991). As tamarisk displaces native vegetation the value of the original habitat is progressively diminished for many native animal species.
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Anderson, B. W., and E. R. Miller. 1992. Suitability of the Rancho Dos Palmas area for revegetation. Draft Report to Metropolitan Water District, Los Angeles. 66 pp. + iv.
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Viljoen, P. J. 1989. Habitat selection and preferred food plants of a desert-dwelling elephant population in the northern Namib Desert, South West Africa/Namibia. African Journal of Ecology 27:227-240.
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